Type of early scientific instrument used for reckoning time and for observational purposes. Astrolabes can be traced to the 6th century AD; they came into wide use in Europe and the Islamic world in the early Middle Ages and were adopted by mariners by the mid-15th century. One widely used variety, the planispheric astrolabe, can be regarded as a rudimentary analog computer. It enabled astronomers to calculate the positions of the Sun and prominent stars with respect to both the horizon and the meridian.
Astrolabe, 11th century.—Oxford Science Archive/Heritage-Images
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