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Astrolabe

A 16th century astrolabe.
A 16th century astrolabe.

The astrolabe was invented by Hypatia of Alexandria and was the chief navigational instrument until the 16th century (when the sextant was invented). Some historians credit the invention of the astrolabe to Hipparchus (2nd century BC) while others credit the Persian Fazari (Richard Nelson Frye: Golden Age of Persia. p163). The 15th Century metal highly precise Astrolabe was developed by Abraham Zacuto in Lisbon, from its quite different and imprecise wood Arab-used precursor.

An astrolabe consists of a circle marked in degrees (similar to a protractor) with a rotating arm attached at its center. When the 0 mark on the circle is aligned with the horizon, and a star (or other celestial body) "sighted" at the end of the movable arm, the position (in degrees) of the star can be read ("taken") off the calibrated circle (hence, "astro" = star + "labe" = to take).

On the plate (mater) are engraved coordinate lines which represent a stereographic projection of the celestial sphere (climate), valid for places at a specific geographic latitude. Over this coordinate grid rotates the rete, which is a framework with spikes whose points represent fixed stars. After adjusting the instrument for the current time, the position of a star can be read from the coordinate grid. Conversely, the instrument can be adjusted to fit the measured position, and the time can be read off the scale.

An 18th century  astrolabe.
An 18th century Persian astrolabe.

The astrolabe therefore is a predecessor of the modern planisphere.

Stereographical projection was first described by Ptolemy (2nd century). The astrolabe reached the Islamic world in the 8th or 9th century, and it was re-introduced to Europe via Islamic Spain in the 11th century (early Christian recipients of Arab astronomy included Gerbert of Aurillac and Hermannus Contractus). The mathematical background was established by Al-Battani in his treatise Kitab az-Zij (ca. 920), which was translated into Latin by Plato Tiburtinus (De Motu Stellarum).

The English author Geoffrey Chaucer (ca. 1343 - 1400) compiled a treatise on the astrolabe for his son, mainly based on Messahalla. The same source was translated by the French astronomer and astrologer Pelerin de Prusse and others. The first printed book on the astrolabe was Composition and Use of Astrolabe by Cristannus de Prachaticz, also using Messahalla, but relatively original.

In the 15th century, the French instrument-maker Jean Fusoris (ca. 1365 - 1436) started selling astrolabes in his shop in Paris, along with portable sundials and other popular scientific gadgets of the day.

See also

References

  • Critical edition of Pelerin de Prusse on the Astrolabe (translation of Practique de Astralabe). Edtiors Edgar Laird, Robert Fischer. Binghamton, New York, 1995, in Medieval & Renaissance Texts & Studies. ISBN 0866981322

 


 

 

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