Re: Iridium flare seasons (and Tselinas )

Bjoern Gimle (
Thu, 13 Aug 1998 14:54:57 +0200

Walter Nissen wrote:
>Less bright glints will occur in the same part of the sky, actually =
>part of an orbital plane, often numerous times per evening, often every =

>night for weeks.  The central area of the glint tracks for any one =
>will tend to bounce around (I'm not sure why this occurs) from place to =

>place to the West and East.  When one happens to land on you, you see =
>extremely bright glint.  Or at least, you do, if the attitude of the =

I don't quite see what you mean here.
Look at it from space instead, along the track during one minute or so:
During this time, an Iridium moves about 3.6 degrees in latitude, or
about 400 km S-N or N-S. Its attitude rotates 3.6 degrees to follow
the horizon shift. The orbit precesses only 0.02 deg/h, and Earth moves
0.04 deg/h relative to the Sun, so the rest of the geometry is quite =
So, if the flare isn't almost grazing the Earth, it will follow a =
similar N-S
track, and so will all other stable Iridiums in that plane do, for =
days, compared to the up to 2*3.6 degrees of flare direction change.

But during this minute, Earth rotates beneath these tracks, 20.7 km at
your latitude. This can be more than one degree of flare angle, and make
a lot of difference in brightness.

At the next regular Iridium pass, you will have moved 180 km away from
the track. Since the Iridium orbital period is not synchronized with the
~1434 minutes it takes to get back to the track position, successive
passes appear randomly distributed.

But the seasonal variations are there. Since the Earth tilts +/- 23 =
wrt. Sun, and the orbit planes precess, there are large variations, not
necessarily identical from year to year.=20

Also remember that your perception of reasonable observing hours
change during the year,

>Focusing on repetition, the Tselina-2's and their Zenit-2 rocket bodies =

>are pretty incredible.  I recall one which passed between Alkaid and=20
>Mizar, the stars at the end of the handle of the Big Dipper, on 8=20
>consecutive nights.  These objects are the Cosmos objects which have=20
>rocket bodies of 10.4 m length in the Molczan file, C* 2360 back thru =
These appear purposely have a Mean Motion of about 14.13, to make
the satellite reappear at the time when the Earth is back at the same
position wrt. the precessed orbital plane, so the satellite passes the
same geophysical structures day after day. This may be for their
internal measurement functions, and also means that the 14th harmonic
of the Earth's gravitational field can be accurately deduced.

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