The night sky contains many wonders, but most of them are static displays. In contrast, occultations are dynamic phenomena that provide a startling demonstration of orbital motion. If you want to getfull value from your telescope, make room in your schedule to watch some of these remarkable events. Even more satisfying is timing them, a simple process that most observers can do.

"To occult" means literally "to hide." A lunar occultation takes place when the Moon's edge creeps up to a star and suddenly snuffs it out. The star reappears just as suddenly on the Moon's opposite side up to an hour or more later. When the Moon is in its waxing phases, the disappearance usually happens on the Moon's dark edge, where it's easy to watch; reappearances are on the bright side, where the star is harder to see. When the Moon is waning, the situation is opposite: stars vanish on the bright limb and reappear from behind the dark edge. In either case reappearances take more planning to watch, because you need to be looking at the correct place on the Moon's limb at the moment the star pops back into view.

A grazing occultation happens when the Moon skims just past a star. Within a mile or two of the edge of an occultation's predicted path, termed its northern or southern limit, you might see the star wink off and on several times as it passes behind hills and valleys near the Moon's poles. Grazes are the most dynamic and interesting lunar occultations.

Planets, asteroids, and comets can also cover stars. These planetary occultations are much rarer than lunar ones due to the tiny apparent sizes and slow motions of the occulting objects. But there are scores of large asteroids, and because they are relatively dim, even faint stars can be seen right next to them. In fact the asteroid itself might not be visible at all in a small telescope; its presence is revealed only when the target star disappears.

For More Information on Occultations:

IOTA's Lunar Occultation Page
IOTA's Asteroid Occultation Page