Astrolabe

astronomyThe astrolabe is a very ancient astronomical computer for solving problems relating to time and the position of the Sun and stars in the sky.

Evidence suggests that the history of the astrolabe begins more than two thousand years ago.  The principles behind the astrolabe projection were almost certainly known before 150 B.C., but the first known astrolabe is believed to have been developed in Alexandria about 160 B.C. by a Greek man named Hipparchus.  A marriage of the planisphere and dioptra, the astrolabe was effectively an analog calculator capable of working out several different kinds of problems in spherical astronomy. Theon of Alexandria wrote a detailed treatise on the astrolabe, and Lewis (2001) argues that Ptolemy used an astrolabe to make the astronomical observations recorded in the Tetrabiblos.

Since that first model history has recorded many variations of astrolabes, including a traditional design for use on land, the Mariner's Astrolabe and the Universal Astrolabe, just to name a few.  By far the most popular type is the planispheric astrolabe, on which the celestial sphere is projected onto the plane of the equator.

There is often some confusion between the traditional astrolabe and the mariner's astrolabe.  While the traditional astrolabe could be useful for determining latitude on land, it is an awkward instrument for use on the heaving deck of a ship or in wind.  The mariner's astrolabe was developed to address these issues by simplifying the devices functionality and compacting it's design.

Astrolabes are used to show how the sky looks at a specific place at a given time.  This is done by drawing the sky on the face of the astrolabe and marking it so positions in the sky are easy to find. To use an astrolabe, you adjust the moveable components to a specific date and time. Once set, the entire sky, both visible and invisible, is represented on the face of the instrument. This allows a great many astronomical problems to be solved in a very visual way.

The astrolabe quickly became the dominant historical astronomical instrument used by classical astronomers, navigators, and astrologers.  Its many uses included locating and predicting the positions of the Sun, Moon, planets and stars; determining local time given local latitude and vice-versa; surveying; and triangulation.

Paper Astrolabe, by Philippe Danfrie, Paris, 1584Brass astrolabes were developed in the medieval Islamic world, chiefly as an aid to navigation and as a way of finding the qibla, the direction of Mecca. The first person credited with building the astrolabe in the Islamic world is reportedly the 8th century Persian mathematician al-Fazari.

The mathematical background was established by the Arab astronomer al-Battani in his treatise Kitab az-Zij (ca. 920 AD), which was translated into Latin by Plato Tiburtinus (De Motu Stellarum). The earliest surviving astrolabe is dated AH 315 (927/8 AD). In the Islamic world, astrolabes were used to find the times of sunrise and the rising of fixed stars, to help schedule morning prayers (salat). In the 10th century, al-Sufi first described over 1,000 different uses of an astrolabe, in areas as diverse as astronomy, astrology, horoscopes, navigation, surveying, timekeeping, prayer, Salah, Qibla, etc.

Universal AstrolabeArzachel (al-Zarqali) of al-Andalus constructed the first universal astrolabe instrument which, unlike its predecessors, did not depend on the latitude of the observer, and could be used from anywhere on the Earth. This instrument became known in Europe as the "Saphaea". The astrolabe was introduced to other parts of Europe via Al-Andalus in the 11th century. Early Christian recipients of Arab astronomy included Gerbert of Aurillac and Hermannus Contractus. The first geared mechanical astrolabe was later developed by Abi Bakr of Isfahan in 1235.

As testament to the astrolabe's capabilities, mathematicians throughout the ages continued to publish work after work on new applications for the device.  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 subject of the astrolabe was Composition and Use of Astrolabe by Cristannus de Prachaticz, also using Messahalla, but relatively original.  In 1370, the first Indian treatise on the astrolabe was written by the Jain astronomer Mahendra Suri and in the 16th century, Johannes Stöffler published Elucidatio fabricae ususque astrolabii, a manual of the construction and use of the astrolabe.

Fusori AstrolabeThe first known European metal astrolabe was developed in the 15th century by Rabbi Abraham Zacuto in Lisbon.

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

Four identical 16th century astrolabes made by Georg Hartmann provide some of the earliest evidence for batch production by division of labor.

Astrolabes are still appreciated today for their unique capabilities, their elegant beauty, and their value for astronomy education.


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