Occultations occur when one celestial body passes in front of another. These frequent and intriguing events are fun to watch, and provide an important way for amateur astronomers to make significant discoveries about objects within our own Solar System as well as the stars beyond.

Just as the Moon passes in front of background stars, so too do planets and minor planets (also called asteroids). However planetary occultations occur less frequently than lunar occultations because the planets appear so much smaller in our sky than does the Moon. Nevertheless, observing occultations of stars by planets has yielded some stunning discoveries - for example, the rings of Uranus, and the atmosphere around Pluto.

While occultations of bright stars by major planets are very rare, occultations by asteroids are a little less so. This is not because any one asteroid has a greater chance of passing in front of a star. Rather, it is because there are so many more asteroids to choose from!

Typically, the occultation shadows of perhaps 100 asteroids will sweep across parts of Australia and New Zealand annually. Each shadow is usually about the same width as the minor planet (typically 100-200 km), although the shadow itself does sweep out a strip many thousands of kilometres long across the Earth. Thus the same asteroid occultation can potentially be observed in New Zealand, and on both the east and west coasts of Australia.

The value in observing minor planet occultations is rather similar to that in observing grazing occultations. If the occultation shadow band is perhaps a few tens to a few hundred kilometres wide, then observers situated within the band and perpendicular to the direction of motion of the shadow will each see the star occulted by a different part of the asteroid. If enough observations are obtained, one can essentially "join the dots" to build up a picture of the shape of the asteroid. Even if only two observers see an event, so long as they are separated by a reasonable distance an average diameter for the asteroid can be deduced - the only direct way in which this information can be obtained. Determining the diameter of a minor planet is important because it can provide clues to the asteroid's density, which in turn tells us something about its bulk composition and thus its origin.

- Definitions are from The Wellington Occultation Section pages

For the latest in occultation information and updates, visit this Impending Occultations Page

You can also view further occultation information at: The Wellington Occultation section