Re: How inaccurate are predictions made months in advance?

JMason001 (JMason001@aol.com)
Sun, 26 Apr 1998 09:49:00 EDT

Jay,

I'm an amateur observer in a suburb of St. Louis who's been watching Mir for
3-4 years now.  I have been reading SeeSat for a couple of months, but this is
my first posting.

My experience is that you can tell a couple months in advance which nights Mir
will be visible.  I have used STSPLUS to determine when Mir will continuously
be in sunlight (this happens twice a year, near the solstice), and found it to
be accurate. 

The main factor here is the rate at which Mir precesses.  Mir's orbit
precesses approximately 5 degrees westward per day.  When you add in the
difference between the sidereal and solar day, which is equivalent to about
another degree, this means that the earth's rotation carries us under Mir's
orbital plane approximately 24 minutes earlier each day.  Rate of precession
depends mostly on inclination and altitude.  Inclination is basically
constant, and Mir's altitude variation is small enough that it doesn't affect
precession rate much, either.

When we pass under the orbital plane shortly before sunrise or shortly after
sunset, Mir will be making visible passes.  We don't see it exactly when we
pass under the plane, of course, because Mir is probably somewhere else in its
orbit.  But, within plus or minus 50 minutes or so of the plane crossing, it
should make a visible pass.

At my latitude (39 N), Mir is generally visible for 4-5 nights in a row.  I
would project forward and find a series of nights that it's supposed to be
visible and pick the middle one for your star party.

Jim Mason
JMason001@aol.com