What are Occultations?

The Moon is a solid object with an appreciable diameter of about half a degree of arc in the sky. This is almost the width of your little finger held out at arm’s length! As it travels around the earth, once in just under a month, it follows that it must occasionally hide any object that lies in its path.

Sometimes the moon passes either partially or totally in front of the sun and this is, in a sense, an occultation. However, astronomers prefer to us the word ECLIPSE for this particular event, as you all know.

In a similar way, the Moon can pass in front of stars and can hide them for up to 1 hour and it is this phenomenon that is referred to as an OCCULTATION.

An occultation of a star by the Moon is a very beautiful sight, and in the case of the brighter stars, one that can be followed in binoculars, provided that you have some means of steadying the instrument (“Elbow shake” is not conducive to good observations). Especially beautiful is the occultation of a bright star by the crescent Moon, with its night hemisphere faintly illuminated with Earthshine (often called, when seen next to the waxing crescent, “The Old Moon in the New Moon’s arms”). In this case we can actually watch the dark edge of the moon gradually getting nearer and nearer to the star until suddenly the light of the star blinks out as it disappears behind the Moon. At such a time as this time we can actually appreciate the awesome movement of a heavenly body.

As the Moon’s true motion across the sky is from west to east, i.e. from right to left (it is the earth’s faster rotation that makes sun, moon and stars rise in the east and set in the west), it follows that a star always disappears behind the LEFT limb (or edge) of the Moon.

The sudden disappearance of a star is always very striking and as well as indicating the Moon’s motion, it also gives proof of the non-existence of a lunar atmosphere; for if an atmosphere were indeed present on the Moon, we should witness a fading in the star’s light before IMMERSION (the term used in occultation studies for the disappearance).

Watching the disappearance of a star is by far easier than watching a star reappear from behind the Moon at the end of an occultation. The problem with EMERSION, as the reappearance of a star is called, is in knowing exactly where the star will appear so that you are looking at the exact spot, a very difficult thing to do. For this reason, where predictions for occultations are given, you will see a column with the heading P.A. (or POSITION ANGLE), and this gives you some idea as to whereabouts on the Moon’s limb the reappearance of the star is due to take place. The position angle for a star’s disappearance is also given.

Position angle is measured in degrees from the Moon’s NORTH POINT, the point on the moon’s limb nearest the North Celestial Pole in the sky, not the moon’s north pole. Position angle is measured in an anticlockwise direction from the North Point through 360 degrees.

Not only are occultations beautiful sights to watch and describe in your observing book, but it is possible to do some very useful scientific recording. This is by the careful timing of the exact moment of a star’s disappearance or reappearance. In order to do this you will need to have an accurate stopwatch, access to a reliable time source and knowledge of your exact geographical location. The Occultation Section can help you with all of these.

The unique information you are able to provide, from your observing site, when submitted to the Occultation Timing Agencies, can be very useful indeed. It is from collecting and analysing these times that our knowledge of the Moon’s movement, and any variations that may occur, can be discovered and monitored. This can lead to more accurate tables being produced for eclipses and the times of the tides and it enables us to keep a careful check on the gravitational constant (G) which appears in equations that describe how astronomical bodies revolve around each other. Occultations have also led to the discovery of double stars which are too close to be separated by optical means.

There is a special type of lunar occultation called a GRAZING OCCULTATION. Here, the star grazes the northern or southern regions of the Moon’s limb disappearing and reappearing in quick succession as it is alternately hidden by mountains and shines again over lunar valleys on the limb. This type of occultation is obviously very useful in determining the contours of the lunar limb with greater accuracy.