Astronomy Picture of the Day

Explanation: Are those green clouds or aurora? Photographed above two weeks ago, puffy green aurora help the Moon illuminate the serene Willow Lake and the snowy Wrangell and Saint Elias Mountains in eastern Alaska, USA. 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 likely occur a few days after a powerful magnetic event has been seen on the Sun.