Re: ISS eclipsed ?

From: Bruno Tilgner (
Date: Mon Jun 11 2001 - 07:17:58 PDT

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    As regards short eclipses of ISS, my understanding is
    that they can only occur just before and after the time
    when ISS is continually sunlit. This was the case between
    23:33 UTC on 31.05. (i.e. 31 May) and 21:22 UTC on 04.06.
    (i.e. 4 June).
    Just before this period, there was an eclipse of 2m31s at
    23:20:25 UTC on 31 May, and the first eclipse after the
    continuous illumination occurred at 21:21:32 on 4 June
    for a duration of 2m 25. Since then, the eclipses have
    become longer and longer.
    The observation of eclipses a few minutes long requires
    that the observer is located where both events, entry into
    and exit from the eclipse are visible. Considering that the
    visible fraction of a pass seldom exceeds 6 minutes or so
    (depending on local horizon circumstances), one has to be
    located at a place where the satellite passes close to the
    zenith. In addition, the longitude has to be such that both
    events occur whilst the satellite is above the horizon.
    This limits considerably the locations from which short
    eclipses in the order of a few minutes can be observed.
    My definition of "eclipse" is somewhat simplistic: if the
    line satellite-Sun intersects the Earth, the satellite is
    in darkness, otherwise it is sunlit. The Earth is assumed
    to be a perfect sphere, and the Sun is a pointlike source.
    This approach does not take into account the partial phase
    of the "eclipse". I am working on a more sophisticated
    approach, but the mathematics are quite complex.
    BTW: the time when a satellite is in continuous sunlight
    occurs when the right ascension of its ascending node (RAAN)
    minus the RA of the Sun are close to 90 degrees (for the
    northern hemisphere) AND when the declination of the Sun
    exceeds a certain value which is a function of the satellite's
    inclination and its orbital height. The phenomenon of short
    eclipses should occur not only for ISS but for many other
    satellites as well.
    Bruno Tilgner
    Saint-Cloud, France
    48.85N 2.02E UTC+2
    ISS etait illuminee sans interruption entre le 31.05 23:33 UT et le
    04.06 21:22 UT. Juste avant et apres cette periode les eclipses
    etaient tres courtes. La derniere avait lieu le 31.05 a 23:20:25
    pour une duree de 2m31s, et la premiere le 04.06 a 21:21:32 UT
    pour une duree de 2m25s. Depuis, les eclipses sont devenues plus
    longues, par exemple 7m06s le 05.06.
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