Re: Bright Events

Bjoern Gimle (
Wed, 10 Jun 1998 07:53:12 +0200

>One question! If I get one mag -8 Iridium flare per month,
>am I lucky, or is that about normal?
>Koos van Zyl
I think you are about right. The operational satellites are spread out
evenly, 11 per orbit plane (6 of them, 30 degrees apart)
With orbital period of 100.4 minutes, these 11 cover an arc of longitude
of 23.9 degrees. There is usually a 12th (spare) satellite in the plane.
Except for the gap between the planes, with northbound and southbound
passes you can say there are 144 passes evenly spaced over the day.
Usually less than half of them are not sunlit. Though there are some
geometries where no reflection hits the ground, there are also some that
send two to the surface. If you have 90 passes, they are four degrees
apart, on the average. This would be 445 km on the equator. Multiply
by cos(latitude) When you are within about 10 km, the flare will be
near max., so this ratio indicates the average frequency.

You can easily check by running IridFlar for a day or more, with verbose
output, a max flare angle of 14-15 degrees, and daylight/night mag limits
of +4 or +5. In 'Peak mag' and 'Maximum flare lat/long' columns you will
see where they hit the ground, and if they are bright enough there.

If there is a geometry where flares occur in your (wide) area, they are
spaced by about 250 km * cos(Lat), so the chances are better than average,
and you can affect it by travelling the indicated W-E distance.

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