Astrolabe: A Beautiful, Mysterious And Sophisticated Computer Of The Ancient World

A. Sutherland - - The astrolabe is a beautiful ancient object. It is fair to say it is an instrument with a past and future. It is also a sophisticated ancient astronomical computer especially created for solving problems related to time and the position of the Sun and stars in the sky.

Astrolabe: A Beautiful, Mysterious And Sophisticated Computer Of The Ancient World

Credit: Adobe Stock - markrhiggins

In most cases, the astrolabes were made of brass and had a 6-inch diameter. Some of the objects were much larger.

The oldest astrolabes date back to two thousand years ago. Among the most popular types is the planispheric astrolabe, on which the celestial sphere is projected onto the plane of the equator. Despite being created at a time when people thought that the Earth was the center of the universe, the astrolabe is a useful ancient tool.

The object operates on the same basis as any other modern computer. You provide it with data, that is, input information, and then you receive output.

Ancient people used astrolabes to find out how the sky looked at a specific place at a given time. It was done by drawing the sky on the face of the astrolabe and marking it, so positions in the sky were easy to find.

The 18th century Persian astrolabe

An 18th Century Persian astrolabe–maker unknown. The points of the curved spikes on the front rete plate, mark the positions of the brightest stars. The name of each star being labeled at the base of each spike. The backplate or mater is engraved with projected coordinate lines. From the Whipple Museum of the History of Science in Cambridge. Image credit: Andrew Dunn - CC BY-SA 2.0

To use an astrolabe, you adjust the moveable components to a specific date and time. Once set, much of the sky, both visible and invisible, is represented on the face of the instrument.

This clever ancient computer was mainly used to find the time during the day or night, the time of a celestial event such as sunrise or sunset, and as a reference for stellar positions.

During the late Middle Ages, it served as a primary education astronomy tool and was sometimes even used for astrological purposes.

The typical astrolabe was not a navigational instrument. However, the mariner's astrolabe was widely used in the Renaissance. The device can measure the altitude above the horizon of a celestial body, day or night, identify stars and planets, determine local latitude on land or calm seas, and survey.

Astrolabe: A Beautiful, Mysterious And Sophisticated Computer Of The Ancient World

Astrolabe made between 1081 and 1082. Credit: History of Science Museum - CC BY-SA 4.0

The astrolabe was important in classical antiquity, the Islamic Golden Age, and the European Middle Ages. The name 'astrolabium' refers to the so-called prismatic astrolabe, invented by the French astronomer André Danjon.

The early astrolabe was invented in the Hellenistic civilization by Apollonius of Perga between 220 and 150 BC, often attributed to Hipparchus, a Greek mathematician, geographer, and astronomer who lived in the 2nd century BC. The essential part of the astrolabe is a brass plate with the lines of the celestial body's height above the horizon. It also has azimuth lines, and the hour wheels were engraved. Through the center of the plate was a line marking the north pole of the sky.

Hypatia (c. 350–370; died 415 AD),  a great ancient scholar, built many astrolabes, but she did not invent them. The astrolabe was already in use at least 500 years before Hypatia was born.

Today, the astrolabe is still admired for its beauty and appreciated for its unique capabilities and its value for astronomy education.

Written by – A. Sutherland  - Senior Staff Writer

Updated on November 18, 2022

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History of Science Museum

James E. Morrison, The Astrolabe