Astrolabe

by C Mitchell

The astrolabe was an ancient astronomical "computer" that was used by navigators, astrologers, and astronomers to solve problems regarding time and the position of the Sun, stars, and other astronomical bodies. It was a sort of a combination between a planisphere, a star chart made up of rotating disks that is used to learn the constellations at different times and dates, and a dioptra, an astronomical and surveying instrument from around the third century BCE. There are several types of astrolabes, but the planispheric astrolabe is the most widely known. On a planispheric astrolabe, the celestial sphere (see Parts/Construction) is projected onto the plane of the equator. There is also the mariner's astrolabe, which was a little different. Developed because the larger conventional astrolabe was difficult to use on ships, the mariner's astrolabe was smaller and was used to measure vertical angles.

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Mariner's Astrolabes

Use
The astrolabe was mostly used for finding the position of astronomical bodies. Other uses include finding the time at local latitudes, surveying, and triangulation.

Parts/Construction

Ancient astrolabes were usually about 6 inches (or 15 cm) in diameter and were made out of brass, but some were a lot bigger or smaller. They consisted of a hollowed out circular interior called the mater that held flat plates called tympans, also called climates. Each tympan was engraved with a stereographic projection of part of the celestial sphere, an imaginary rotating sphere that shares an axis with the earth and encompasses the sky and space around earth. Like a planisphere, (one of the successors of the astrolabe) it had a rotating disk-like framework located above the mater, called a rete, which represented the sky and had planetary bodies such as stars and planets on it. One full rotation represented one day. The edge of the mater usually had the degrees of an arc and/or hours of time on it.

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History
The astrolabe is most often thought to have been invented by Hipparchus, a Greek mathematician, astronomer, and geographer from the Hellenistic period. No one knows the exact dates of his birth and death, but his birth is estimated at c.a 190 BC and Ptolemy attributes astronomical observations to him from 147 BC to 127 BC.

The first brass astrolabe was developed by Muslims in the Medieval Islamic world. The first one is usually attributed to an eighth century Persian mathematician named Muhammad al-Fazari. The Muslims used them to help with navigation and to find the qibla so that they could pray in the direction of Mecca.

Abū Ishāq Ibrāhīm al-Zarqālī is believed to have made the first astrolabe that could be used anywhere in the world, unlike earlier versions. It was not introduced into the Christian world until about the 11th century, and eventually became known to them as a "Saphaea". The spherical astrolabe was also invented in the Middle Ages, by Islamic astronomers.

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A Spherical Astrolabe

It wasn't until the fifteenth century that Abraham Zacuto, a Rabbi from Lisbon developed the metal spherical astrolabe, which was greatly improved in accuracy compared to earlier wooden models.

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