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The Iridium communication satellites have a peculiar shape with three polished door-sized antennas, 120 degrees apart and at 40 degree angles with the main bus. The forward antenna faces the direction in which the satellite is travelling. Occasionally an antenna will reflect sunlight directly down to the Earth, creating a predictable and quickly moving illuminated spot of about 10 km diameter. To an observer this looks like an extremely bright flare in the sky with a duration of a few seconds. Ranging up to -8 magnitude (rarely to a brilliant -9.5), some of the flares are so bright that they can be seen at daytime; but they are most impressive at night. This flashing has been some annoyance to astronomers, as the flares occasionally disturb observations and can damage sensitive equipment.
Daily predictions for all satellites brighter than magnitude:
(brightest) 3.5 | 4.0 | 4.5 (dimmest)