High above clouds over the Atlantic Ocean, a night begins for astronomers at the Roque de los Muchachos Observatory on the top of La Palma, 2400 meters above see level. Minutes after the sunset crepuscular rays appear stunningly in the western horizon. They are the rays of sunlight that appear to radiate from a single point in the sky. These rays, which stream through gaps in clouds are diverging columns of sunlit air separated by darker regions which are shadowed by clouds. There are about a dozen major telescopes on this excellent observing site including the the 10.4 m GTC (Gran Telescopio Canarias) on the far left, the the 4.2m William Herschel Telescope (closer at left), the Liverpool Robotic Telescope and Mercator telescope (middle right), and the 2.5m Isaac Newton Telescope or INT (right end). INT was the first telescope on the mountain in early 1980s and was once the telescope of Royal Greenwich observatory before moving to La Palma.