Astronomy Picture of the Day Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. 2002 October 29 A Lunar Rille Credit: Apollo 10, NASA Explanation: What could cause a long indentation on the Moon? First discovered over 200 years ago with a small telescope, rilles (rhymes with pills) appear all over the Moon. Three types of rilles are now recognized: sinuous rilles, which have many meandering curves, arcuate rilles which form sweeping arcs, and straight rilles, like Ariadaeus Rille pictured above. Long...