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Astronomy From Ancient Times To 1975

Throughout history, mankind has been interested in the appearance and movement of objects in the sky: the Sun by day, the Moon, stars and planets by night. All cultures have held beliefs about the sky and some gained empirical knowledge through patient observation, for example knowledge of the patterns with which particular objects appear in the heavens. Some early civilisations regarded the Sun and Moon as gods. All advanced civilisations needed to develop an accurate calendar to determine when to hold religious festivals and to regulate agriculture and trade, and this provided one of the first important practical benefits of studying the sky.

Very early in the history of astronomy, the science was linked with astrology; indeed, in early times, the two were so intertwined that it is impossible to separate them. Copernicus was one of the first to adopt a purely scientific approach to astronomy. Later, Kepler and Newton enhanced his approach, as a result of which astronomy and astrology were at last separated.

The development of the telescope in the late sixteenth century opened a vast potential for new astronomical discoveries and subsequent inventions extended this enormously. Scientists determined the distances of the stars, their chemical and physical properties and their overall distribution, the shape of the galaxy and our position within it. The work of Edwin Hubble (1889-1953) and Walter Baade (1893-1960) revealed the existence of countless millions of other galaxies, of all shapes and sizes. Since the end of the Second World War, many new branches of astronomy have been developed. Radio astronomy has provided a completely new outlook on the universe and, together with gamma-ray, X-ray, infra-red and ultra-violet observations, has provided vast quantities of information on all manner of celestial objects. More recently, quasars, pulsars and Active Galactic Nuclei (AGN) have been discovered. Such objects radiate vast amounts of energy by mechanisms which are not yet understood.

This page provides an overview of key elements of the development of astronomy from ancient times up to 1975.

Assyrian Astronomy

Assyrian_tablet.jpg Neo-Assyrian cuneiform tablet, 7th century BC from Nineveh, describing observations of Venus.

Mesopotamia, the land between the Tigris and Euphrates rivers in what is nowadays Iraq, was the birthplace of civilization almost 10,000 years ago. The oldest records of the study of astronomy are in the form of clay tablets dating from circa 4000 BC found from ancient Sumeria, the southernmost region of Mesopotamia.

Assyria was a later civilisation, flourishing around 1000 BC in the same geographic area, which inherited many of the astronomical myths and legends from Sumeria. Historians have learned about Assyrian astronomy mainly by studying thousands of fragments of clay tablets. The tablets also provided insight into the customs, business life, culture and religion of the Assyrians. The majority of the tablets are now in the British Museum.

During the Assyrian period, astronomy was, as with all early civilisations, largely astrological in nature. Movements of bodies in the sky were believed to be significant for individuals and, in particular, for kings and empires. The Assyrians therefore studied the motions of the Sun, Moon and planets very carefully, looking for omens, good or bad. They regarded the heavenly objects as gods. The priests therefore took most interest in the sky. Of all heavenly objects, the planets were the most inexplicable: their motions were restricted to a band of the sky and they would change direction, approach and recede from one another and change brightness. The Assyrians named the heavenly bodies as follows:

The Assyrians believed that eclipses were important omens. They predicted lunar eclipses with reasonable accuracy and in their predictions accounted for effects like the invisibility of a lunar eclipse occurring in daylight.

Babylonian Astronomy

Babylonian_tablet.jpg Babylonian tablet recording observations of Halley's Comet in September 164 AD.

Babylonian astronomy came to prominence after the Assyrian Empire began to weaken through wars. The Babylonians were the first to undertake astronomy as a science and they originated many of the names of groupings of stars and constellations used today, for example Taurus, Aquila and Leo. However, the boundaries of the constellations known to the Babylonians were different from those in use today. In Babylonian times, many bright stars were associated with days of the year.

Through very careful observation, Babylonian astronomers constructed accurate calendars, based on two systems: an eight year cycle and a 19 year cycle. In each system, there were some years with 12 months and some with an extra 13th month. In the eight year cycle, the extra month was inserted at intervals of three, three and two years and in the 19 year cycle, it was inserted at intervals of three, three, two, three, three, three and two years. Writings discovered by archaeologists show that the Babylonians used the eight year system early during their civilisation, around 530 BC and a century later, the 19 year system, presumably because it more accurately aligned with the true length of the year. It is not known how many days the Babylonians assigned to each month.

Babylonian astronomers collected a large amount of data on the positions and motions of the planets and Moon. From this, they were able to compile ephemerides for the bodies and were able to predict their rising and setting times over current, previous and future epochs. They were able to predict both lunar and solar eclipses with great accuracy and demonstrated far deeper understanding of the theory of eclipse predictions than did the Assyrians.

For both the Assyrians and the Babylonians, the basis of predicting eclipses was understanding the interval between successive eclipses. For an eclipse to occur, the Sun, Moon and Earth have to be approximately in line in space, a configuration termed syzygy; this can occur in a one or two month period twice a year, when the Sun is near the ascending or descending node of the lunar orbit.

There are multiple periodicities of the lunar orbit and these translate into different series of eclipses. The shortest series is associated with the Semester, a period of 177 days. Eclipses taking place at intervals of one Semester belong to the same Semester series. A Semester series consists of seven or eight eclipses of varying types: one or two partial (P) followed by two or three central (annular (A) or total (T)) followed again by one or two P. The most famous series is associated with the Saros, a period of 18.03 years. Eclipses taking place at intervals of one Saros belong to the same Saros series. Each Saros series contains between 69 and 86 eclipses and begins with several P, followed by several A/T and ends with several P again.

The Babylonians certainly knew about the Saros series. (It is not clear that the Assyrians understood it.) They formulated a table, fragments of which are now held in the British Museum, to calculate the occurrence of eclipses for previous, current and future times. Archaeologists Johann Nepomuk Strassmaier (1846-1920) and Joseph Epping (1835-94) studied the table and named it the Saros canon.

Egyptian Astronomy

Little is known about Egyptian astronomy. The Egyptians used sundials and water clocks to tell the time, so perhaps had little need to develop astronomy for this purpose and, as a result, left no trace of investigation of the science. The only significant stellar circumstance upon which the Egyptians relied was the annual appearance of Sirius just before dawn at the summer solstice, 21 June, heralding the coming rising of the Nile, upon which agriculture depended.

Chinese Astronomy

Su_Song_Star_Map.jpg Star map of Su Song.

The historical record indicates that astronomy was under active study in China from at least two millennia BC. Astronomy there began, as in many other countries, from a need to measure time: the beginning of seasons was denoted by certain constellations being in known positions at sunset, for example, mid-summer when Antares was due south at sunset and winter when the tail of Ursa Major pointed downwards. At this time, the country was relatively isolated from the Middle East and Mediterranean but, during the first century AD onwards, it enjoyed scientific contact with the Roman Empire, Iran and Greece by way of India. This resulted in a stimulus to the study of astronomy and, by the middle of the first century AD, the Chinese had estimated the length of the year to within 11 minutes and the length of the synodic month to within 23 seconds. Even though early Chinese astronomers achieved high standards of astronomical knowledge, over a period of many centuries their astronomical calendars became inaccurate. The Chinese did not resolve this problem until the Jesuits came to China in the 16th Century and were commissioned to construct a more accurate calendar.

Astrology played a major part in the astronomy of early China and, as with the Assyrians, priests were the only people to study the sky in detail. They assigned names associated with the emperor and state to the brighter stars and groupings of stars (for example Emperor, Palace and names of members of the royal household). They constantly studied the skies searching for omens concerning the rulers of the state. They grouped the stars and constellations into four large groups, and the zodiacal stars into 28 mansions, encompassing the path of the Moon through the sky.

Early Chinese astronomers exhibited little understanding of the motions of the planets and Moon. They developed a mythology to explain solar eclipses, and legend has it that two court astrologers, Hsi and Ho, were executed for failing to predict a solar eclipse. However, the early astronomers compiled star charts, among the most famous of which is that by Shih-Shen (circa fourth century BC), containing descriptions of 122 constellations and 809 stars. (His chart pre-dates the catalogue of the Greek, Hipparchus.) The astronomical data compiled by the early astronomers is of great value as no other records exist from around this period. For example, they recorded the appearance of comets in the years 989, 1066, 1145 and 1301, now known to be apparitions of Halley's Comet although, at the time, it was not appreciated that the appearance was of the same body.

Arguably the most important astronomical observation made by the Chinese was of a supernova which, during June 1054, shone so brightly that it was visible in daytime. The supernova remnant is the famous Crab Nebula in Taurus, one of the most interesting objects for observation by both amateurs and professionals alike.

The earliest extant printed star map from China dates from approximately this period. It is the celestial atlas of Su Song (1020-1101), printed in 1092, depicting the heavens in an equidistant cylindrical projection.

Greek Astronomy

Ptolemy.png Claudius Ptolemy (c. 90-168) imagined by an early Baroque artist.

Fragmentary writings have survived from the classical Greek period, and it is possible to piece together the main ideas of the time on the subject of astronomy. Many Greek philosophers contributed to the subject, each bringing his own ideas on the motions of the planets, Moon and stars. Many adapted writings by earlier philosophers and this resulted in the original ideas and opinions of many appearing to be scanty or contradictory. However, the works of the Greek philosophers Plato and Aristotle have been preserved well, and both were highly esteemed in later times.

Astronomy in Greece proceeded, as with other cultures, from the need for accurate timekeeping and an accurate calendar. In common with the Babylonians, the Greeks discovered the inequality of the seasons, whereby the speed of the Sun through the sky varies throughout the year, resulting in the periods between solstices and equinoxes differing by several days.

The Greeks developed sophisticated mathematics, particularly in the field of geometry, and were the first to use mathematics to describe the motions of the planets. However, the Greek passion for mathematics overshadowed observational astronomy to such an extent that their notions of planetary motion were generally unrealistic. In fact, there were many conflicting ideas in Greek philosophy about the motion of bodies in the solar system. The famous mathematician Eudoxus was first to formulate a mathematical model of planetary motion. In keeping with the ideas current at the time, he considered every planet fixed to a sphere, a so-called homocentric sphere, which rotated around the Earth. To explain the irregular motions of the planets, he proposed the use of more spheres, all rotating in a regular manner around the Earth. The theory produced reasonably accurate predictions of planetary positions for Jupiter and Saturn, but not for Mars, Venus or Mercury. However, Eudoxus did not have sufficient accurate observational data to appreciate this. To improve the model, Callippus added another sphere for the planets Mercury, Venus and Mars. Two more spheres were subsequently added for the Sun and Moon.

The most original ideas concerning planetary motion came from Heraclides and Aristarchus. They proposed a heliocentric system with all the planets orbiting the Sun in circular orbits. However, other astronomers and philosophers of the period did not recognise that the system could account for many of the difficult aspects of planetary motion, as a result of which it was abandoned for some seventeen centuries before coming to prominence once more.

Using the Greek mathematics of geometry, Aristarchus attempted to find the distance of the Sun from the Earth. He found the distance to be 19 times the Moon-Earth distance; his figure was very inaccurate - the accepted modern value is 390 times. He then estimated the relative volumes of the Sun and the Earth: his estimate was between 254 and 368 times, compared with the accepted modern value of 1.3 million times.

During the second century BC, Hipparcus of Nicaea invented the epicyclic system, utilising a set of circular motions to represent the paths of heavenly bodies in the sky. Although not physically realistic, the system could represent the motions of the planets and Moon to within an error of less than one arcmin. Ptolemy, one of the greatest astronomers of ancient times, who lived in Alexandria during the time of the Roman emperor Hadrian in the second century AD, favoured the epicyclic system and refined it, so that ever since it has been known by his name. For over 16 centuries the system was accepted as an explanation of the heavens - its accuracy, combined with its positioning of the Earth at the centre of the cosmos, were the main factors in its favour. Questions as to why the Earth was stationary and at the centre of the universe were generally not addressed; the Church had sufficient power and influence to suppress dissent and anyone expressing belief in a heliocentric theory was liable to be burned at the stake as a heretic.

Note also that Ptolemy was also the first observational astronomer as the term is understood in the modern sense; he compiled the most accurate star catalogue of the time, comprising 1028 objects forming the classical 48 constellations.

Nicolaus Copernicus (1473 - 1543)

Nikolaus_Kopernikus.jpg Portrait of Copernicus in 1580 (artist unknown).

Nicolaus Copernicus was born in Thorn, on the river Vistula, in Poland, on 19 February 1473. His father was a merchant who had moved from Crakow in 1458. When Copernicus was ten, his father died and his uncle, Lucas Watzelode, took charge of his upbringing. In 1849, Watzelode became bishop of the diocese of Ermland.

In 1491, Copernicus entered the University of Crakow, renowned for its flourishing schools of mathematics and astronomy. At university, among his other studies, he became acquainted with astronomy, via the medium of astrology, and Greek. In 1496, he finished his course at Crakow and moved to Italy to study canon law and medicine. In 1500, he started lecturing on astronomy at Rome. In 1503 he took his doctoral degree in canon law and, in 1505, went to stay at Heilsburg as companion and physician to his uncle, Watzelode, who was still bishop of Ermland.

At the end of the 15th and beginning of the 16th centuries, there was much debate about the works of the Greek philosophers. One result of this was the discovery of many inaccuracies in the Alfonsine Tables of the positions of the heavenly bodies, which were based on a geocentric model of the universe. Copernicus, able to read the works of the Greek philosophers in the original text, noticing the inaccuracies of the Tables, began to formulate a new theory of the motion of the Sun, Moon and planets. His work was prompted by ideas from several Greek philosophers: Nicetas who, according to Cicero, perceived the motion of the Earth; Philoleus who believed that the Earth made a daily orbit around a central fire and Heraclides who, with Euphantus, thought that the Earth rotated about its axis once a day. Copernicus formulated several hypotheses:

  1. The objects in the Solar System have no single centre.
  2. The Earth is not the centre of the Universe but only the centre of the orbit of the Moon.
  3. All objects other than the Moon orbit the centre of the Sun.
  4. The Earth rotates once a day on its axis.
  5. Retrograde motion of a planet in its orbit is due to the motion of the Earth around the Sun.

The few years after 1505 at Heilsburg were very difficult, as Watzelode was a very firm administrator and Copernicus had many administrative duties to perform. However, in 1512, Watzelode died and Copernicus went to live in Frauenburg, becoming a canon to Frauenburg Cathedral. He stayed at Frauenburg for about 30 years, living all this time in a few rooms in one of the turrets in the wall that surrounded the cathedral. The turret also served him as an observatory. While in Frauenburg, in 1514, Copernicus circulated a manuscript summarising his new ideas among his friends. For the next several years, he made observations of the positions of the planets and stars, and used them to develop his theories.

It took Copernicus some 36 years to fully develop a new theory to replace the Ptolemaic system. He finally evolved a theory whereby the Sun and background stars were fixed, the planets including the Earth orbited the Sun, and the Moon orbited the Earth. Copernicus assumed that all the orbits were circular and, in order to make the theory fit observations accurately, had to introduce a system of deferents and epicycles, similar to those used in the Ptolemaic system. Copernicus worked on a book explaining his ideas and conclusions, De Revolutionibus Orbium Coelestium. By about 1529, he finalised the manuscript but, realising that the work would create a storm, did not publish it.

In 1539, Georg Joachim, known as Rheticus, a young professor from Wittenburg, went to Frauenburg to find out more about Copernicus' new theory. He was so impressed that he wrote an account of the theory and had it published in 1540; the account was known as Narratio Prima. It received such a good reception that Copernicus agreed to publish De Revolutionibus. The publisher was Andreas Osiander from Nerumberg. He realised the trouble that publication could cause and, to defuse any potential difficulties, inserted a preface at the beginning of the book (without Copernicus knowing) to the effect that the theories were merely hypothetical.

The completed publication reached Copernicus on 22 May 1543 as he lay in a coma, apparently suffering from a stroke, just hours before he died. He never read the publication. At first, De Revolutionibus caused little excitement but, later, when its full implications were appreciated, as predicted, it provoked a strong and prolonged hostile reaction from the Church.

However, Copernicus' theory marked the start of modern theories of the Solar System. Astronomers realised that Copernicus had solved the major problem of the movement of the planets, and there was a resurgence in observing in order to test the new heliocentric theory. Tycho Brahe was one of the astronomers who made many accurate observations of the positions of the stars and planets. Kepler later used Brahe's observations in his formulation of hhis famous laws of planetary motion. It was Galileo's telescopic observations of Venus which finally confirmed the heliocentric nature of the Solar System, and later Newton provided the theoretical underpinning of Kepler's laws.

Tycho Brahe (1546 - 1601)

Tycho_Brahe.jpg Tycho Brahe.

Tycho Brahe was born in Denmark in 1546, the son of a Danish nobleman. His family sent him to study law at Leipzig University, but he was an enthusiastic astronomer and made secret nightly observations and studies of the science. He was convinced of the truth of astrological predictions and often compiled horoscopes. In order to make astrological predictions more reliable, set out to undertake research to provide a better understanding of the motions of the stars and planets.

On 11 November 1572, the appearance of a bright "new star" or supernova in the constellation Cassiopeia caught Brahe's attention and stirred the interest of a great many people. It proved that the old notion that the heavens were unchanging was not true. Brahe immediately began observing the supernova and recorded many estimates of its brightness relative to that of other nearby stars in Cassiopeia. The supernova soon began to fade and, after about two years, disappeared from sight.

The appearance of the supernova stimulated a general interest in building an observatory. In response, King Frederic of Denmark gifted Brahe the island of Hven, near Copenhagen. Upon the island, Brahe built the observatory of Uraniborg, equipping it with instruments of his own design, which were a considerable improvement on previous models. His instruments were astronomical quadrants, ranging in size from small, hand-held models to large devices with radii in excess of two metres, able to measure positions to within 10 arcsec.

Brahe used the equipment at his observatory to compile the most accurate star catalogue of the era. It superseded the catalogues of Hipparcus and Ptolemy that had lasted well over 1000 years. Brahe was not convinced by Copernicus' heliocentric model of the Solar System and hoped to use his numerous measurements of the positions of the planets to prove it wrong. Ironically, it was the accuracy of his observations that, in the hands of Kepler, revealed the truth of the heliocentric approach and the true structure of the Solar System.

Johannes Kepler (1571 - 1630)

Johannes_Kepler.jpg Johannes Kepler, 1610 (artist unknown).

Johannes Kepler was born at Weil-der-Stadt, Wurttemburg on 27 December 1571. He entered university at Tübingen where he studied mathematics, astronomy and theology. While a student at Tübingen, he heard a lecture on the heliocentric interpretation of the Solar System given by Michael Maestlin (1550 - 1631). In 1594, he was appointed Professor of Mathematics at the Protestant School in Graz, Austria. In 1596, he published his ideas on planetary orbits and distances, his explanation involving superimposing regular polyhedra within the planetary orbits. He assumed that each planetary orbit could be represented by a sphere and between each pair of successive spheres he placed one of the regular polyhedra with its edges touching the exterior sphere and its faces tangential to the interior sphere. The ratios of the inner and outer spheres gave the relative distances of the planetary orbits. This model, although in vogue at the time, has no physical basis and is now discredited.

In about 1594, Kepler began writing to fellow astronomer Tycho Brahe. In 1599, Brahe went to work in Prague, having been forced out of Denmark due to differences with the Danish court and noblemen. In 1600, Kepler, by this time collaborating with Brahe in Prague, suffering under Catholic persecution of Lutherans, was dismissed from his post at Graz and moved with his family to stay with Brahe in Prague. In 1601, Brahe and Kepler met with Emperor Rudolph II, and Kepler became acknowledged as Brahe's assistant. Brahe died in 1601 and Kepler was appointed his successor as Mathematician to the Emperor. But he received no salary for this post and lived a life of poverty even after gaining other appointments at Linz and becoming astrologer to the military leader and politician Albrecht Wenzel Eusebius von Wallenstein (1583-1634).

In October 1604, a supernova erupted in the constellation Ophiuchius and became an object of study by astronomers in Europe, China and Korea. In Europe, Kepler and Fabricius identified its position so accurately that, in 1943, it was possible to identify a small patch of nebulosity with the original supernova remnant. The supernova came to be called Kepler's Star in his honour.

Kepler's most important contribution to astronomy was his formulation of three laws of planetary motion, which he developed on the basis of Brahe's voluminous and accurate observations:

  1. Every planet moves in an ellipse with the Sun at one focus. This law ousted the ancient belief that the orbits of the planets were circular.
  2. The radius vector joining the planet to the Sun sweeps out equal areas in equal times.
  3. The square of the sidereal period of a planet is proportional to the cube of its mean distance from the Sun.

Kepler published the first two laws in 1609 in an article in the book Astronomia Nova - Commentaries on the Motions of Mars, and the third ten years later in Harmonices Mundi. Although the laws give an accurate description of planetary motion, the causal, physical description was not provided until, in 1687, Isaac Newton published Philosophiae Naturalis Principia Mathematica, characterising the force of gravity.

Kepler wrote other books too. In 1621, he published The Epitome of the Copernican Astronomer, detailing the movements of the planets and Moon in great depth. His last book, published in 1627, The Rudolphine Tables, named after his patron, Emperor Rudolf II, gave the positions of the Sun, planets and Moon based on his work and that of Tycho Brahe. So accurate were the Tables that they provided the basis of standard astronomical predictions for the next one hundred years.

Galileo Galilei (1564 - 1642)

Galileo_Galilei.jpg Portrait of Galileo by Justus Sustermans, 1636.

Galileo Galilei was born in Pisa, Italy on 15 February 1564 (the year was recorded at the time as 1563, since New Year's Day fell on 25 March, the Feast of the Annunciation). He worked diligently at school studying Medicine and Aristotelian Philosophy and, in September 1581, entered the University of Pisa. At Pisa, he made the important discovery that the oscillation of a pendulum is of constant period, which led him many years later to greatly improve clocks and other timekeeping equipment.

Galileo became professor of mathematics at Pisa in 1589 at the age of 25. He propounded the novel theory that all falling bodies, heavy or light, fall to the Earth with equal velocity. Legend has it that he proved the theory experimentally by dropping a hundred pound ball and a one pound ball simultaneously from the top of the leaning Tower of Pisa. No contemporaneous account of the demonstration exists, but Galileo recounted the story in his old age.

In 1591, Galileo resigned his chair and moved to Florence. In the following year he was nominated Professor of Mathematics at the University of Padua. His time at Padua was a productive one: he was very successful as a lecturer, attracting students from all over Europe, and made several discoveries and inventions, among them a type of thermometer, the proportional or sector compass and, most famously of all, great improvements to the design of the refracting telescope.

At the end of the seventeenth century, the spectacle-maker Hans Lippershey of Middelburg (1570-1619) found that two lenses held in line could magnify distant objects. News of the discovery soon spread across Europe. When news reached Galileo, he proceeded to construct small instruments, the most powerful of which provided a magnification of x32 and, in 1610, began to use them in serious astronomical investigations. Among many findings he discovered that the Moon, instead of being self-luminous and smooth as had been assumed previously, had an uneven surface marked with numerous hills, valleys and mountains. The mountains were of particular interest: they cast shadows in the direction away from the Sun, providing conclusive proof that the Moon shone from reflected solar light. He discovered the phases of Venus and saw the Milky Way as countless thousands of stars. He also began a series of observations of Jupiter which resulted, on 07 January 1610, in the discovery of the planet's four large satellites, subsequently named Io, Europa, Ganymede and Callisto, and referred to collectively as the Galileans in his honour. He discovered sunspots and, by timing their progress across the face of the Sun, estimated the rotation period of the Sun as about 27 days.

Through his astronomical discoveries, Galileo came to believe in the heliocentric or Copernican view that the Sun was at the centre of the Solar System. His observations of the phases of Venus provided particularly strong support of this belief: in the Ptolomaic or geocentric view it was impossible for Venus to exhibit phases. The ecclesiastical authorities supported the geocentric view of the heavens and were enraged by Galileo's support for the Copernican view. On 26 February 1616, Cardinal Bellarmine, representative of Pope Paul V, admonished Galileo against further support for the Copernican view. In 1633, the ecclesiastical authorities under Pope Urban tried Galileo for his beliefs and he was forced to speak aloud on his knees an abjuration of his beliefs in front of an Ecclesiastical Tribunal. He was not imprisoned, for by then he was in his late sixties and not a well man, but he was forced to live the remaining nine years of his life in retreat in the village of Arcetri near Florence.

Galileo died on 08 January 1642 and is buried at the church of Santa Croce in Florence.

Giovanni Cassini (1625 - 1712)

Jean_Dominique_Cassini.jpg Cassini (date and artist unknown).

Giovanni Domenico Cassini (also known by the French form of his forenames, Jean Dominique) was born at Perinaldo, Italy on 08 June 1625. He was the first of five successive generations of astronomers. He studied mathematics and astronomy at a Jesuit college - his original reason for studying astronomy was said to be a desire to prove that astrology had no scientific validity. In 1650, he was offered the post of Professor of Astronomy at Bologna University; he accepted the offer and held the position until 1669.

While at Bologna, Cassini's main area of study was the Solar System. In 1665 and 1666, he determined the rotation periods of Jupiter and Mars. Two years later he published a table of the motions of Jupiter's Galilean satellites: this table was later to help Ole Rømer in his study of the speed of light (Rømer announced to the French Academy of Sciences on 21 November 1676 that he had amassed evidence proving that the speed of light was finite). Cassini was one of the first astronomers to study the zodiacal light, concluding (correctly) that it was due to dust in interplanetary space.

By 1669, Cassini's reputation was famous throughout Europe to such an extent that Louis XIV invited him to become director of the Paris Observatory. Cassini redesigned much of the observatory and removed ornamentation causing a hindrance to observations. At Paris, Cassini worked with Christian Huyghens and together they made many important advances in astronomy.

Cassini's telescopes were very cumbersome: they were refractors with very long focal lengths, some over 30 m. However, this did not appear to limit his observations! He discovered four satellites of Saturn between 1671 and 1684 and, in 1675, discovered a division in the planet's ring system which was subsequently named in his honour. Cassini believed that the rings were composed of a countless myriad of small particles, but few astronomers of the time concurred. The nature of the rings remained in doubt until 1857 when James Clerk Maxwell (1831-79) proved that they could only be stable if they indeed comprised innumerable small particles.

Arguably, Cassini's most important work was the determination of the parallax of Mars in 1672. Mars was observed from two locations: Paris and French Guiana. The parallax of the planet was used to calculate its distance and, since the relative distances of all the planets from the Sun was known from Kepler's laws, the scale of the Solar System.

Cassini died on 14 September 1712.

John Flamsteed (1646 - 1719)

John_Flamsteed.jpg Flamsteed by Godfrey Kneller, 1702.

John Flamsteed was born at Danby in Derbyshire on 19 August 1646. He suffered bad health throughout his life. At the age of 15, his poor health caused him to leave school and led to the pursuit of his hobby, astronomy. He began to construct his own astronomical instruments and, by 1670, received attention through the publication of several papers on astronomical topics. This in turn led Flamsteed to become acquainted with Newton and to gain entrance to Cambridge University.

During the mid-seventeenth century, maritime exploration and overseas trade were becoming widespread. However, the inability to determine longitude accurately remained a severe limitation to effective navigation at sea. King Charles II established a committee, which included Flamsteed, to investigate methods for determining longitude at sea. Flamsteed concluded that no method would work until there was an accurate star map available, so he petitioned the King to establish an observatory for this purpose. Charles II agreed to Flamsteed's request, appointing him Astronomer Royal in 1675.

An observatory was built for Flamsteed, on a hill at Greenwich, to the design of Christopher Wren. The King imposed a limit of only £500 for the building. Money was raised by selling off military gunpowder to merchants who were able to re-treat it and sell it on for uses less onerous than ordnance. Building materials were obtained from a variety of establishments that were being demolished, including wood from a gate house at the Tower, and bricks, iron and lead from a fort at Tilbury.

Unfortunately the King had not made provision for the purchase of instruments for his observatory so Flamsteed had to equip it at his own expense, assisted by whatever funds he could raise from friends and well-wishers. His annual allowance from the public purse was only £100 and, by 1683, he had to supplement this by taking private pupils in astronomy and mathematics.

Flamsteed had to supervise all the observing work himself until his father's death, when he was able to employ Abraham Sharp, an instrument maker and tireless worker at processing observational results. He built a large mural instrument for the observatory which speeded up the work of measuring stellar altitudes. He was one of the first observers to use a telescope in combination with a graduated arc for measuring angles. By 1703 he had completed more than 30,000 stellar positional measurements with a greater accuracy than had been achieved before.

As Flamsteed had to provide his own instruments, he regarded the results of his observations as his private property. Some contemporary astronomers, notably Newton and Halley, took the opposing view that, as Flamsteed received a salary from public funds, his results were public property and should be published as quickly as possible. However, Flamsteed was not prepared to publish results until they had been fully corrected for potential errors and reduced to a standard form. The resulting delays caused much bad feeling. In 1708, Halley obtained several of Flamsteed's observations and published them. Flamsteed's response was to burn as many of the publications as he could find - in total over 300!

Flamsteed's most significant work was a catalogue of the positions of 2935 stars, to much greater accuracy than had been achieved previously. The work was based on 40 years of meticulous observations did not appear in its final form until 1725, six years after his death. It consisted of three volumes, the second two being completed by Abraham Sharp and Joseph Crossthwait. The catalogue gave Greenwich an international reputation for precise observations that it has held ever since.

Isaac Newton (1643 - 1727)

Isaac_Newton.jpg Newton by Godfrey Kneller, 1689.

Isaac Newton was born on 04 January 1643, the son of a Lincolnshire farmer. Although from relatively humble beginnings, he went to Cambridge University in 1661, where he excelled at mathematics, and then went on to make many contributions to mathematics and science. His main contribution to astronomy was a mathematical theory describing the force of gravity that attracts all objects in the universe.

The idea that there was an attractive force between particles of matter in the universe was mature by the time of Newton. Copernicus had spoken of a mutual attraction between parts of the Earth as the cause of its spherical shape and had assumed that a similar force was present between astronomical bodies. Kepler had spoken of a "power" that tended to attract bodies to one another, which he believed was the reason the planets remained in orbit about the Sun and the Moon in orbit about the Earth. He believed that the ocean tides on Earth, raised by the Moon, provided proof of its existence.

On leaving Cambridge, in 1665, Newton returned to his home village of Woolsthorpe and started work on the laws governing the motion of objects, eventually turning his attention to the motion of the Moon around the Earth. The word gravity had been in use for some time, referring to a mutual affection between bodies that tended to draw them together. Newton wanted to define gravity in an exact manner using mathematics and to understand how the attractive force varied with the distance between the bodies. His ultimate aim was to use an understanding of gravity to make comprehensible the observational facts of lunar and planetary orbits.

Around this time, other scientists were also trying to understand lunar and planetary motions in terms of a force of gravity, but the mathematics involved proved to be a formidable stumbling block. Robert Hooke was one of Newton's main rivals. Hooke, like Newton, suggested that the force of gravity between two objects decreases inversely as the square of the distance between them. Hooke and Newton exchanged much correspondence on the subject until Newton realised that Hooke was getting close to a solution. Spurred on by the competition, Newton continued to work on the problem which he solved during the next five years.

Hooke often met with Christopher Wren and Edmund Halley to discuss astronomy. After a discussion of gravitation, Wren offered a valuable book as a prize to whoever found a mathematical description of the force. Hooke declared that he had a solution but was slow to produce it. Meanwhile, Halley visited Newton in Cambridge and discovered that Newton had already worked out the solution. Halley persuaded Newton to present his work to the Royal Society, which he did in December 1684. Through the efforts of Halley, Newton's work was published as Philosophiae Naturalis Principia Mathematica in July 1687 after much haggling with Hooke before it appeared in print.

Newton also contributed two other important innovations to astronomy. He discovered that white light was composed of colours that were made visible by a prism. Early refracting telescopes were hampered by chromatic aberration associated with their lenses; Newton invented the reflecting telescope which relied on a mirror rather than an objective lens and thus avoided chromatic aberration.

Edmond Halley (1656 - 1742)

Edmond_Halley.gif Edmond Halley, portrait by Thomas Murray, c. 1687.

Edmond Halley is remembered principally nowadays for the periodic comet named after him (Comet 1P/Halley). However, he pursued a wide range of scientific interests and held posts varying from sea captain to Astronomer Royal; his contribution to astronomy was considerable, but he was largely overshadowed by his contemporary, Sir Isaac Newton.

Halley was born on 08 November 1656 in Haggerston, east London. He went to school at Saint Pauls and, in 1673, went on to Queen's College, Oxford where his interest in astronomy developed. At the age of 19, only part-way through his degree, he began working for John Flamsteed, the Astronomer Royal. Influenced by Flamsteed's aim to compile a catalogue of the northern sky, Halley decided to catalogue stars in the southern hemisphere. He gained the support of King Charles II, who granted free passage aboard a vessel of the East India Company and, in November 1676, sailed together with a colleague for St Helena in the South Atlantic. During Halley's stay in St Helena, he observed a transit of Mercury on 07 November 1677 and catalogued the positions of some 360 stars. On returning to England in 1679 he published his observations as the Catalogus Stellarum Australium, containing detailed positions of 341 stars. The Catalogus received wide acclaim. Not only was it the first catalogue of southern hemisphere stars; it was also the first mapping of stars compiled using a telescope. It established Halley's scientific reputation, prompted Cambridge University to award him an honorary degree, and the Royal Society to elect him a Fellow.

In 1679, Halley suggested that observations of a transit of Mercury or Venus across the Sun's disk could be used to estimate the Earth-Sun distance and the scale of the Solar System. Because of the small size of Mercury and the attendant difficulty of timing its transit accurately, Halley decided to concentrate on Venus, for which the next transits would occur in 1761 and 1769 (transits of Venus occur in pairs separated by eight years). Although he would certainly be dead before the next transits of Venus, he nonetheless encouraged astronomers to observe them. Indeed, following Halley's exhortation, the next transits saw numerous expeditions by astronomers worldwide and a major international effort to analyse their observations to estimate the scale of the Solar System.

In 1684, Halley worked with Sir Isaac Newton on the laws of planetary motion. In 1685, he was elected Clerk to the Royal Society and started editing the Society's journal, Philosophical Transactions. He asked the Society to finance publication of Newton's Principia, but his request was declined, as the Society had just financed publication of Historia Piscium (History of Fishes), by John Ray and Francis Willughby, a book which was not selling well. (In fact, the Society owed Halley fifty pounds salary but could not afford to pay him so instead sent him fifty copies of Historia Piscium. Halley was renowned for having a strong sense of humour but whether he saw the joke in this matter is uncertain!) In 1687, Halley therefore personally financed publication of Newton's Principia, the most significant scientific publication of all time.

During his term as Clerk, Halley wrote many scientific papers on subjects ranging from trade winds to astronomy. In 1686, he published an account of trade winds and monsoons for mariners. He also investigated the salinity of the oceans and was able to estimate fairly accurately the age of the Earth by measuring the rate at which salinity increased. He also at this time investigated planetary perturbations of Jupiter and Saturn and the slow secular acceleration of the mean motion of the Moon.

At this time, there was considerable interest in finding a quick and reliable way of determining longitude at sea. Halley proposed a method based on an understanding of variations in the Earth's magnetic field. He contacted the Admiralty about the matter and officials there took a great interest in the proposal and commissioned him to the rank of captain, giving him command of a small ship, the Paramour, for research into the technique. His first voyage in the ship began in November 1698 at Portsmouth, but was cut short by a mutiny! His second trip, completed in mid-1700, was more rewarding, and covered the Atlantic as far south as the Falkland Islands. He discovered that the Aurora Borealis was related to the Earth's magnetic field.

Halley was one of the first astronomers to apply Newton's laws of motion to comets. In 1705, he published Astronomiae Cometicae Synopsis which listed observations of 24 bright comets that had appeared between 1397 and 1698. He also articulated his theory that the comets of 1456, 1531, 1607 and 1682 were the same object, and that the body would appear again in 1758. As he knew that he would not be alive to see the return of the comet, he wrote in his diary:

If the Comet should return according to my prediction, about the year 1758, impartial posterity will not refuse to acknowledge that this was discovered by an Englishman.

The comet was first seen again on Christmas Day 1758. It was given the name Halley's Comet in recognition of his outstanding work on astronomy. The comet indeed returns every 76 years, as he predicted.

During 1718, Halley observed Sirius, Aldebaran and Arcturus and compared their positions in the sky with Ptolomey's Star Atlas; noticing that the positions did not agree, Halley thus discovered the phenomenon of stellar proper motion.

Halley also did much work in the field of geometry and, in 1704, became Savilian Professor of Geometry at Oxford.

The highlight of Halley's career was his appointment to the post of Astronomer Royal in 1719, at the age of 64, succeeding Flamsteed. Halley held the post for 20 years. From as early as 1684, he had observed regular deviations of the Moon from its predicted motion and, as Astronomer Royal, he continued his work on the Moon's motion and observed the Moon through one entire saros cycle of eighteen years. He died before he could fully analyse his observations but they proved to be of great value to later astronomers in calculating the complex nature of the Moon's motion. The results of his observations were published in 1749 and included tables of the Moon and planets which he had prepared as early as 1719.

Halley died at the age of 86 on 14 January 1742.

Charles Messier (1730 - 1817)

Charles_Messier.jpg Charles Messier at age 40 in 1770.

Charles Messier was born on 26 June 1730 in the small French town of Badonviller. He was born into a large family, being the tenth of twelve children. He received only a basic education, while he was comparatively young. During his teens he developed an interest in astronomy, especially the discovery of comets. Finding few prospects of employment at home, at the age of 21, he left to seek work in Paris.

Messier's sole skills at this time were neat handwriting and a little knowledge of draughtsmanship! He eventually found employment with Nicholas Delisle, who had established an observatory in the Hotel du Cluny under the auspices of the French Navy. After a few years at the observatory, Messier's position earned him an official title, Clerk of the Depot of the Navy, which included a small salary. Though his main duties were keeping the observatory's records, Messier found night-time observing more to his liking.

In 1705, Halley predicted that the comet which he had observed in 1682 would return in late 1758 or early 1759. Many observers started searching for the comet more than a year before its predicted return and Delisle had drawn up a chart for Messier to use in his search. Messier actually started to look for the comet nearly two years before its expected reappearance: he found it on 21 January 1759, but was not the first to sight it. A German astronomer, Johann Georg Palitzsch (1723-88), had seen the comet some four weeks earlier on the evening of Christmas Day 1758.

About a year later, Delisle retired, bequeathing Messier the observatory and equipment for his own use. Messier then devoted his work almost exclusively to the discovery and observation of comets. He discovered a comet on 03 January 1764 and was fortunate enough to discover another one two years later by a chance naked-eye observation. Between 1760 and 1798 he discovered 15 comets.

Although in his work Messier concentrated on comets to the almost total exclusion of all else, today he is remembered for his catalogue of nebulae and star clusters. In 1758, whilst observing a comet in Taurus, he discovered a nebulous object that resembled a comet. He made a note of its position for future reference. In 1760 he discovered a second nebulous object, in Aquarius. By May 1764, he decided to make a list of as many such objects as he could, so as to avoid potential confusion with comets. Several earlier astronomers had discovered nebulous objects so Messier noted the positions of these too. By the end of 1764, he had compiled a list of 40 objects, 22 of which he had discovered. He discovered the 41st object in January 1765. The list, with a few additions, was published in 1769. The final version of the catalogue was printed in 1781, containing 103 objects. Messier's nebulae and star clusters are simply known today by their catalogue number prefixed with the letter 'M'.

For his services to astronomy, Messier was elected to many societies and academies all over Europe, including the Royal Society, London, the Academy of Stockholm, and the Academy of Sciences, Paris.

Messier died on 12 April 1817 at age 86.

William Herschel (1738 - 1822)

William_Herschel.jpg William Herschel in 1785 by Lemuel Francis Abbott.

William Herschel was born in Hanover, Germany, in 1738. He was the son of a Hanoverian bandsman and counted many musicians among his relatives. He entered the Hanoverian Foot Guards as an oboe player. In 1755, he visited England with the Guards and decided that he would like to stay there. He returned to the Continent but, after the defeat of the German army at Hastenbeck, resigned from the Guards and, in 1757, left Germany for England, where he settled in Leeds as a music teacher. He later became an organist in Halifax, and, in 1766, organist at the Octagon Chapel in Bath.

In Bath, Herschel taught music. In the city, his early interest in astronomy was reawakened when he read the book "Astronomy Explained Upon Sir Isaac Newton's Principles" by James Ferguson (1710-76), published in 1756, and soon he was devoting all his spare time to reading and studying astronomy and mathematics. Not being able to afford a telescope, he constructed one, a Newtonian reflector and, in 1774, began serious astronomical observations. His interest in astronomy began to occupy so much of his time that he reduced the number of students in his music classes to about seven a day.

On Tuesday 13 March 1781, while Herschel was observing the night sky with a 5.7 inch reflector of his own construction, he found an object in Taurus, near the border with Gemini, which showed a definite disk and which he considered to be either a nebulous star or perhaps a comet. On observing the object four nights later, he commented: I looked for the Comet or Nebulous Star and found that it is a Comet for it has changed its place. He continued observing the object until he had captured sufficiently many positions to permit accurate calculation of its orbit. During April 1781 he announced his discovery to the Royal Society. The Astronomer Royal at the time, Nevil Maskelyne, after observing the object for some time concluded that it was behaving more like a planet than a comet.

When sufficiently many measurements of the position of the object were available, three continental mathematicians, Simon Laplace and Jean Bochart de Sarron in France and Anders Lexell in Russia, calculated the orbit of the body. They determined that the object had an almost circular orbit at a distance from the Sun approximately double that of Saturn. At first, Herschel called the new object Georgium Sidus in honour of George III but, today we know it as Uranus, named after a classical god. Subsequent analysis of historical records showed that in fact Uranus had been observed before its true nature was known. John Flamsteed had recorded it as a star of magnitude six during 1768-79; Le Monnier had made eight observations of it; and Tobias Meyer had recorded its position accurately at the beginning of 1756.

Herschel's discovery of Uranus brought his name to the attention of other astronomers. George III, who was himself interested in astronomy and had built an observatory at Kew, asked to meet Herschel and requested the latter to bring his telescope to Greenwich. The instrument with which Herschel discovered Uranus aroused much interest among other astronomers when it was set up at Greenwich. It had a much greater magnification than other telescopes, and everyone familiar with telescopes thought that it was the highest quality instrument that they had ever seen. Herschel was elected a Fellow of the Royal Society and awarded the Copley Medal for his discovery. When Herschel took the telescope to Windsor for examination by the Royal Family, George III appointed him King's Astronomer and made him independent of his musical profession by giving him an annual salary of £250 and an observatory site at Datchet. The salary enabled Herschel to live quite comfortably: he gave up music altogether and moved to Slough where he became a full time astronomer and telescope maker. In 1788 he married Mary Pitt, a lady of considerable wealth, enabling him to devote his life to astronomy without further concern over his income.

Herschel was a prodigious maker of astronomical telescopes. He constructed approximately 400 mirrors of various sizes and sold some 69 telescopes. One of his mirrors was the largest constructed at the time. After beginning his full-time astronomical work at Datchet, Herschel succeeded in obtaining £4000 from the privy purse for construction of his observatory. He had a telescope built with a mirror 48 inches in diameter. The telescope was constructed at Slough and, though it was used for the first two years after completion, it proved to be too cumbersome for constant use and gradually became obsolete.

Using smaller instruments, Herschel made many important discoveries through the next 30 years. He produced a catalogue of approximately 2500 nebulae and similar objects and made many series of measurements on double and variable stars. After his discovery of Uranus, his most important work was on the distribution of stars in the galaxy and the shape of the latter. For this, he undertook star counts in approximately 3400 selected areas. Herschel thought that the galaxy was shaped like a rectangular box that was split open at one end; from the Earth, within this box, the stars would appear as a luminous band across the sky, the Milky Way.

In 1787 Herschel found two of the moons of Uranus, Oberon and Titania, and in 1789 he discovered two moons of Saturn, Enceladus and Mimas. The huge crater which dominates the face of Mimas is named after him. He also discovered infra red radiation using a thermometer held at the red end of a spectrum produced by a prism. In 1816, he was knighted.

In 1822, Herschel died. After his death, his sister Caroline Lucretia (1750-1848), who had for several years before been his assistant, carried on his work. She constructed a catalogue of star clusters and nebula that Herschel had discovered. Herschel was also survived by his son, Sir John Herschel (1792-1871), who continued developing and updating his father's catalogues. In time, John Herschel went on to discover more clusters and nebula than his father had done, doing much of his work in the Southern Hemisphere.

Johann Bode (1748 - 1826)

Johann_Bode.jpg Johann Bode.

Johann Elert Bode was born in Hamburg on 19 January 1747. He became interested in astronomy at an early age and, throughout his life wrote many books on the subject, the first being published in 1766 when he was aged only 19.

Between 1774 and 1779, Bode discovered several nebulae of various types. At the end of December 1774, he found two nebulae in Ursa Major, now known as the galaxies M81 and M82 (at the time of discovery their true nature was unknown). He discovered more objects in 1775 (including M53) and 1777 (including M92) and, later the same year, published a catalogue of some 75 objects. However, many of the entries were asterisms or non-existent objects copied from early catalogues compiled by Hevelius and others. Only about 50 of the entries were in fact nebulae or star clusters, and even among these, there were several positional errors. However, despite its flaws, Bode's catalogue had some utility, and Messier used it, together with those of early observers, to compile the most comprehensive and accurate catalogue of nebulae and star clusters of the time, the final version of which appeared in 1784.

In 1781 William Herschel discovered the planet Uranus. He wanted to name the planet Georgium Sidus (The Georgian Star) in honour of George III. It was Bode, who became greatly interested in the discovery, who proposed the name Uranus. During the subsequent months Bode started a search for earlier (pre-discovery) observations of the object. He found that Tobias Mayer in 1756 had observed the planet and listed it as a star in his catalogue of 1775 and that Flamsteed had been first to observe it, as early as 1690. The early observations helped in calculating the planet's orbit accurately.

Bode is remembered primarily for his "law" of planetary distances from the Sun. It relates the semi-major axis a of each planet outward from the Sun as follows:

The "law" is now of only historical interest, its importance diminishing after the discovery of Neptune (for which the distance from the Sun deviated from the predicted figure). The "law" was originally proposed by Johann Titius, who published it in 1772. Bode popularised it in his book, The Knowledge of the Starry Heavens, and became pre-eminently associated with it.

In 1786, Bode became Director of Berlin Observatory, a post that he held for nearly 40 years, retiring in 1825. In 1801 he published a comprehensive star atlas with the title Uranographia. The atlas proved popular and contained many new constellations defined by Bode which, unfortunately for him, were ever officially adopted. The only remnant of Bode's new constellations today is the Quadrantid Meteors of early January. The radiant of the shower is in the northern part of Boötes: Bode had asigned this area the name of Quadrans Muralis.

Bode died on 23 November 1826.

Friedrich Bessel (1784 - 1846)

Friedrich_Bessel.jpg Friedrich Bessel painted in 1839.

Friedrich Bessel was born on 22 July 1784 in Minden, Prussia. He started working life as an accountant. He was an self-taught astronomer, his first significant astronomical achievement coming in 1804 when he recalculated the orbit of Halley's Comet and sent his conclusions to Olbers. Olbers was impressed with the work and obtained a post for Bessel at an observatory.

During the next six years, Bessel gained much fame from his work, both in astronomical and court circles. Amongst his admirers was King Frederick William III of Prussia, who offered him a position in control of construction of a new observatory at Konigsberg. When the observatory was completed, in 1813, Bessel became its first Director, remaining in post until his death.

Soon after starting work at Konigsberg Observatory, Bessel began to make accurate positional measurements of stars for inclusion in a new star catalogue. The completed catalogue was an extension and revision of an early one compiled by James Bradley. On its completion in 1818, Bessel had recorded some 63,000 accurate star positions. His measurements were precise enough to reveal irregularities in the proper motions of Sirius and Procyon, from which he surmised that they must each have an object in orbit around them. This surmise proved correct and in the second half of the nineteenth century, the companion stars were discovered as follows:

Bessel is best remembered today for being first to determine a star's distance from parallax measurements. He chose a star that had a large proper motion and and hence was likely to be relatively close to Earth. He reported his results in 1838, quoting a parallax of 0.31 arcsec for the star 61 Cygni. (The modern accepted value is 0.3 arcsec. Bessel was not alone in measuring stellar parallax: Struve and Henderson were also attempting measurements, the three astronomers working independently of one other.)

Towards the end of his life Bessel started researching the anomalous motion of Uranus. He calculated the masses of Jupiter and Saturn with greater accuracy than had been achieved previously, and was thereby able to eliminate their gravitational influence as the cause of the irregularities of Uranus.

Bessel died in March 1846, some six months before Neptune was discovered.

Caroline Herschel (1750 - 1848)

Caroline_Herschel.jpg Caroline Herschel.

Caroline Lucretia Herschel was born on 16 March 1750, eleven years after William, her more famous brother. William settled in England in 1757. He took up various musical posts, as organist and teacher of repute and, in 1772, was able to bring Caroline to England from the family home in Hanover. Caroline was probably appreciative of the move as at Hanover she had to run the family household alone!

During Caroline's first few years in England, she supported her brother's musical career, having been trained as a concert singer. However, William had wide interests, and his prrincipal hobby was astronomy, especially the construction of telescopes. Caroline enthusiastically supported his hobby activities. After many failures, the Herschels became expert grinders of lenses and mirrors, producing ultimately the best telescopes of the day. When grinding some of the larger mirrors, William would remain at work for up to 16 hours; during these lengthy sessions, Caroline would physically feed him and occasionally would read aloud to him from books such as Don Quixote and Arabian Nights.

In 1782, William was granted a pension by King George III, enabling him to retire from his career in music and concentrate on astronomy full time. William undertook systematic sweeps of the sky, being greatly assisted by Caroline, who acted among her many other duties as note-taker. Without the immense assistance that Caroline provided, William would almost certainly have made fewer observations and fewer discoveries.

In order to give Caroline more independence in her own astronomical research, William built her a telescope, but she was able to use it only when William did not require assistance! In 1786, William visited Hanover and his absence gave Caroline a chance to complete some sky sweeps of her own. On 01 August 1786, she discovered a comet, establishing her membership of the astronomical community. Between 1788 and 1797 she discovered a further seven comets. Unfortunately, three of the comets were either simultaneously or previously discovered by other observers: the sixth and seventh were found by Messier and Mechain respectively and the eighth, in 1797, was discovered by Stephen Lee on the same night.

One of Caroline's most important contributions to astronomy was the indexing of the star catalogue compiled by the first Astronomer Royal, John Flamsteed, together with a list of omissions. This work was later published. Caroline returned to Hanover after her brother's death in 1822. In Hanover, she prepared a catalogue of nebulae and star clusters discovered by William during his sky sweeps. The catalogue was never published but was of use to her nephew, John, in his astronomical researches. The Royal Astronomical Society awarded Caroline its Gold Medal in recognition of the work involved in compiling the catalogue. While in Hanover, Caroline also took a great interest in the astronomical career of her nephew John.

Caroline lived to the age of 98, dying in 1848.

Friedrich Struve (1793 - 1864)

Friedrich_Struve.jpg Friedrich Struve.

Friedrich Georg Wilhelm von Struve was born on 05 April 1793 in Altona, Germany. In 1808, at the age of 15, he was forced to decide whether to stay in Germany and risk being conscripted into Napoleon's army, then in occupation, or to flee the country. He decided on the latter course, leaving Germany to stay for a short time in Denmark before moving to Russia where he settled for the remainder of his life.

Once in Russia, Struve enrolled at the University of Dorpat (now known as Tartu). In 1815 he became Director of Dorpat Observatory, an institution very well equipped by the standards of the period. In 1824, a ten-inch refractor by Fraunhofer was installed at Dorpat, equatorially mounted and driven by one of the first clock-drives. Struve then commenced the research for which he became famous, the study of double stars.

He started a comprehensive survey of the sky, as far south as declination -15°. He finalised the survey and published a star catalogue in 1827; it included about 120,000 stars, some 2200 of which were multiple. During the years 1825-27, he constructed a travelling wire micrometer which he used to measure accurately the positions of the various components of the multiple stars which he discovered. Following publication of the first catalogue, he wrote two books on multiple stars, published in 1837 and 1852. The first book included a list of additional multiple stars, increasing the total to 3112.

Between 1834 and 1837, Struve determined the parallax of Vega, arriving at a parallax angle of 0.26 arcsec. (The modern value is under half this figure at 0.12 arcsec.) Bessel is usually credited with determination of the first stellar parallax, of 61 Cygni in 1837. In fact it is probable that Struve preceded this date by a year or so; however, Bessel's results gained more rapid acceptance by the astronomical community.

After 24 years as Director of Dorpat Observatory, Struve was invited by Tsar Nicholas I of Russia to become Director of a new observatory at Pulkovo. The observatory was situated about ten miles south of St. Petersburg and was built and equipped to Struve's specifications. Struve worked there for over 20 years, concentrating on double star studies. His son, Otto, assisted in the observations. In 1861, Struve retired from the Directorship of Pulkovo and was succeeded by Otto.

Struve lived to the age of 71, dying in November 1864.

Pietro Secchi (1818-78)

Pietro_Secchi.jpg Pietro Secchi.

Pietro Angelo Secchi was born on 29 June 1818 in the town of Reggio Emilia in northern Italy. At the age of 15 he entered the Jesuit Order, where he continued to study his principle interest, astronomy. As a member of the Jesuits he taught in many of the schools of the Order up to 1848 when, due to religious persecution then prevalent on the Continent, he was forced to leave Italy. He settled briefly in the UK before going on to the US where he taught at Georgetown University, Washington DC.

When the political climate changed in Europe, Secchi returned to Italy, taking up the post of Director of Rome Observatory. His principle interest was the new field of astronomical spectroscopy. In this he was a pioneer and contemporary of Huggins in England. Secchi carried out a systematic observational programme, recording the spectra of approximately 4000 stars between 1864 and 1868.

Up to the time of Secchi's work, the only information known about stars was their position, brightness and colour. Secchi noticed that stellar spectra had wide diversity, indicating that stars had different chemical compositions. During 1867, he suggested that stars be classified by their spectra and grouped stars into four spectral classes. This first attempt at spectral classification has subsequently been expanded to ten spectral classes.

Secchi was also a pioneer in the use of photography for astronomical research. During an eclipse in 1851, he took photographs of the Sun, recording the various phases. By 1859, he had photographed the entirety of the Moon's visible surface.

Secchi died on 26 February 1878.

William Huggins (1824 - 1910)

William_Huggins.jpg William Huggins by John Collier (d.1934), date unknown.

William Huggins was born in London on 07 February 1824. He had no scientific education, joining the family business of clothes and fabric merchants on leaving school. His first interest was microscopy, but his astronomical pursuits slowly took precedence. By 1856, he had built an 8-inch refractor, obtaining the lens from a leading American lens maker, Alvan Clark. Three years later, he sold the family business to concentrate exclusively on astronomy.

During the first half of the nineteenth century, several famous physicists and chemists (among them Joseph von Fraunhofer (1787-1826), Gustav Kirchhoff (1824-87) and Robert Bunsen (1811-99)) had experimented with prisms and the dispersion of light. Although their work led to the modern science of spectroscopy, the pioneer scientists only dabbled in astronomical applications, their prime interest being chemical, and they left it to others to develop the importance of spectroscopy as an astronomical tool. During 1859, Huggins attended a lecture organised by the Pharmaceutical Society, which included a demonstration of new spectroscopic techniques. He decided that the new science of spectroscopy was just what he was looking for: a new tool for astronomical research. At the lecture, he approached Professor William Allen Miller (1817-70), a leading English spectroscopist. No spectroscopic equipment sensitive enough for astronomical use was available at the time and Miller, aware of the immense technical problems of constructing such equipment, was sceptical about Huggins' intentions. Undaunted, Huggins set about the task of building an astronomical spectroscope; Miller provided encouragement but devoted little time to the project.

After much difficulty, Huggins succeeded in pioneering the new branch of astronomy and, after four years of work, by 1863 had amassed sufficient data to present a paper jointly with Miller at the Royal Society on the spectral lines of several of the brighter stars. He presented a more complete report the following year, containing a major discovery. Both of the Herschels and Rosse had observed many nebulae that could not be resolved into separate stars: all three concluded that this was due to the limited aperture of the equipment they had at their disposal. By the end of August 1863, Huggins had obtained the spectrum of the planetary nebula NGC 6543 in Draco; the spectrum was unusual, having only a single bright, emission line. This demonstrated that some nebulae were luminous gas clouds, and not comprised of stars.

About seven-and-a-half years after starting his research, Huggins presented a paper on his findings to the British Association. The paper contained results that represented a major advance for astronomy:

Huggins married in 1875 and was greatly assisted in his work by his wife. About this time, he succeeded in photographing stellar spectra, an approach that enabled him to investigate the spectra of faint stars. In succeeding years, the Huggins were able to determine the radial velocities of some thirty stars.

Huggins was knighted in 1897 in recognition of his work. He continued observing until 1908 and died two years later.

Giovanni Schiaparelli (1835 - 1910)

Giovanni_Schiaparelli.jpg Giovanni Schiaparelli, 1890s photograph.

Giovanni Virginio Schiaparelli was born on 14 March 1835 at Savigliano, Italy. He gained a degree from the University of Turin in 1854 and on leaving university went to study under Encke at Berlin Observatory and then under Struve at Pulkovo Observatory in Russia in 1859.

Five years later Schiaparelli was appointed Director of Milan Observatory, a position which he retained until retirement. The majority of his work was associated with the Solar System. During the 1860s, he discovered a connection between Comet 1862 III and the Perseid meteor shower. (His work built upon that of the English astronomer John Adams who had calculated the orbit of the Leonid meteor swarm, demonstrating that it was comet-like.)

In 1877, Mars was at a favourable opposition, being at its minimum distance from Earth of 56 million kilometres. (This was the opposition at which Asaph Hall, observing from the US Naval Observatory in Washington, discovered the satellites of the planet, Phobos and Diemos.) From observations at this opposition and subsequent ones, Schiapparelli inadvertently instigated one of the biggest astronomical misconceptions of all time. Through careful micrometer measurements, he convinced himself that some of the features on Mars were straight lines, arranged in a complicated pattern. He published reports referring to these lines in Italian as canali, which should have been translated into English as channels but was inevitably mis-translated as canals, implying artificial structures which had been created by intelligent life on the planet. The popular press of the time gave the latter notion much publicity.

Several astronomers carried the idea of canals on Mars far beyond Schiaparelli's original drawings. The most notable exponent of Martian canals was Percival Lawrence Lowell (1855-1916), who claimed to have observed over 500 of them! Lowell's canals were probably the result of optical illusions combined with an over-imaginative mind. Many of Lowell's contemporaries reported seeing no such features.

Schiaparelli retired from Milan Observatory in 1900. Up to his death in 1910 he compiled an extensive survey of early astronomical history, concentrating in particular on Babylonian astronomy.

Norman Lockyer (1836 - 1920)

Norman_Lockyer.jpg Norman Lockyer, pre-1897.

Joseph Norman Lockyer wan born in Rugby on 17 May 1836. In 1857 he started a career as a clerk at the War Office. His employment included editing Army Regulations; the job held no interest for him and he instead directed his efforts towards astronomy. His interest in the science began as a hobby but subsequently was to become his full-time profession. He started observing with a 6.25" refractor by Thomas Cooke of York.

For the first few years, Lockyer observed the planets, singling out Mars for special study. When he heard of the work being done by Kirchhoff and Bunsen he decided to take up spectroscopy. He worked to develop spectroscopy independently of Huggins, although the two were active in the field at the same time. Lockyer concentrated on spectroscopy of the Sun, leaving the study of the spectra of remote stars to others.

Lockyer was the first to study the spectra of sunspots. By 1866, he had amassed sufficient evidence to conclude that sunspots appeared dark against the Sun's disk for two principal reasons:

  1. Sunspots emitted much less light than their surroundings (previously widely accepted).
  2. Sunspots absorb more sunlight than their surroundings.

Two years later he discovered that prominences could be observed in daylight by widening the slit at the front of the spectroscope and directing the light through the edge of the prism. Prior to this, prominences had only been seen fleetingly during total solar eclipses. Pierre Janssen (1824-1907), and a little later Huggins, also independently discovered this method of observing prominences in daylight.

In 1868 a total solar eclipse was visible from India. Janssen went to India to observe the phenomenon, having agreed to share observational results with Lockyer. Janssen obtained a spectrum of the Sun and noticed a previously unidentified spectral line. He forwarded his results to Locker, who concluded that the line belonged to a new element that had not yet been discovered on Earth. Locker named the element Helium: William Ramsay (1852-1916) discovered it on Earth at the turn of the 20th Century.

During 1869, Lockyer founded the science magazine Nature, taking up the editorship as a spare-time job, as officially he still worked for the War Office. The following year he was appointed secretary to a Royal Commission on Scientific Instruction and the Advancement of Science. Six years later the Commission reached its conclusions; one of its recommendations was to set up an observatory for solar study. The observatory was built at the Department of Science and Art of the Royal College of Science in South Kensington. (This centre has since been renamed, now being the Imperial College of London University.) Lockyer was transferred from the War Office to direct the new observatory. Now able to pursue astronomy on a full time basis, he extended his work from solar research to stellar spectra.

Lockyer received a knighthood in 1897 for services to astronomy. Four years later he retired, moving to Sidmouth, where, although in his seventies, he quickly established a new observatory. He continued observing until his death in August 1920.

Edward Barnard (1857 - 1923)

Edward_Barnard.jpg Edward Barnard circa 1897.

Edward Emerson Barnard was born in Nashville, Tennessee on 16 December 1857. He received only a mediocre education, following which he became interested in photography, making it for a few years his profession. From an early age he was interested in astronomy, being a serious sky observer in his leisure time. In August 1877, the American Association for the Advancement of Science (AAAS) held a meeting in Nashville. Barnard had recently acquired a 5" refractor, and he attended the AAAS meeting to seek advice on how best to use it. He returned from the meeting having talked to Professor Simon Newcomb (1835-1909) who suggested that he should search for comets.

The meeting with Newcomb marked the beginning of Barnard's world fame as a discoverer of comets. After about four years of searching, he discovered his first comet on 12 May 1881, near α Pegasi. The object was visible for only two nights before being lost from view. There was, however, only a four month gap until Barnard found his next comet, 1881 VI, which brought his name to prominence. Subsequent years proved even more fruitful, as Barnard discovered 15 more comets during the years 1882-92. He discovered every new comet in 1891!

During 1883, Vanderbilt University awarded Barnard a fellowship in astronomy. After only a short period at Vanderbilt, he was given charge of the University Observatory. Later in the year, whilst observing a lunar occultation of β Capricorni, he noticed that the star flickered prior to disappearance instead of disappearing instantly. On the basis of the observation, Barnard proposed that the star could be binary. The Dearborn Observatory, Chicago, which housed a larger telescope, subsequently confirmed the hypothesis. Also in 1883, Barnard re-discovered the Gegenschein (sunlight scattered by minute dust particles in the inner Solar System), which had first been reported in 1854, but had attracted little attention.

After completing his University course, Barnard was offered, and accepted, the post of assistant astronomer at the new Lick Observatory on Mount Hamilton, California. While there, he made three important discoveries in 1892:

In 1895, Barnard moved to take up the post of Professor of Astronomy at Chicago University, where he began working at Yerkes Observatory with the famous 40" refractor. At Yerkes, Barnard continued his program of photographing the Milky Way. Along with Wolf, he was one of the first to realize that the dark patches in the Milky Way were, in fact, clouds of gas and dust obscuring more distant stars. In 1917, the Carnegie Institute in Washington published a photographic atlas of selected Milky Way regions based on his photographs.

During 1916, Barnard discovered a star in Ophiuchus with very fast proper motion. It moves approximately ½° (the apparent diameter of the full moon) in a period of only 18 years. This object has become known as Barnard's Star, holding the record for the greatest proper motion of any star.

Barnard lived to the age of 65, dying in February 1923.

Bernhard Schmidt (1879 - 1935)

Bernhard_Schmidt.jpg Bernhard Schmidt.

Bernhard Woldemar Schmidt was born on 11 April 1879 on the small island of Nargen (only 8 km long by 3 km wide) situated some 20 km off the coast of Estonia. Life on Nargen was dominated by the Lutheran Church and farming and it is remarkable that an influential figure in the field of optics should emerge from such an isolated environment. Despite his parents' emphasis on a strict Lutheran upbringing, Schmidt's instinctive interest in science soon became apparent. When he was eleven he was already experimenting with gunpowder. In one experiment, while packing gunpowder into a metal tube, it exploded and he lost his right hand and forearm, being fortunate not to die. Despite this devastating accident he maintained and developed his interest in mathematics and physics. In the years that followed, he became interested in optics and, working from drawings of a camera in a book, he ground a lens from the bottom of a bottle, mounted it in a cigar box, and with some photographic plates from his friend the village chemist, succeeded in taking photos. This was a sign of greater things to come!

In Schmidt's late teens he enrolled as an engineering student in Gothenburg, Sweden where he specialised in optics. While studying there he came across the work of the German optician Stehl and, after completing his studies, left for Germany to seek him out. Stehl had worked at Mittweida but, by the time Schmidt arrived there, had gone elsewhere. Despite the absence of Stehl, Schmidt found Mittweida to his liking and stayed on, supporting himself by making mirrors and selling them to local astronomers. Initially he ground mirrors only for amateurs but, once professionals realised how good was his product, they too began placing orders. Starting in 1900, he produced mirrors up to about 200 mm in diameter. In 1905, he constructed a 400 mm mirror which far surpassed anything then available and, as his skill developed, he figured 300 mm, 500 mm and 600 mm objectives for Leipzig, Potsdam and Hamburg observatories. It is remarkable that he carried out all his work with his left hand and never used machines.

Schmidt's reputation spread rapidly and he was offered several jobs by the great German optical companies of the day. However, Schmidt wished to maintain his independence and declined every offer. A man who disliked regimentation, he worked only as the mood took him.

By 1920, he had ground several mirrors for Hamburg Observatory at Bergedorf and, in 1926, the director of the observatory, Richard Schorr (1867-1951), eventually persuaded him to join the staff, albeit as a "voluntary colleague". Schmidt maintained his irregular, independent style of work, often leaving the optical workshop to roam in the nearby woods.

From the start of his time at Bergedorf, Schmidt was set on overcoming the limitations of conventional telescopes. In 1929, he went on an eclipse expedition to the Philippines with the astronomer Walter Baade (1893-1960). During the trip, Schmidt told Baade that he had at last solved, in principle, the problem of producing a reflecting telescope that not only had a large aperture but also had a wide field of view. Schmidt's design has a large mirror at the bottom of the tube, a thin glass plate at the top end and a film-holder with a curved surface facing the mirror in the middle. Baade, realising the importance of the new design, urged Schmidt to build one as soon as possible as did Schorr on hearing the details. Despite the encouragement of his colleagues, however, Schmidt continued his apparently aimless walks in the woods insisting that he first had to solve the problem of how to grind the complex curves involved in his design. In late 1929, he announced that he had solved the problem, and began construction work.

Schmidt's ability to work was phenomenal, once he started. On one occasion, Baade visited to find him sleeping after 36 hours continuous work. Schmidt completed his first camera in early 1930 and soon used it to produce fine photographs. It had a 350 mm glass plate and a 430 mm mirror and, with a focal length of 635 mm, achieved a photographic speed of f/1.7, incredibly fast for such a large instrument. It seems that Schmidt had to work very close to the limits imposed by the breaking strain of glass in order to produce the instrument.

Photos from the camera initially failed to impress European astronomers but, as soon as Edwin Hubble (1889-1953) of Mount Wilson saw them, he immediately asked Schmidt what was the largest camera that could be built. The answer turned out to be 1.2 m diameter for the glass plate and 1.8 m diameter for the mirror. The two large cameras of Mount Palomar and Siding Springs observatories are of this size: anything bigger would run into technical problems.

Schmidt continued work until his death on 01 December 1935. (He died in Hamburg from a lung infection.) The 1.2 m Schmidt camera at Mount Palomar was completed in the late 1940s and continues to be of immense value to astronomers; a great tribute to Schmidt's insight and optical genius.

Ejnar Hertzsprung (1873 - 1967) and Henry Russell (1877 - 1957)

H+R.jpg Ejnar Hertzsprung (top) and Henry Russell (bottom).

Ejnar Hertzsprung was born on 08 October 1873 and Henry Norris Russell on 25 October 1877. Both men independently discovered the relationship between the absolute magnitude and the colour of stars. The result was the Hertzsprung-Russell, or H-R diagram which, in both its original and modern forms, has greatly assisted astronomers to understand stellar evolution.

Hertzsprung was educated as a chemical engineer and worked from 1898 to 1901 in St. Petersburg. In 1902 he returned to his native Copenhagen with a great interest in astronomy. After some seven further years he was appointed an astrophysical lecturer at Göttingen University. He was one of the first to advance the idea of absolute magnitude. (The absolute magnitude of a star is the magnitude that it would present if it were situated at the standard distance of 10 parsecs (32.6 light-years) from Earth.) Absolute magnitude enables stars of differing luminosities to be directly compared.

Hertzsprung specialized in stellar photography, particularly of double stars, and in estimating stellar magnitude from photographs. This led him to publish, in 1905, in a semi-populist photographic journal, his ideas about stellar colour and absolute magnitude. Unfortunately, the article went unnoticed for nearly ten years.

In 1910, he met with American astronomer Henry Norris Russell, who had independently reached very similar conclusions concerning the relationship between stellar colour and absolute magnitude. The two astronomers merged their results as the Hertzsprung-Russell (or H-R) diagram, published in 1913.

During 1911, he discovered that the Pole Star was a Cepheid variable, varying by 0.2 magnitude in a period of about four days.

In 1935, he became a professor at Leiden, Holland. On retirement, he returned to his native Denmark, dying on 21 October 1967.

Russell was educated at Princeton University, New Jersey, receiving his doctorate in 1900. He worked for a short time in England before returning to teach at Princeton. His research led him to the discovery of the luminosity-colour-spectral class relationships of stars. He presented his results at a meeting of the American Association for the Advancement of Science in December 1913 and published the work in 1914, some nine years after Hertzsprung.

In 1929, Russell analysed in great detail the composition of the Sun from its spectrum; he was one of the first to do so. He was rather surprised to find that its composition was mostly hydrogen, with helium, oxygen and nitrogen being the most important trace elements.

Russell died on 18 February 1957.

Significant Astronomical Discoveries And Events

Discovery / Event
ca. 3000 BC
Earliest recorded Babylonian observations of eclipses, planets and stars.
ca. 2500 BC
Egyptian pyramids constructed, oriented north-south by the stars.
ca. 2000 BC
Babylonian story of creation. Enuma Elish. Stonehenge built in southern England, aligned with the stars.
ca. 1000 BC
Beginnings of Chinese and Hindu astronomical observations.
ca. 700 - 400 BC
Greek story of creation. Theogeny of Hesiod. Hebrew story of creation. Greek philosophers Thales, Pythagoras and Meton note the regularity of celestial motions.
ca. 400 - 300 BC
Greek philosophers Plato, Eudoxus and Calippus develop the concept of celestial motion on spheres. Aristotle develops the idea of four elements and the concept that heavy things fall, light things rise.
ca. 300 - 100 BC
Aristarchus proposes that the Earth moves. Eratosthenes measures the size of the Earth. Hipparchus makes accurate positions of the position of stars.
ca. 150 AD
Ptolemy writes The Almagest, a treatise on astronomy, summarising the geocentric theory: it explains the motion of the planets by the circular motion of deferents and epicycles.
ca. 1400
Ulugh-Beg in Samarkand re-observes star positions.
Copernicus in Poland postulates that the Earth and planets move around the Sun because this involves fewer circular motions. This revolutionary idea later provokes strong opposition.
ca. 1600
Tycho Brahe accurately measures the motions of the planets. Kepler uses these measurements to show that the orbits of the planets are ellipses rather than combinations of circles. Galileo uses one of the first telescopes to observe the satellites of Jupiter and the crescent shape of Venus, providing strong supporting evidence for the model of Copernicus. He also establishes that falling weights would all be accelerated to the same degree if there were no air resistance to retard larger objects.
ca. 1680
Newton combines findings from Kepler and Galileo, together with observations of the Moon and comets, to formulate the fundamental laws of mechanics and gravitation. He also studies light, its colour and spectrum. By this time, accurate pendulum clocks are in use.
Edmund Halley notes periodic reports of a bright comet every 76 years and concludes that they relate to an object moving around the Sun in a highly eccentric, elliptical orbit. He predicts the return of the comet in 1758. The comet duly reappears (16 years after his death) and is subsequently named Halley's Comet in his honour. In the modern era, Halley's Comet had an apparition in 1985-86.
Kant postulates that the Sun and planets formed from a giant cloud of gas, in the form of a spiral nebula, which coalesced under its own gravity.
William Herschel builds large telescopes, discovers the planet Uranus and explains the Milky Way as a flat disk of stars surrounding the Sun.
1700 - 1800
Mathematical astronomy flourishes, involving many Europeans - Cassini, Bradley, d'Alembert, Laplace, Legrange and others - who apply Newton's mechanics to celestial motion with remarkable precision.
Joseph Fraunhoffer discovers dark lines in the spectrum of the Sun, paving the way for the invention of spectroscopy.
Sir William Herschel dies.
Johann Bode dies. He is best remembered for popularising a mathematical relationship between the distances of planets from the Sun.
Friedrich Bessel determines the distance of the star 61 Cygni.
Sir John Herschel, son of Sir William Herschel, travels to Cape Town and produces the first extensive catalogue of stars in the southern hemisphere.
J W Draper takes the first astronomical photograph; his subject is the Moon. By 1905 astronomical photography is well established and used with telescopes ranging in size up to 40" in aperture, recording stars of brightness only 0.000 01 that visible to the naked eye.
Doppler explains the effect of motion on the spectrum of light.
William Parsons, 3rd Earl of Rosse, discovers the spiral shape of galaxies.
Jean Couch Adams and Urbain Jean Joseph Leverrier predict the position of a new planet beyond the orbit of Uranus. Johann Gottfried Galle and a student assistant, Heinrich d'Arrest, working at the Berlin Observatory in Germany, find the planet. It is named Neptune.
William Parsons gives the name Crab Nebula to the remnants of the supernova of 1054 (the supernova was observed by Chinese astronomers).
William Bond is first to photograph a star.
Pietro Secchi is one of the first to photograph the Sun during an eclipse.
Gustav Kirchhoff begins developing the discoveries of Fraunhoffer and invents the spectroscope.
Sir William Huggins uses a spectroscope to analyse starlight.
George Bond is first to photograph the double star Mizar (in the constellation Ursa Major).
After studying stellar spectra, Pietro Secchi suggests establishing spectral classes for stars.
Sir William Huggins discovers that the star Sirius is receding from the Earth.
Hermann Vogel uses spectroscopy to analyse planetary atmospheres. William Huggins, though, had started such observations some time earlier.
The Italian astronomer Giovanni Virginio Schiaparelli reported observations of Martian canali, a term mis-translated into English as canals, implying an artificial construction. This initiated huge interest in the possibility of intelligent life on Mars utilising advanced engineering to transport scarce water around the planet.
1800 - 1900
Navigation is now a precise and important practical application of astronomy. Accurate observations of stellar positions reveal that annual parallax is due to the Earth's motion around the Sun, confirming the Copernican model of the Solar System and providing a method of estimating the distance of the nearer stars. Other precise measurements show that the stars are themselves moving through space.
1850 - 1900
The laboratory study of light together with physical theory shows that spectral analysis can be used to determine the temperature and chemical composition of a light source (e.g. a star).
George Hale invents the spectroheliograph for observing the distribution of chemical elements in the Sun.
George Hale initiates his idea for building a 40" refracting telescope.
George Hale's telescope is completed. It is still the largest refracting telescope in the world. It was named after its chief financier, Charles Yerkes.
Thomas Chrowder Chamberlin and Forest Ray Moulton speculate that the planets were formed after another star passed close to the Sun and pulled some of the material of the latter into orbit around the Sun.
Henrietta Swan Leavitt discovers the period-luminosity law for Cepheid variables, which can be used to estimate the distance of such stars.
1902 - 1920
Albert Einstein establishes the theory of relativity. Large reflecting telescopes are built at the Mount Wilson Observatory in California.
Vesto Slipher is first to observe that the galaxy M31 in Andromeda is approaching the Earth, whereas most others are receding.
Henry Russell publishes the discovery of the relationship between a star's colour and its luminosity. He arrives at the same conclusion as Ejnar Hertzsprung, but independently. The resulting Hertzsprung-Russell diagram provides significant advantage in the understanding of stellar evolution.
1915 - 1920
Albert Einstein develops the General Theory of Relativity. Harlow Shapley studies globular star clusters and identifies Cepheid variables within them to estimate their distances. Shapley goes on to estimate the dimensions of the galaxy: just as Copernicus moved the centre of the universe from the Earth to the Sun, so Shapley moved the Earth a long way from the centre of the galaxy!
Clyde Tombaugh discovers Pluto.
1910 - 1940
Slipher and Edwin Hubble find that most other galaxies are receding from the Milky Way. Willem de Sitter, Arthur Eddington, Georges Henri Joseph Édouard Lemaitre and others apply relativity theory to explain this recession.
1930 - 1960
Hans Bethe, George Gamow and others in the US apply the results of nuclear physics to explain the source of stars' energy. Many astrophysicists work on theories of the formation of stars from giant clouds of interstellar gas and their subsequent evolution. Carl Friedrich Freiherr von Weizsäcker, Gerard Peter Kuiper, Harold Clayton Urey and others develop a theory of the origin of the solar system from a giant gas cloud.
Karl Jansky discovers radio waves emanating from sources in the galaxy.
Walter Baade discovers over 300 Cepheid variables in the Andromeda Galaxy and uses them to derive a more accurate estimate of its distance.
1947 - 1960
The first instruments are launched above the atmosphere in the US for astronomical observations.
Walter Baade derives a new period-luminosity law for Cepheid variables. This effectively doubles the size of the known universe. Walter Baade finds that our own galaxy, the Milky Way, is of only average size, thus de-throning it from holding special significance due to its size.
USSR launches the first artificial satellite, Sputnik, into Earth orbit.
Soviet scientists launch the first space probe to hit the Moon.
Soviet cosmonaut Yuri Gagarin makes the first manned spaceflight around the Earth.
1961 - 1966
Radiotelescopes locate quasi-stellar radio sources (quasars) which are found to have large optical red-shifts, like the most distant galaxies.
1962 - 1967
Rocket-borne instruments flying above the Earth's atmosphere detect cosmic X-ray sources.
1964 - 1965
US space probes Ranger 7 and Ranger 8 obtain first close-up photographs of the lunar surface.
Mariner 4 takes photographs of the surface of Mars from a distance of approximately 17,000 km. The photographs show a cratered surface.
Soviet space probe Luna 9 makes the first soft-landing on the Moon.
Surveyor 3 carries out the first physical analysis of the lunar surface.
First optical discovery of a pulsar (M1 in the Crab Nebula). Neil Armstrong becomes the first man to reach the Moon.
Mariner 9 enters orbit around Mars.
Pioneer 10 makes a successful fly-by of Jupiter.
Pioneer 11 makes a successful fly-by of Jupiter. Mariner 10 makes a successful fly-by of Venus and Mercury.

Joe Walsh, Roy Gooding, Roy Adams, Mike Harlow