Astronomy and Space

Observation and Measurement

What is an astrolabe?

Invented by the Greeks or Alexandrians around 100B.C.E., an astrolabe is a two-dimensional working model of the heavens, with sights for observations. It consists of two concentric, flat disks, one fixed, representing the observer on Earth, the other moving, which can be rotated to represent the appearance of the celestial sphere at a given moment. Given latitude, date, and time, the observer can read off the altitude and azimuth of the sun, the brightest stars, and the planets. By measuring the altitude of a particular body, one can find the time. The astrolabe can also be used to find times of sunrise, sunset, twilight, or the height of a tower or depth of a well. After 1600, it was replaced by the sextant and other more accurate instruments.


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