(This article originally appeared in the Q4 1988 STAR news letter.)
Timing Occultations
By Mark Lang
Observing an occultation is both exciting and scientifically valuable. I would like to get more RAC members active in observing occultations, so hopefully this article will show those who haven’t tried one how easy it can be. Weather permitting, observers can time several more occultations this year.
The most common type of occultation is a total lunar occultation. The Moon, in it’s eastward motion across the sky, covers then uncovers stars in its path. For example, on Monday January 22, 1989 the Moon occulted Regulus at approximately 8:45 pm. Several RAC members timed this event on their own.
The second most common type of occultation is a grazing lunar occultation. This is a special class of lunar occultation that happens at the North or South pole of the Moon. During these grazes, the star being occulted is so near the poles that as the Moon passes, unseen mountains on the moon alternately hide then reveal the star. This type of graze happened locally on Friday January 13th (a meeting night!).
The last type of occultation is an asteroid occultation. In this type of occultation, an orbiting asteroid conceals then exposes a star. The next local asteroid occultation happens on March 17 at 11:03 pm when the asteroid Bamberga, at magnitude 11.8, occults for up to 15 seconds a 9.0 magnitude star in the constellation Leo. The track of the occultation, which is the path that the shadow of the asteroid casts on the Earth as the asteroid moves, is predicted to cross directly across North Carolina from southeast to northwest. Unfortunately, the Moon will be up at altitude 41 degrees and it will be 84% sunlit. This, however, should not prohibit viewing the occultation.
Only modest equipment is needed to observe any type of occultation. You need a portable radio that receives the WWV time signals, a tape recorder, and your telescope. The radio can be purchased at Radio Shack for about $40.00 and it also receives local weather broadcasts. The tape recorder is a normal battery operated model that uses either standard or micro cassettes. Always make sure that you have fresh batteries for your radio and tape recorder. In cold weather, try to keep the radio and tape recorder warm as long as possible to prevent battery drain. Almost any size telescope works, but at least a six inch is preferable when observing faint asteroids and stars. An equatorially mounted telescope generally is preferred to an alt-az type of mounting. You can concentrate on watching the star and not on moving the telescope at the critical time to re-center the star.
The occultation coordinator (usually me), organizes an expedition to the site of the occultation. For a lunar graze, we must position ourselves inside a track that is only a few miles wide. I receive predictions from IOTA, the International Occultation Timing Association, which allow me to plot the graze track on large scale maps. I can then position observers at the optimum locations. If I have enough observers, they are positioned at 0.1 mile increments perpendicular to the graze track. Asteroid tracks are usually much wider and positioning is not so critical. For the March 17 occultation, almost any location in the state of North Carolina will do. Observers should be spaced evenly across a line perpendicular to the center line of the track; otherwise timing data will be duplicated.
Once the observing station is established, the participant sets up a telescope. It should be placed so that the moon or asteroid will not be obscured by trees or other obstructions. If possible, have the telescope set up 30 minutes before the predicted start of the occultation. Make sure that the radio is receiving the WWV time signals, otherwise your observations will be useless. Ensure that the tape recorder can record your voice and the time signal in the background. Be as comfortable as possible and try to be relaxed. If you have enough tape, start recording 10 minutes before the occultation is predicted to occur. Watch the star carefully in the telescope. As the Moon or the asteroid approaches, the star disappears. At that instant call into the tape recorder “out” or something that signifies to you that the star has disappeared. Continue to watch carefully for the star to reappear and call “in” when this happens.
On lunar grazes, the star can disappear and reappear many times. Try not to talk while the minute time signal is announced on the radio because this hinders the reduction of the data on the tape. When you are sure that the star has reappeared for the last time, let the tape recorder continue to record until you hear the next minute time signal. Also record on the tape your location, the size and f-ratio of the telescope, the magnification of the eyepiece you used, the approximate temperature, and the sky conditions.
After the occultation, all the observers go to a predetermined location to play back their tapes and discuss the evening’s events. The occultation coordinator collects all the tapes and gets the timings and reports them to IOTA. Anyone who successfully observes a graze or an asteroid occultation will tell you that it’s one of the most exciting astronomical events to observe!
