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Subject: Eclipse of Io
From: Herbert Prinz (hprinz@XXX.XXX)
Date: Sun Oct 06 2002 - 12:04:33 EDT
Hi Mitch, and Everybody,
A few hours ago, those of us who were in the right place could watch a
nice eclipse of Io, the first moon of Jupiter. The eclipse was visible
in North America east of ca. 120 W, in South America west of ca. 60 W,
and from the North Pole(!). The atmosphere was beutifully clear and the
Moon (our Moon) helped greatly by staying totally out of the way.
I observed with a 12" Newtonian reflector, 1m focal length and a 7mm
occular.
At 09:46:05 UT I first became aware of the fading of Io. Also in the
field of view happened to be an unnamed star of mag.8, against which I
could compare Io. (Io is mag. 4.8 ) Of course, you can always compare
the eclipsing moon against the other ones, but they are all about the
sime brightness, so it is much more obvious when you can watch how Io
starts ought significantly brighter and ends up fainter than a nearby
star.
At 09:47:46 Io had about the same app. magnitude as the comparison star.
At 09:48:37 things got exciting when Io had become a tiny point that was
about to disappear any second. But it still took another 50 seconds to
09:49:25 until Io was finally gone. This is the amazing thing, that it
takes so long for that last dot to disappear. Now I am used to it, but
in the beginning I would get really tense, trying to look so long fully
concentrated, afraid to blink.
According to the IMC ephemeris, entrance into penumbra was at 09:45:47
UT, exterior contact with umbra at 09:46:30 and interior contact at
09:50:05.
One really can't expect to notice much during the penumbral phase of the
event. I am not sure how meaningfull my first observation of fadingis.
Of the time it takes from ext. to int. contact, a third had passed when
I noticed the decrease of app. magn of Io from 4.8 to 8. Finally, I was
40 seconds early with my observation of disappearance. (should be
interior contact, but in practice never is). This fits well with my
previous experience. Sometimes I am a minute early. I reckon 40s is my
telescope correction in very good weather and a minute in not so good
weather.I am also that much late on reappearances, sometimes even more
as they are harder to observe.
Herbert
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