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    Re: Eclipse of Io
    From: Ken Gebhart
    Date: 2002 Oct 8, 19:56 -0500

    Your story made me wonder about the following:
    Doesn't celestial mechanics compute the actual position of the various
    bodies in the solar system instead of just their appearance to us on
    Earth?  If so, the time for light to reach us would give us a different
    visual witness of an event.  Could this account for the disparity in time
    that you experienced?
    Ken Gebhart
    Herbert Prinz wrote:
    > 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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