Astronomy Picture of the Day

Explanation: Aurorae can make spectacular sights. Photographed above last weekend, flowing multi-coloured aurorae helped illuminate a busy sky above Tromsø, Norway. Besides the spectacular aurora pictured above, the photographer caught three satellites streaks, one airplane streak, and a friend trying to capture the same sight. Although aurorae might first appear to be moonlit clouds, they only add light to the sky and do not block background stars from view. Called northern lights in the northern hemisphere, aurorae are caused by collisions between charged particles from the magnetosphere and air molecules high in the Earth's atmosphere. If viewed from space, aurorae can be seen to glow in X-ray and ultraviolet light as well. Predictable aurorae might occur a few days after a powerful magnetic event has been seen on the Sun.