From: roelle@uars_mag.jhuapl.edu (Curtis Roelle)
Subject: Re: has anybody seen ...
Organization: Johns Hopkins University
Date: 22 Dec 92 16:05:24 GMT

ALPAR@TRMETU.BITNET writes:

>  Hi friends
> During last total lunar eclipse on 12/09/1992 I observed a star very close to
>moon . The temperature was about -10 degrees centigrades so on my telescopes ob
>jective there was a thin ice layer so I couldn't decide if it was a real star o
>r an internal reflection . Neither on my photos nor on video cassete I couldn't
> see any star close to moon . The star was about 50 arc seconds west from the m
>oon maybe that was why I couldn't define the star on computer simulations .
>  Has anybody seen that star ? Can anybody tell me which star it was (HD number
> or something like that) ?

>                                                 Alper Ates

Nearly every night the moon passes in front of, and occults stars --
nothing unusual or rare about it.  One particular type of lunar
occultation called a "graze" can occur when the star appears to pass
tangentially along the northern or southern limb, or edge, of the
moon.  The star will be seen to appear and disappear as it is blocked
by lunar mountains and seen through lunar valleys.

By having timings from individual observers positioned on various
chords along the path, coupled with the predicted libration at that
time, it has been possible to deduce contours that assist in mapping
the moon's polar regions.  It seems that the poles are a region of the
moon that has been for the most part ignored by lunar missions (manned
and unmanned) whose tendancy was to map and photograph the equatorial
zone.  The last lunar graze I observed was just this past Friday
morning.  The graze paths are narrow, usually on the order of 1-2
miles.  The path was only about 6 miles from home (and unfortunately
intermittent clouds were a problem).

Another type of lunar occultation is "total", where the leading edge
of the moon covers a star.  Total lunar occultations can be thought of
as a total eclipse of a distant sun.  Total occultations are usually
observed at the star's disappearance or at reappearance.
Disappearances are easier to time than reappearances, because during
the reappearance, the star is not visible until it has already
reappeared, making it difficult for the eye to make a rapid response.
It is also difficult to know just where along the limb to expect the
reappearance to occur.

Any of these events, grazes, disappearances, or reappearances are
easier to view against a dark limb than a bright limb, because a
bright limb tends to overpower the light of the star.  However, events
may also be hard to view against a dark limb since by being dark it is
hard to determine exactly where the limb is located.  Often
"Earthshine", or illumination of the unlit portion of the moon by
sunlight reflected off the earth, is helpful in that regard.

For this reason a total lunar eclipse provides a rare opportunity to
observe and time occultations against a lunar limb that is neither
bright nor totally dark.  Also, note that for any given total
occultation usually one event (disappearance or reappearance) occurs
on a dark limb while the other event occurs on the bright limb.
However, during a total lunar eclipse, both events can be observed on
a limb darkened by Earth's shadow.

I had received some IOTA predictions for stars located along the
moon's path during the eclipse. However, I have since discarded that
material and therefore cannot directly answer your question.

You may perhaps be able to obtain your answer by contacting the
International Occultation Timing Association (IOTA).  As far as an
e-mail address you might want to contact David Dunham, at
DunhamDW@space2.spacenet.jhuapl.edu.  He prepares and distributes
occultation predictions and produces a yearly column in the January
issue of S&T magazine.  Because of lunar parallax, in order to
determine which star you were looking at it would be necessary to know
the time of your observation and your geographic location.  Also, the
approximate position angle (or cusp angle) of the star in question
would be helpful as well.

Curt Roelle
roelle@sigi.jhuapl.edu  






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