DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY DR JEREMY BURGESS / SCIENCE PHOTO LIBRARY
An astronomer with his astrolabe, an illustration from the Nuernberg Chronicles of the World, published in 1493. The astrolabe was a primitive type of instrument used, in the form shown here, to measure the altitudes of stars (their angular heights above the horizon). The altitude of a standard star could be used to determine the time of night.
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