More about Lunar Occultations

Detailed predictions for 18 standard stations in North America (Halifax; central Massachusetts; Montreal; Washington, DC; Toronto; Miami; Atlanta; Chicago; Kansas City; Winnipeg; Austin; Denver; New Mexico & Arizona; Edmonton; Los Angeles; northern California; Vancouver; and Honolulu) are given in the Observer’s Handbook of the Royal Astronomical Society of Canada. About 15 occultations of stars as faint as 5th magnitude are listed for each location. The handbook also has graze maps for nearly 200 occultations of stars to magnitude 7.5 for the U.S., Canada, and northern Mexico.

The International Lunar Occultation Centre in Japan prepared the predictions for the Observer's Handbook. It collects and analyzes timings; you can obtain the necessary report forms and instructions at IOTA's Web site. Observers from Europe and the United Kingdom are encouraged to join IOTA/ES.

Occultations by Planets and Asteroids

Accurate timings of planetary and asteroidal occultations can reveal fundamental new information about the size, shape, and atmosphere (if any) of the occulting body, so such timings are especially sought by IOTA. The more timings we receive from widely spaced observers, the better we can reconstruct the object's profile.

Asteroids are so distant that the locations of their occultation tracks are uncertain by hundreds of miles. So even if you're far from the predicted path, you should watch for an event. You might even record a brief occultation by a small moon circling an asteroid, an arrangement we've known can exist ever since the Galileo spacecraft discovered Dactyl orbiting Ida. More accurate predictions often become available days or hours before an occultation, when the asteroid and target star can be imaged on the same CCD frame. These updated predictions can be found on IOTA's Web site.

IOTA maintains a list of occultation observers so that we can inform them by e-mail or telephone of last-minute path shifts onto their sites. Please contact David Dunham by e-mail if you would like to be included. Those with CCD equipment and astrometric reduction software are especially needed to update predictions.

Finder charts for particularly good asteroid occultations usually appear in the Calendar Notes section of Sky & Telescope. When setting up to watch for an asteroid occultation, allow at least a half hour to locate the target star. Even better, practice finding it on a previous night. Many observers who thought they'd left enough time have ended up in a mad rush and missed good events by just a few minutes.

Some events involving 8th-magnitude or brighter stars can be videorecorded with telescopes as small as 8 inches using an ordinary low-light surveillance camera, available for less than $200. Tenth-magnitude stars might be reached this way with 12-inch scopes. If shortwave time signals are recorded on the audio track, we can recover event times accurate to 0.03 second. IOTA strongly encourages such recordings.