Q.

Why is the direction of an Iridium flare sometimes opposite to the direction to the flare centre? Which way should I look?

A.

The direction of an Iridium flare (i.e. its azimuth and elevation angles) is the direction to look in the sky to see the flare. This has nothing to do with the direction to the flare centre, which is the direction to the point on the earth's surface where the flare would appear at its brightest. So if, for example, a flare is scheduled to appear with an azimuth of 92° (i.e. in the east), and the flare centre is 20 km to the west, this means you would have to travel west for 20km to see the flare at its brightest, but then still look east in the sky to see it. Another way to think about this would be the flare beam coming from the satellite in the east, passing over your head and striking the ground 20km to the west of you.

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