An astrolabe (Greek: ἁστρολάβον astrolabon 'star-taker')[1] is an historical astronomical instrument used by classical astronomers, navigators, and astrologers. Its many uses include 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 introduced by Arabs[2] and used primarily for astronomical studies, as well as in other areas as diverse as astrology, geography, navigation, Qibla, Salah prayers, surveying, and timekeeping. In the European nations, astrolabes were used to construct horoscopes, for astronomical studies, and for navigation.
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.
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An early astrolabe was invented in the Hellenistic world in 150 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 Lewis (2001) argues that Ptolemy used an astrolabe to make the astronomical observations recorded in the Tetrabiblos.[3]
Astrolabes continued in use in the Greek-speaking world throughout the Byzantine period. About 550 AD the Christian philosopher John Philoponus wrote a treatise on the astrolabe in Greek, which is the earliest extant Greek treatise on the instrument.[4] In addition, Severus Sebokht also wrote a treatise on the astrolabe in Syriac in the mid-seventh century.[5] It was undoubtedly from such Eastern Christian scholars, either Greek or Syriac-speakers, that Muslim scholars were first introduced to the astrolabe, just as they were introduced by such Eastern Christians to other Greek scientific instruments and texts (including other works by Philoponus). Severus Sebokht refers in the introduction of his treatise to the astrolabe as being made of brass, indicating that metal astrolabes were known in the Christan East well before they were developed in the Islamic world or the Latin West.[6]
Astrolabes were further developed in the medieval Islamic world, where Arabic astronomers introduced angular scales to the astrolabe,[7] adding circles indicating azimuths on the horizon.[8] It was widely used throughout the Muslim 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 eighth century mathematician, Muhammad al-Fazari.[9] 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 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.[10]
Abū Ishāq Ibrāhīm al-Zarqālī (Arzachel) of Al-Andalus constructed the first universal astrolabe 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 Western Europe via Al-Andalus in the 11th century.[11]
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.[12] 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."[13] The first geared mechanical astrolabe was later invented by Abi Bakr of Isfahan in 1235.[14]
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.[15]
The first known metal astrolabe known in Western Europe was developed in the fifteenth century by Rabbi Abraham Zacuto in Lisbon. Metal astrolabes improved on the accuracy of their wooden precursors. In the fifteenth 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.
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.[16] 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.[17]
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.
An astrolabe consists of a fragile 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 the word's Greek roots: "astron" (ἄστρον) = star + "lab-" (λαβ-) = to take.
"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."
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ASTROLABE (from Gr. oi rrpov, star, and Xa(3€Iv, to take), an instrument used not only for stellar, but for solar and lunar altitude-taking. The principle of the astrolabe is explained in fig. 2. There were two kinds, - spherical and planispheric.
FIG. 2. - Principle of the Astrolabe. If a .JP solid circle be fixed in any one position and a tube be pivoted on its centre so as to move; and if the line C D be drawn upon the circle pointing towards any object Q in the heavens which lies in the plane of the circle, by turn ing the tube A B towards any other object P in the plane of the circle, the angle B 0 D will be the angle subtended by the two objects P and Q at the eye.
The earliest forms were "armillae" and spherical. Gradually, from Eratosthenes to Tycho, Hipparchus playing the most important part among ancient astronomers, the complex astrolabe was evolved, large specimens being among the chief observa tory instruments of the 15th, 16th and even 17th centuries; while small ones were in use among travellers and learned men, not only for astronomical, but for astrological and topographical purposes. Nearly every one of the modern instruments used for the observations of physical astronomy is a part of the perfected astrolabe. A collection of circles such as is the armillary sphere, if each circle were fitted with a view-tube, might be considered a complete astrolabe. Tycho's armillae were astrolabes. In fact the modern equatorial, and the altitude and azimuth circle are astrolabes in the strictest and oldest meaning of the term; and Tycho in one of his astrolabes came so near the modern equatorial that it may be taken as the first of the kind.
The two forms of the planispheric astrolabe most widely known and used in the 15th, 16th and even 17th centuries were: (I) the portable astrolabe shown in fig. I (Plate). This originated in the East, and was in early use in India, Persia and Arabia, and was introduced into Europe by the Arabs, who had perfected it - perhaps as early as A.D. 700. It combines the planisphere and armillae of Hipparchus and others, and the theodolite of Theon, and was usually of brass, varying in diameter from a couple of inches to a foot or more. It was used for taking the altitudes of sun, moon and stars; for calculating latitude; for determining the points of the compass, and time; for ascertaining heights of mountains, &c.; and for construction of horoscopes. The instrument was a ingenuity, and was called "the mathematical jewel." Nevertheless it passed out of use, because incapable of any great precision.
(2) The mariner's astrolabe, fig. 3, was adapted from that of astronomers by Martin Behaim, c. 1480. This was the instrument used by Columbus. With the tables of the sun's declination then available, he could calculate his latitude by meridian altitudes of the sun taken with his astrolabe. The mariner's astrolabe was superseded by John Hadley's quadrant of 1731. Authorities. - Chaucer, Treatise on the Astrolabe (Skeat's edition of Chaucer); J. J. StOffler, Elucidatio Fabrice ususque Astrolabii, &c.; Thomas Blundeville, His Exercises (1594); F. Ritter, Astrolabium; W. H. Morley, Description of Astrolabe of Shah Husain; M. L. Huggins, "The Astrolabe" (Astrophysical Journal, 1894); Penny Cyclopaedia, article "Astrolabe;" R. Grant, History of Physical Astronomy. (M. L. H.)
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[[File:|right|thumb|An astrolabe from the 16th century]] The astrolabe was a tool used by explorers to help them figure out where they were. It worked by using the positions of the stars or sun. This is not to be confused with the mariner's astrolabe which was used on ships to calculate their distance above and below the equator by measuring the distance of the sun and stars above the horizon.
An early astrolabe was made in the Hellenistic world in 150 BC. It is often attributed to Hipparchus. A marriage of the planisphere and dioptre, the astrolabe was a calculator able to work out many kinds of problems in astronomy. Theon of Alexandria wrote a complex book on the astrolabe. But Lewis (2001) says that Ptolemy used an astrolabe to make the astronomy observations recorded in the Tetrabiblos.[1]
   | A working model of the Dr. Ludwig Oechslin's Astrolabium Galileo Galilei watch - www.rogivue.com - News |
| | Ulysse Nardin Astrolabium Galilei Galileo: A Detailed Explanation - www.rogivue.com - News |
| | Keith's Astrolabe, a software astrolabe simulator and tutorial written in Java - Keith's astrolabes: a Java program |
| | Fully illustrated online catalogue of world's largest collection of astrolabes - MHS Astrolabe home page |
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