Occultations - Brian Loader


Astronomically, an occultation occurs when one body, such as the moon or a planet hides another object, often a star, by passing in front of it. A third player in the action is an observer in the right place to see or, nowadays, record the event! To make use of the occultation the observer needs to make, among other things, an accurate timing of when the star disappears behind the moon or planet and/or when it reappears again.

The use of the times of occultations has varied over the ages. Some of these uses will be covered in later articles. Originally they were observed visually using an accurate time piece referenced to an accurate time source. All this has changed in the 21st century with the use of video and GPS sourced times giving a much more reliable and certainty of the event and its time. This has resulted in a widening of the use of occultation observations and in their application to other fields. Even to playing a minor part in the Kepler mission’s aim of discovering planets orbiting other stars. Much software has been developed to assist in observing and analysing the results of occultation observations ... available free.

All this and more will be covered in later articles. For now we’ll have a look at just what occultations are.

Occultations by the Moon

The most common object causing an occultation is indeed the moon. This is due to its relatively large apparent size compared to planets and also to its motion right round the sky in about 27 days so that, in the course of a month, it moves across a considerable area of sky. Even so only a limited number of stars can ever be occulted by the moon, they need to be within a few degrees of the ecliptic. Occultations by the moon can be seen, using a medium size telescope, on almost any clear night the moon is in the sky. The exception is near full moon when the glare from the moon makes it difficult to see any but the brightest stars.

Occultations can only be successfully observed at the dark limb of the moon, unlit by sunlight. At the sunlit limb a star will be swamped by the brightness of the moon so that it can only be seen when some distance from the limb. This applies to all but the very brightest stars.

Between New Moon and Full Moon the occultations at the “dark” limb, will be disappearances of the star behind the Moon. In fact the “dark” limb is lit by Earth light, that is sunlight reflected from the Earth onto the Moon. The Earth lit limb can be quite bright particularly before the first and after the last lunar quarters. Before full moon the reappearance of the star occurs at the sunlit moon limb. Between Full Moon and New Moon the opposite applies, it is the reappearances which take place at the unlit limb of the moon, the disappearances taking place at the sunlit limb.

Disappearances are easier to observe than reappearances. They mostly occur in the evening and it is usually easy to see the star before it disappears. At a reappearance (observed after full moon), the star is not visible before the event. So it is necessary to have a good idea of where it will appear, that is where to look, round the limb of the moon. Added to that problem, most reappearances from occultation occur after midnight, a less convenient time.

In either case accurate predictions of when the disappearance or reappearance will occur are needed. Fortunately such predictions are readily available.

In effect, during an occultation a shadow of the moon (or planet) is cast onto the Earth by the occulted star. The size of the “shadow” is the size of the moon or planet as projected onto the Earth. Since the moon is smaller than the Earth, only about the size of Australia, a particular occultation is only visible from a limited region of the Earth.

On 2017 July 31 the 5.9 magnitude star ZC2141 was occulted by the Moon with the latter just past first quarter. The map shows the region of the Earth from which the occultation was visible. It is delineated by the pair of curved, coloured lines. Between the white lines the occultation took place at night, so New Zealand was well placed for viewing the disappearance of the star. Just to the west of this region is an area between dark blue lines. In this area the occultation took place just after sunset while the sky was still bright. Left of that again, limited by red spots is the region for which the occultation occurred before sunset, so making it unobservable at least in small telescopes. So in this case the Moon was in the sky when the occultation took place for all parts of Australia but being during the daytime it was unobservable.

As seen from anywhere outside the areas between the pair of lines the moon would have missed the star without occulting it. The two turquoise loops show the areas where the occultation was in progress at moonrise, to the west (left) and moonset to the east (right). And of course the occultation took place at different times as seen from different places, earliest to the west and latest to the east.

Occultations by planets and asteroids

Planets may also at times occult a star but instances involving the major planets are rare, both because their apparent size is so much smaller than the moon, and their apparent rate of movement is much less. Asteroids are much smaller so occultations by a particular asteroid are even rarer. But there are thousands of asteroids for which occultation predictions are available. As a result, on average, about one occultation a week by an asteroid is visible from any one site. Of course a good fraction of these will not be seen due to cloud!

Because asteroids are faint objects it is possible to see much fainter stars occulted by them than is the case with occultations by the moon. And, importantly, both the disappearance and the reappearance of a star occulted by an asteroid can be seen. Normally the star is hidden for only a very few seconds, sometimes less than one. The path along which the occultation can be seen is narrow, its width being the diameter of the asteroid as projected onto the Earth.

An example of one such predicted occultation was by the 80km wide asteroid (464) Megaira which occulted a magnitude 13.3 star on 2017 December 6. The maximum expected duration of any occultation was 7.1 second.

The map shows the expected path of the occultation, the strip along which it was expected to occur is shaded. In this case the asteroid was moving from east to west taking just under 10 minutes to move across the segment of the Earth shown. Parts of the South Island of New Zealand and northern Victoria were on this path. The width of the expected path was just under 160 km.

One of the problems involved in observing these events is locating the star to be occulted. There is no moon to show an intending observer where to point the telescope! Also occultation predictions are not as accurate as those for the moon. As a result an observer can be in the predicted path, but sees nothing happen to the star. This happened for one observer of the Megaira event who was close to the middle of the predicted path. In all there were 6 intending observers for the occultation. Several had cloud with the result that no one saw the occultation. Despite these sort of problems asteroidal occultations are an interesting, potentially useful and sometimes challenging type of event.

Occultations of moons of other planets

As many will know, among the planets Jupiter regularly occults and eclipses its own satellites, particularly the four large Galilean moons. Eclipses and occultations are related phenomena but are in fact completely different.

When Jupiter occults one of its satellites, an observer will see the satellite disappear behind Jupiter’s disk to reappear some time later at the other side of the planet. At an eclipse a satellite moves into the Jupiter’s shadow cast by the Sun. As it does so the satellite disappears from view as it darkens, reappearing some time later when it emerges from the shadow.

Depending on the relative positions of Sun, Earth and Jupiter, the eclipse may appear to take place some distance from Jupiter itself. Some times only one phase of the eclipse is visible, the other taking place while the moon is behind, that is occulted by, Jupiter as seen from the Earth. An occultation on the other hand always takes place at the limb of Jupiter. The brightness of the sunlit disk of Jupiter makes their observation a little more difficult than one by a dark asteroid.

Occultations of most of Jupiter’s four largest satellites occur every orbit of the satellite. Similar events can occur for the moons of other planets, but they are far less frequent and more difficult to observe.