Lunar occultations timed to an accuracy of a few tenths of a second have various scientific uses, particularly in refining our knowledge of lunar topography (Sky & Telescope: November 1988, page 480). But once the laser altimeter data from the 1992 Clementine mission to the Moon are properly analyzed, photoelectric and video timings accurate to a few hundredths of a second will be in most cases the only amateur timings still on the cutting edge.

If you time only a few events, download accurate time from the U.S. Naval Observatory's master clock at http://nist.time.gov/timezone.cgi?Eastern/d/-5/java. Alternatively, if you have a shortwave radio, you can use time signals from WWV (which broadcasts at 2.5, 5, 10, 15, and 20 MHz) or CHU (3.330, 7.335, and 14.670 MHz). Timing methods are reviewed in the September 1990 Sky & Telescope, page 288.

You can get an accurate video timing by running a camcorder continuously to record the time signals before and after the occultation (or perhaps during it if you use a portable phone). You can aim the camcorder down a telescope's eyepiece to record the event. This should be easy if the occulted star is quite bright. Even for purely visual events, a camcorder can be used just to record audio — your voice on top of time signals — if you don't have a tape recorder. See "Camcorder Timing Tips" for more information about this technique.

Within a mile or two of the edge of a lunar 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. This is a grazing occultation. Observers spaced across this graze zone will time different sequences of events, which can be analyzed to map the lunar limb profile. Such timings remain important because the Clementine laser altimeter could not map the lunar polar regions. And they are easy to make — just call out the successive disappearances and reappearances to a camcorder or tape recorder while time signals are being recorded simultaneously in the background. The time base can be a regular radio broadcast from a car radio, if someone else records the broadcast on top of WWV or CHU.

For most graze occultations, we receive data from only one or two observers. Many more are needed to properly record the lunar profile as presented to Earth, which is never exactly the same from one day to the next. About two dozen grazes each year are visible in a 6-inch telescope within 100 miles of a given location. Some are observed by organized groups of amateurs spaced a few hundred yards apart in a line; newcomers are especially welcome on these expeditions. Contact the International Occultation Timing Association (IOTA).