DAN SCHECHTER / SCIENCE PHOTO LIBRARY DAN SCHECHTER / SCIENCE PHOTO LIBRARY
Solar tower. Mount Wilson Observatory, California, USA. The tower, built in 1912, is still used today to observe the Sun. Mirrors at the top of the tower reflect sunlight down the central shaft to create a 42- centimetre-diameter image in the observation room at ground level. The sunlight is analysed using a spectrograph that extends 75 feet below the tower. The tower is named for its 150-foot focal length, not its height of 160 feet. The tower's height is designed to protect its view from distortion by ground heat. Mount Wilson Observatory, established in 1904, is at an elevation of 1742 metres in the San Gabriel Mountains.
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