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Most artificial satellites have curved parts that reflect sunlight back to Earth, from where they appear as small points of light that move across the sky in a few minutes in a straight line. Because most satellites rotate around their axis in a few seconds and aren't perfect spheres, the brightness of satellites often goes up and down in a regular period of a few seconds. If a satellite enters the shadow of the Earth, then it suddenly no longer reflects sunlight to Earth, so it seems to disappear, as seen from Earth. If a satellite leaves the shadow of the Earth, then it becomes visible again just as suddenly. If the satellite has an orbit around the Earth that is sufficiently high, then it can be seen even many hours after sunset or before sunrise.