Satellite flare (also known as satellite glint) is the phenomenon caused by the reflective surfaces on satellites (such as antennas or solar panels) reflecting sunlight directly onto the Earth below and appearing as a brief, bright "flare".
The Iridium communication satellites have a peculiar shape with three polished door-sized antennas, 120° apart and at 40° angles with the main bus. The forward antenna faces the direction the satellite is travelling. Occasionally, an antenna reflects sunlight directly down at Earth, creating a predictable and quickly moving illuminated spot on the surface below of about 10 km diameter. To an observer this looks like a bright flash, or 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 in the daytime; but they are most impressive at night. This flashing has caused some annoyance to astronomers, as the flares occasionally disturb observations and can damage sensitive equipment.
When not flaring, the satellites are often visible crossing the night sky at a typical magnitude of 6, similar to a dim star.
Viewing satellite flares
While satellites may be seen by chance, there are websites which provide location specific information as to when and where in the sky a satellite flare may be seen.