Lunar Iridium Flare Observed!!!

Ron Lee (
Fri, 09 Jan 1998 18:33:50 -0700

You basically have to have a SUPERflare-
>type geometry, weather that cooperates, a sun that is well below the
>horizon (and not illuminating the satellite), and a gibbous moon with a
>line of sight to the satellite (but preferably in a different part of the
>The moon will only have the proper phase about a third of the time, so
>the lunar flare occurrences are going to be rare.
>Somehow, everything converged at once.  And you even got a low-range
>case to boot. 

The stuff is Rob Matson's comment about seeing an Iridium flare illuminated
solely by moonlight.   Rob and I have been trying for about two months to 
predict and observe such an event.  This morning everything clicked into
place and a lunar Iridium flare WAS positively observed.

Most of you are familiar with the great Iridium flare programs developed
by Randy John and Rob Matson.   At some point in the early flare process,
someone raised the issue of observing lunar Iridium flares.

Rob did the work to define the conditions.  Two initial attempts were
unsuccessful...likeley due to faintness.   This morning ay 1021 UT, Rob 
predicted a 6.8 magnitude lunar flare at an elevation of about 77 degrees.
the range was just under 800 km.

I set my 8" about 30 minutes prior to the expected passage. Fortunately,
the max flare point was almost due north of Algieba (Leo) and the stars
were easy to identify.  As an aside, his new Hipparcos star data would 
have been sufficient to position the telescope.  However, I still used
Guide 6.0 to print out a star chart to ensure proper positioning.

It was perhaps a little west of the expected track but well within
expected errors...maybe a max of two arcminutes.   My recollection
is that it popped into view as opposed to being visible during the
entire passage through the eyepiece.  It was visible as it passed
the southern field of view....possibly brighter than it was in the
center of the field of view.  I had centered the telescope on the
expected maximum point.   This may reflect the individual pointing
errors of the MMAs and/or observational discrepancies.

Estimated magnitude was 7.5 (plus/minus 0.5).  I did not time the 
passage but a glance at my watch shortly after showed it being on 
time (several second ambiguity).    I was more concerned with trying
to see a faint object than timing it.

Seeing a satellite with lunar illumination only is certainly a first
for me.   It was a challenge mostly due to the need to wait for ideal
conditions.   I was very excited to see Rob's prediction come true,
although it should be no surprise considering the accuracy of his other

These events will be much rarer (for an 8" telescope) than the visual,
sunlite Iridium flares...however, it will still be a challenge to observe
a few more to try to get the magnitude estimation better defined.  This one
was essentially right on considering the uncertainties of my magnitude

Ron Lee