Orwell Astronomical Society (Ipswich)

Jeremiah Horrocks and the Transit of Venus

The orbit of the planet Venus lies within that of the Earth and so, when the geometry is right, Venus will appear to pass across the Sun's disk as seen from the Earth: this gives rise to the phenomenon known as a Transit of Venus (ToV). At the time of writing (May 2004), it is 122 years since the last ToV (on 06 December 1882) and it is a sobering thought that nobody alive today has ever observed one. The next ToV will occur on 08 June 2004. To gain a historical perspective on the ToV, let's look at Jeremiah Horrocks, a young Lancashire man, who secured his place in astronomical history by predicting a ToV and then making the first ever-scientific observation of the phenomena.

Jeremiah Horrocks (or Horrox, as he later signed himself) lived from around 1619 (his exact date of birth is unclear) until 1641 and within his all-too short life made an indelible mark on the science of astronomy. Even the irascible Sir Isaac Newton was to credit his contribution to the science. Horrocks was born in Toxteth, Liverpool of a farming and watch-making ancestral background, which gave his family a reasonable standing locally. Little is known of his education, but we do know that he entered Emmanuel College, Cambridge as a Sizar (poor working/servant student) at the age of 13. His evident mathematical ability was largely unsatisfied by the regime at Emmanuel and in his own time he read widely, in the works of Continental thinkers such as Copernicus, Tycho Brahe, Kepler, Galileo and others, of the movement away from Ptolemaic astronomy. Horrocks held Tycho Brahe’s observations and Kepler’s subsequent analysis of them in especially high esteem. In 1635, at age 16, Horrocks left Emmanuel without taking a degree, which is not to say that he "dropped out" – merely that he had acquired a broad knowledge base and that he did not require formal qualifications. Moreover, in those days there was a financial obligation to the College in the taking of a formal degree that Sizar students like Horrocks would not have been able to meet.

Back home at Toxteth, Horrocks began to correspond with a self-educated Manchester-based clothing trader, William Crabtree (1610 – 1644), who shared his interest in the work of the aforementioned European scientists. The correspondence was to last for the few remaining years of Horrocks' life. At the time, the astronomical ephemerides in common use were the tables of Belgian mathematician Philip Lansberg, but both Horrocks and Crabtree had begun to realise that they contained many errors in predicted positions of the planets. They were much in awe of Kepler’s discovery of the elliptical nature of the orbit of Mars, and Horrocks was spurred on to try to explain the erratic nature of the Moon’s orbit around Earth. Eventually he suggested that the Sun caused perturbations in the Moon’s orbit and correctly described its elliptical nature, work (as already touched upon) that would later be mentioned by Newton when he formulated his gravitational theories. Other work, discussed in correspondence with Crabtree, included the study of planetary motions – particularly those of Jupiter and Saturn – and acknowledgement that comets moved in elliptical orbits and were natural objects rather than harbingers of doom as described by astrologers! Horrocks also tried to measure the diameters of stars using different sized pinholes in cardboard to frame them. By those methods, both men came to realise that there was no satisfactory way to measure the apparent diameters as stars were but points of light. Confirmation came in 1637 when they jointly observed a lunar occultation of the Pleiades and observed that the light from occulted stars disappeared instantaneously.

A change in Horrocks’ domestic circumstances came in 1639, when he went to live in the village of Hoole, a few miles NW of Preston, for a year or so. Folklore has it that he was appointed curate there, but no evidence exists to substantiate this and Victorian "sanitisation" of popular historic figures probably accounts for it. Horrocks may, however, have had puritanical leanings although not be ordained.

Kepler had predicted that transits of Venus would occur in 1631 and again in 1761. However, the 1631 event was not visible from Europe and was consequently unobserved. Horrocks, after studying the positions of Venus in the sky and comparing them with Lansberg's tables, came to realise a mistake in Kepler’s calculations insofar as transits of Venus occur in pairs separated by an eight year gap – and consequently another transit would occur on 24 November 1639. His realisation came barely a month before the event and he hastily despatched a letter to Crabtree, telling him of his discovery and imploring him to observe the Sun on the predicted date.

On Sunday 24 November 1639, Horrocks was much engaged in the rituals of Sunday worship, but was nevertheless able to spend over four hours observing. He had purchased a small telescope (likely a refractor of around 5 cm aperture) and set up an arrangement whereby the telescope projected the Sun’s image into a darkened room and onto a 15 cm diameter circle on paper, upon which he had carefully inscribed 360 degree markers. He saw nothing until about half an hour before sunset and then had the supreme moment of triumph when he saw the jet black disk of Venus, relatively tiny in relation to the Sun. In the next half hour he made three very careful measurements and was later able to calculate the ingress and egress times of the transit, the angular velocity of Venus and, by trigonometry, arrive at value for the distance of the Sun from the Earth. Crabtree, meanwhile, further south near Manchester, was horribly clouded out but just before sunset the clouds parted sufficiently for him to discern, in a state of great excitement, the silhouette of Venus against the disk of the Sun. He articulated his joy thus:

rap’t in contemplation he stood, motionless, scarce trusting his senses, through excess of joy [1].

Horrocks and Crabtree had observed a phenomena never previously seen, and which could not again be witnessed for 122 years.

For some weeks following the transit, Horrocks made meticulous measurements of the angle of separation of Venus from the Sun in the pre-dawn sky and subsequently wrote a treatise on his observations entitled Venus In Sol Visa (Venus in Transit Across the Sun) but it was not published for another 23 years – and then by the Danzig astronomer, Johannes Hevelius, who had somehow come by the manuscript.

The following popular image of Jeremiah Horrocks observing the transit of Venus on 24 November 1639 is thought to be much embellished by Victorian sentiment. It depicts Horrocks "wearing the cloth", although there is no record of this being the case. No true portrait of Horrocks is known to exist.

Horrocks died suddenly on 03 January 1641, while in his early 20s, but we do not know the cause of his death. Crabtree, too, died young at the age of 34 years in July 1644.

Horrocks' achievement was outstanding by any measure and his half hour observation of Venus against the Sun’s disk surely ranks as one of the seminal moments in the astronomical history of Britain.

Footnote

Sources

Ken Goward, FRAS

Original: Newsletter May 2004
Updated: 26 March 2023