An Occultation Primer

Moon occults Spica
Spica (upper left) grazes the Moon's north polar region. 'We observed the occultation of Spica early in the morning of November 30, 1994, from Central Tokyo,' writes photographer Motomaru Shirao. 'The appearance of Spica was very beautiful, because the Moon was thin and the earthshine was obvious.' Spica's icy blue-white color contrasted with the Moon's yellowish white.
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 get full 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.

Moon occults Aldebaran
When the waning gibbous Moon occulted 1st-magnitude Aldebaran on October 19, 1997, Sky & Telescope's Rick Fienberg made pairs of short-exposure images that show the Moon well (but don't show the star) and long-exposure images that show the star (but overexpose the Moon). He then combined them digitally to make a time-lapse movie (90K GIF) showing the Moon covering and uncovering Aldebaran over about two hours. Fienberg used a 500-mm f/5.6 telephoto lens piggybacked on an equatorially mounted telescope with a motorized drive. Click on the image to see the sequence.
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 and asteroids can also cover stars. These planetary occultations are 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.

In every January issue, Sky & Telescope publishes a preview of upcoming lunar occultations for the year. Predictions and finder charts for the best planetary occultations are published in the magazine from time to time year-round.

The International Occultation Timing Association (IOTA) Web site also maintain lists of upcoming lunar and asteroidal occultation events. As the appointed time of an occultation approaches, be sure to check for last-minute updates.

Sky & Telescope editors are always eager to receive copies of observing reports and images of occultations. You can send them by e-mail to observers@SkyandTelescope.com.