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(This article originally appeared in the Q4 1988 STAR news letter. -Ian)
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!
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