Astrolabe
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- For the ship of Dumont d'Urville, see L'Astrolabe and for the bay in Papua New Guinea named after this ship, see Astrolabe Bay.
The astrolabe is a historical astronomical instrument used by classical astronomers and astrologers. It was the chief navigational instrument until the invention of the sextant in the 18th century. Its many uses included locating and predicting the positions of the Sun, Moon, planets and stars; determining local time given local longitude and vice-versa; surveying; and triangulation. Astrologers of the European nations used astrolabes to construct horoscopes. In the Islamic world, they are and were used primarily for astronomical studies, though astrology was often involved there as well.
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[edit] History
Most historians credit the invention of the astrolabe to Hipparchus (2nd century BC), and some to Hypatia of Alexandria.
Brass astrolabes were developed in much of the 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 Fazari (Richard Nelson Frye: Golden Age of Persia. p. 163). The mathematical background was established by Al-Battani in his treatise Kitab az-Zij (ca. AD 920), which was translated into Latin by Plato Tiburtinus (De Motu Stellarum). The earliest known example is dated AH 315 (AD 927/8).
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). Al-Zarqall of Andalusia constructed one such instrument in which, unlike its predecessors, did not depend on the latitude of the observer, and could be used anywhere. This instrument became known in Europe as the Saphaea.
The astrolabe 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 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.
The first known European metal astrolabe was developed in the 15th century by Abraham Zacuto in Lisbon. Metal astrolabes improved on the accuracy of their wooden precursors. 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.
In the 16th century, Johannes Stöffler published Elucidatio fabricae ususque astrolabii, a manual of the construction and use of the astrolabe.
[edit] Astrolabes and clocks
The first mechanical astronomical clocks were influenced by the astrolabe; in many ways they could be seen as clockwork astrolabes designed to produce a continual display of the current position of the sun, stars, and planets. For example, Richard of Wallingford's clock (c. 1330) consisted essentially of a star map rotating behind a fixed rete.
Many astronomical clocks, such as the famous clock at Prague, use an astrolabe-style display, adopting a stereographic projection (see below) of the ecliptic plane.
In the late 1990s Swiss watchmaker Dr. Ludwig Oechslin designed and built an astrolabe wristwatch in conjunction with Ulysse Nardin.
[edit] Construction
An astrolabe consists of a hollow disk, called the mater, which is deep enough to hold one or more flat plates called tympans, or climates. A tympan is made for a specific latitude and is engraved with a stereographic projection of lines of equal azimuth and altitude representing the portion of the celestial sphere which is above the local horizon. The rim of the mater is typically graduated into hours of time, or degrees of arc, or both. Above the mater and tympan, the rete, a framework bearing a projection of the ecliptic and several pointers indicating the positions of stars, is free to rotate. Some astrolabes have a narrow rule which rotates over the rete, and may be marked with a scale of declinations.
As the rete is rotated, the stars and the ecliptic move over the projection of the sky coordinates on the tympan. A complete rotation represents the passage of one day. The astrolabe is therefore a predecessor of the modern planisphere.
On the back of the mater there will often be engraved a number of scales which are useful in the astrolabe's various applications; these will vary from designer to designer, but might include curves for time conversions, a calendar for converting the day of the month to the sun's position on the ecliptic, trigonometric scales, and a graduation of 360 degrees around the back edge. Another ruler, called the alidade, is attached to the back face. When the astrolabe is held vertically, the alidade can be rotated and a star sighted along its length, so that the star's altitude in degrees can be read ("taken") from the graduated edge of the astrolabe; hence "astro" = star + "labe" = to take.
[edit] Images
[edit] See also
- Antikythera mechanism
- Armillary sphere
- Astrarium
- Astrology
- Astronomical clock
- Equatorium
- Islamic astronomy
- Orrery
- Planetarium
- Prague Orloj
- Sharafeddin Tusi, the inventor of the linear astrolabe
- Torquetum
[edit] 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 0-86698-132-2
- John North. God's Clockmaker, Richard of Wallingford and the invention of time. Hambledon and London, 2005.
- King, Henry Geared to the Stars: the evolution of planetariums, orreries, and astronomical clocks University of Toronto Press, 1978
