# Re: ISS eclipsed ?

From: Bruno Tilgner (Bruno_Tilgner@compuserve.com)
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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