Orwell Astronomical Society (Ipswich)
Each day, the Moon travels approximately 13° along its orbit, moving from west to east in the sky. This motion causes it occasionally to occult background stars: the eastern limb of the Moon, as it advances through the sky, obscures the star and, some time later, the star reappears from behind the western limb. Because the Moon has no atmosphere, generally disappearance happens suddenly with no preliminary fading and reappearance is equally sudden. Two exceptions to this rule occur in the case of stars with large angular diameter and stars composed of two or more close components.
The Moon's motion is confined to a strip of the sky centred on the ecliptic (the plane of the Earth's orbit around the Sun) with a width of ±6.75°, referred to as the zodiacal band. The following extract from a star chart illustrates the limits of the zodiacal band as it passes through the constellation Taurus. The sizes of stars in the map are exaggerated: in reality they appear simply as points of light. The Moon is shown to scale inside the small square: note how small it appears in relation to the piece of sky illustrated. The arrow (to scale) indicates the extent of the Moon's average daily motion.
There are approximately 850 naked eye stars (magnitudes down to 6.0) which the Moon can occult. Glare from the Moon itself tends to obscure the stars, especially when the lunar phase is close to full, and frequently necessitates use of binoculars or a telescope. However, if the Moon is very young or very old and the star is of first or second magnitude, it is sometimes possible to observe an occultation with the naked eye. Table 1 summarises the brightest stars subject to lunar occultations and the equipment typically required to observe. Among those listed are four first magnitude stars: another first magnitude star, Pollux, lies just outside the band of stars which can be occulted.
|Magnitude||Stars in Zodiacal Band||Visibility of Occultation|
|1||Aldebaran, Regulus, Spica, Antares||Naked eye / binoculars|
|2||β Tauri, γ Geminorum, δ Scorpii, σ Scorpii||Naked eye / binoculars|
|3||20||Naked eye / binoculars|
|5||201||Binoculars / telescope|
Table 1. Stars in the zodiacal band subject to lunar occultations.
Timing a lunar occultation disappearance or reappearance establishes a relationship between the following quantities:
Until the latter half of the 20th Century, professional astronomers observed and timed lunar occultations in order to improve theories of the lunar orbit, to refine knowledge of the lunar limb and to study the rotation of the Earth. Observations of occultations also sometimes revealed errors in the position or proper motion of stars and the existence of unsuspected double or multiple stars. Nowadays, advances in determination of the lunar orbit, lunar mapping and stellar astrometrics have rendered lunar occultations largely redundant from the point of view of professional astronomers. However, they continue to provide an interesting and rewarding spectacle for amateur observers.
The ability of an astronomer to observe an occultation depends on the following parameters:
The Moon occults planets as well as stars. In fact, all the planets with the exception of Venus are confined within the zodiacal band of ±6.75° defined by the Moon's motion. Venus is the exception, and can stray up to ±9.8° from the ecliptic. Occultations of the brighter planets are visible to the naked eye; the fainter planets will require a telescope.
For a given location on the Earth's surface, a lunar occultation of a major planet will occur almost every two years on average. However, the occurrence is very irregular, with clusters of events separated by lengthy periods with no events.
Occultations of planets may be predicted in broadly the same way as occultations of stars. However, the motion of the planet needs to be accounted for in calculating the details.
If the Moon passes centrally in front of a star, it can obscure the latter for more than an hour. However, generally the centre of the lunar disk passes to the north or south of the star, causing a shorter obscuration. In the limit, the north or south polar limb of the Moon briefly obscures the star - this case is termed a grazing lunar occultation or graze. During a graze, mountains and valleys on the lunar limb can pass in front of the star, alternately obscuring and revealing it, making it appear to flash off and on. Such an event is typically visible over a strip of land on the Earth's surface only a few hundred metres wide, referred to as the graze track. On one side of the graze track, an observer will witness a proper occultation, while on the other side he/she will witness a "near miss". In an average year there are a dozen or so grazing occultations that can be observed from the British Isles. Generally, a south polar graze is more spectacular than a north polar graze because the Moon’s southern limb is the more rugged of the two.
Circumstance of occultations, which are not grazes, for the location of Orwell Park Observatory are predicted as follows:
The occultation package Occult by the Australian amateur David Herald, is used to calculate graze predictions from 2014 onwards. For earlier years, predictions are based on the software programme graz written by Jean Meeus, enhanced to use the NASA JPL ephemeris DE-405 or DE-430 and the star catalogues listed above together with electronic Watts charts. Occult enables a much more accurate approach primarily through providing access to lunar limb data from the 2007 Japanese Kaguya (officially Selene) mission; this significantly improves the overall accuracy of graze predictions.
Occultation predictions for Orwell Park Observatory.
Predictions of occultations may be found in the monthly astronomy magazines and on the websites of the British Astronomical Association and International Occultation Timing Association.
"Astronomy On The Personal Computer", 2nd edition, O Montenbruck and T Pfleger, Springer-Verlag, 1994.
C B Watts, "The Marginal Zone of the Moon", USNO, 1963.