Astrolabe

The astrolabe is a 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.

In the medieval Islamic world, they were used primarily for astronomical studies, as well as in other areas as diverse as astrology, navigation, surveying, timekeeping, Salah prayers, and Qibla. Astrologers of the European nations used astrolabes to construct horoscopes.

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.

A Brief History

An early rudimentary astrolabe was invented in the Hellenistic world in either the first or second centuries BC and is often attributed to 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 Harvcoltxt|Lewis|2001 argues that Ptolemy used an astrolabe to make the astronomical observations recorded in the "Tetrabiblos". [Harvcoltxt|Evans|1998|pp=155 "The astrolabe was in fact an invention of the ancient Greeks."
Harvcoltxt|Krebs|Krebs|2003|p=56 "It is generally accepted that Greek astrologers, in either the first or second centuries BC, invented the "astrolabe", an instrument that measures the altitude of stars and planets above the horizon. Some historians attribute its invention to Hipparchus"]

Brass astrolabes ( _ar. اسطرلاب asterlab, ostorlab) 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, Muhammad al-Fazari. [Richard Nelson Frye: Golden Age of Persia. p. 163] The mathematical background was established by the Arab astronomer, Muhammad ibn Jābir al-Harrānī al-Battānī (Albatenius), in his treatise "Kitab az-Zij" (ca. 920 AD), which was translated into Latin by Plato Tiburtinus ("De Motu Stellarum").The [http://www.soas.ac.uk/gallery/Previous/IslamicPatronage/19.html 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. [Dr. Emily Winterburn (National Maritime Museum), [http://www.muslimheritage.com/topics/default.cfm?ArticleID=529 Using an Astrolabe] , Foundation for Science Technology and Civilisation, 2005.]

Abū Ishāq Ibrāhīm al-Zarqālī (Arzachel) 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. [M. T. Houtsma and E. van Donzel (1993), " [http://books.google.com/books?id=GEl6N2tQeawC E. J. Brill's First Encyclopaedia of Islam] ", Brill Publishers, ISBN 9004082654] Early Christian recipients of Arab astronomy included Gerbert of Aurillac and Hermannus Contractus.

The spherical astrolabe, a variation of both the astrolabe and the armillary sphere, was invented during the Middle Ages by astronomers and inventors in the Islamic world. [Emilie Savage-Smith (1993). "Book Reviews", "Journal of Islamic Studies" 4 (2), p. 296-299. quote|"There is no evidence for the Hellenistic origin of the spherical astrolabe, but rather evidence so far available suggests that it may have been an early but distinctly Islamic development with no Greek antecedents."] The earliest description of the spherical astrolabe dates back to Al-Nayrizi (fl. 892-902). In the 12th century, Sharaf al-Dīn al-Tūsī invented the "linear astrolabe", sometimes called the "staff of al-Tusi", which was "a simple wooden rod with graduated markings but without sights. It was furnished with a plumb line and a double chord for making angular measurements and bore a perforated pointer."MacTutor|id=Al-Tusi_Sharaf|title=Sharaf al-Din al-Muzaffar al-Tusi] The first geared mechanical astrolabe was later invented by Abi Bakr of Isfahan in 1235.Silvio A. Bedini, Francis R. Maddison (1966). "Mechanical Universe: The Astrarium of Giovanni de' Dondi", "Transactions of the American Philosophical Society" 56 (5), p. 1-69.]

Peter of Maricourt in the last half of the thirteenth century also wrote a treatise on the construction and use of a universal astrolabe ("Nova compositio astrolabii particularis"). However, given the complicated nature of the instrument, it is highly unlikely that any were actually constructed; at least none survive.

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. [cite book | last = Glick et al., eds. | title = Medieval Science, Technology, and Medicine: An Encyclopedia | year = 2005 | publisher = Routledge | id = ISBN 0415969301 | pages = 464 | quote =]

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

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. Ibn al-Shatir constructed the earliest astrolabic clock in the early 14th century. [David A. King (1983). "The Astronomy of the Mamluks", "Isis" 74 (4), p. 531-555 [545-546] .] At around the same time, Richard of Wallingford's clock (c. 1330) consisted essentially of a star map rotating behind a fixed rete, similar to that of an astrolabe.Fact|date=September 2008

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 [http://www.ulysse-nardin.com/watch.jsp?ID_Page=100011_100002_10001J 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" or "label" 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. The "alidade" is attached to the back face. An alidade can be seen in the lower right illustration of the Persion astrolabe above. 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.

ee 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

Notes

References

*.

* Alessandro Gunella and John Lamprey, "Stoeffler's Elucidatio" (translation of Elucidatio fabricae ususque astrolabii into English). Published by John Lamprey, 2007. [e-mail]

*.

*.

* 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

* [http://www.astrolabes.org The Astrolabe]
* [http://www.autodidacts.f2s.com/astro/ Keith's Astrolabe, a software astrolabe simulator and tutorial written in Java]
* [http://www.rogivue.com/xoops/modules/news/article.php?storyid=13 A working model of the Dr. Ludwig Oechslin's Astrolabium Galileo Galilei watch]
* [http://www.rogivue.com/xoops/modules/news/article.php?storyid=14 Ulysse Nardin Astrolabium Galilei Galileo: A Detailed Explanation]
* [http://www.mhs.ox.ac.uk/astrolabe/ Fully illustrated online catalogue of world's largest collection of astrolabes]
* [http://mathdl.maa.org/convergence/1/?pa=content&sa=viewDocument&nodeId=1187&bodyId=1326 Gerbert d'Aurillac's use of the Astrolabe] at [http://mathdl.maa.org/convergence/1/ Convergence]