ASTROLABE

The 'astrolabe' is a historical astronomical instrument used by classical astronomers and astrologers. 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.
There is often confusion between the astrolabe and the mariner's astrolabe. While the astrolabe could be useful for determining latitude on land, it was 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.

Contents

A Brief History

Astrolabes and clocks

Construction

See also

References

External links

A Brief History

The astrolabe was invented by the ancient Greeks in either the first or second centuries B.C.E. and is often attributed to Hipparchus. A marriage of the planisphere and dioptra, the astrolabe was effectively an analog calculator capable of working out many different kinds of problems of spherical astronomy. Theon of Alexandria wrote a detailed treatise on the astrolabe, and argues that Ptolemy used an astrolabe to make the astronomical observations recorded in the ''Tetrabiblos''.^[1]
Others suggest ancient India as the origin of the astrolabe.
Brass astrolabes (Persian: استرلاب asterlab, ostorlab) were developed in Persia, 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.^[2] 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 known example 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). Arzachel (al-Zarqali) of al-Andalus constructed one such instrument 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 introduced to other parts of 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.
In 1370, the first Indian treatise on the astrolabe was written by the Jain astronomer Mahendra Suri.^[3]
The first known European metal astrolabe was developed in the 15th century by Rabbi 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.

Astrolabes and clocks

At 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 1985 Swiss watchmaker Dr. Ludwig Oechslin designed and built an astrolabe wristwatch in conjunction with Ulysse Nardin.

Construction

An astrolabe consists of a hollow disk, called the ''mater'' (mother), 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 circular 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 plane and several pointers indicating the positions of the brightest 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.
The ''rete'', representing the sky, has the function of a star chart. When it is rotated, the stars and the ecliptic move over the projection of the 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.

See also

★ Antikythera mechanism

★ Armillary sphere

★ Astrarium

★ Astrology

★ Astronomical clock

★ Cosmolabe

★ Equatorium

★ Islamic astronomy

★ Orrery

★ Planetarium

★ Prague Orloj

★ Sextant

★ Sharafeddin Tusi, the inventor of the linear astrolabe

★ Torquetum

References

1. "The astrolabe was in fact an invention of the ancient Greeks."
"It is generally accepted that Greek astrologers, in either the first or second centuries B.C.E., invented the ''astrolabe'', an instrument that measures the altitude of stars and planets above the horizon. Some historians attribute its invention to Hipparchus"
2. Richard Nelson Frye: Golden Age of Persia. p. 163
3. Medieval Science, Technology, and Medicine: An Encyclopedia, , , Glick et al., eds., Routledge, 2005, ISBN 0415969301


★ .

★ Alessandro Gunella and John Lamprey, ''Stoeffler's Elucidatio'' (translation of Elucidatio fabricae ususque astrolabii into English). Published by John Lamprey, 2007. lamprey@frii.com

★ .

★ .

★ John North. ''God's Clockmaker, Richard of Wallingford and the invention of time.'' Hambledon and London, 2005.

★ Critical edition of ''Pelerin de Prusse on the Astrolabe'' (translation of ''Practique de Astralabe''). Editors Edgar Laird, Robert Fischer. Binghamton, New York, 1995, in Medieval & Renaissance Texts & Studies. ISBN 0-86698-132-2

★ King, Henry ''Geared to the Stars: the evolution of planetariums, orreries, and astronomical clocks'' University of Toronto Press, 1978

External links

★ The Astrolabe

★ Keith's Astrolabe, a software astrolabe simulator and tutorial written in Java

★ A working model of the Dr. Ludwig Oechslin's Astrolabium Galileo Galilei watch

★ Ulysse Nardin Astrolabium Galilei Galileo: A Detailed Explanation

★ Fully illustrated online catalogue of world's largest collection of astrolabes

★ Gerbert d'Aurillac's use of the Astrolabe at Convergence