By reflecting sunlight toward the Earth, an Iridium communication satellite flares in the sky above the historic Alamut Fortress. Found in the central Alborz Mountains of Iran, Alamut Castle (2160 m) was built into the rock in the 9th century. The name means Eagle's Nest. Home of the legendary Assassins featured in the adventure movie Prince of Persia, Alamut was also historically a center for libraries and education. For a time, it was the residence of important 13th century Persian scholar and astronomer Nasir al-Din al-Tusi (or simply known as Tusi). Satellite flare or 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". With highly reflecting triple antennas, the Iridium communication satellites create the brightest flares in the night sky, some bright enough to illuminate night landscape for a short time. They create predictable and quickly moving illuminated spots of about 10 km diameter. To an observer this looks like a very bright flare in the sky with duration of a few seconds. However this flashing has been also a problem for serious deep sky astronomical study, as the flares occasionally disturb observations. When not flaring, the satellites are often visible crossing the night sky at a typical magnitude of 6, similar to a dim star.