Astrolabe

The astrolabe is classed among the early scientific instruments used for reckoning time and for observational purposes. Its name means either star-taker or the one that catches the heavenly bodies. The instrument is traced to the 6th century and was in wide use from the early Middle Ages.

The name of the instrument derives from the Greek astrolabos, a compound that joins the word for star to that for taking, so that the device is named for its office of seizing the positions of the stars. The astrolabe is an object probably invented by the Greeks in the 2nd century BC, and knowledge of it was transmitted to the Muslims through the translation of Hellenistic and Byzantine texts into Arabic. The device may have been invented by Hipparchus of Nicea in the 2nd century BCE.

The principle on which the instrument rests is stereographic projection, by which the celestial sphere is mapped on the plate of an astrolabe, allowing the 3-dimensional sphere to be represented on a flat plate; because each latitude needs its own projection, most astrolabes come with several plates.

The instrument consists of a solid body, the mater, into which fit a series of plates and a revolving web-like star map called the rete in Europe and the ankabut, or spider, in the Islamic world. The mater, Latin for mother, is the main body of the astrolabe; its edge is called the limb, and the hollowed-out part is called the womb and contains the latitude plates. In Chaucer's own description the moder of the astrolabe is the thickest plate, pierced with a large hole, receiving in its womb the thin plates compounded for diverse climates and the rete shaped like a net. The standard planispheric astrolabe includes a set of thin brass plates, one for each terrestrial latitude, ruled with the horizon coordinate net for its latitude, under the rete. The rete, sometimes called the star-net, is a cut-out plate that carries the star pointers and ecliptic ring and can be rotated over the latitude plates underneath.

The alidade is a rotating bar on the back of an astrolabe, fitted with vanes with pin-holes that serve as sights; altitude is measured by lining up a star in the two holes and reading the degrees on the scale around the edge. At night the Pole Star was sighted through pinholes in the two vanes on the pivoting alidade; by day the astrolabe was held below the waist so that a beam of sunlight passed from the top pinhole onto the bottom one. The rule is a bar rotating across the front of the astrolabe that locates positions on the plate or rete and relates them to the scale of hours on the limb. A pin through the centre holds the alidade to the back and the rete to the front, secured by a wedge called a horse.

Most of the 41 star pointers on the Whipple rete are labelled with the names of stars, many of them in Arabic, reflecting the Arab influences on medieval European astronomy. The back of the Shah Abbas astrolabe contains 10 scales, among them altitude, cotangent, declination, azimuth of qibla, sine and cosine, and shadow square.

Brass is the most common material, and surviving examples also appear in copper or iron, with gold or silver inlay on finer pieces. The British Museum holds a brass astrolabe dated 1241/2 AH inlaid with silver and copper, carrying the maker's mark Abd al-Karim al-Asturlabi, the Astrolabist.

The planispheric astrolabe allowed an astronomer to compute the positions of the Sun and the prominent stars with respect to the horizon and the meridian, functioning as a rudimentary analog computer. It served to tell the time and to measure the heights of stars and buildings. The face of the instrument could be used as a clock: by moving the position of the Sun, or a pointer to a star, to the circle of its observed altitude, the hour could be read off. The back carried concentric circles for calendar calculations, including saints' days and feast days. The alidade was used to measure altitude by lining up a star in its two holes and reading the degrees. The astrologer worked with three tools: the astrolabe, the ephemeris table, and the dust board.

The earliest extant treatise on the astrolabe, covering both its construction and its use, was written by al-Khwarizmi. The earliest surviving astrolabes and astrolabe treatises belong to the 9th century. This development unfolded alongside the translation of scientific texts in Baghdad during the early Abbasid era, when the city became a major center of study under al-Mansur and al-Ma'mun, while local rulers in Cairo, Rayy and Isfahan likewise supported research. The specific requirements of Islam drove the refinement of such instruments: the astrolabe served as a two-dimensional map of the heavens by which the direction of Mecca and the times of prayer could be found. By the later 9th century it was in use throughout the Islamic world from Spain to India, and it later reached Christian Europe, with knowledge passing through Islamic Spain from around 1000 CE. In the Middle Ages the instrument became widespread in the Arab world, the Byzantine Empire, India and Europe.

Al-Biruni produced treatises on constructing an astrolabe and on using its different kinds, an epistle on the instrument, a treatise on time-keeping and the mechanical calendar, and the al-Qanun al-Masudi. Al-Battani was skilled in making instruments, and the final chapter of his astronomical work discusses instrument construction. Al-Sufi's Book of the Images of the Fixed Stars describes the forty-eight constellations on the basis of observations at Isfahan. The linear astrolabe was invented by Mozaffar ibn Muhammad Tusi.

Al-Zarqali's azafea, known in Europe as the Saphea Arzachelis, rests on stereographic mapping, but its centre of projection is one of the equinoctial points rather than the North Pole. Because of this choice of projection, the instrument is universal: it requires no special latitude plates.

Al-Zarqali led the Toledan school of astronomers and produced the Toledan Tables, containing observations of the sun, moon and planets. Those tables were widely used in Europe until the Alphonsine Tables appeared around 1270 in Castile. His patron Qadi ibn Said gave him books to study, and after two years he had mastered mathematics and astronomy and joined the Toledo group of astronomers in 1062 as its head.

The mariner's astrolabe was a simplified instrument built for use at sea: it was heavier than its predecessors and had parts of the disc cut away to reduce wind resistance, and it served to determine the ship's latitude from the height of the Pole Star or of the sun.

A mariner's astrolabe recovered from Mounts Bay in Cornwall was found in 1982 and purchased in 1989. It is missing its alidade and suspension ring, and its cast brass frame has a mater made from a single casting with a wedge-shaped bottom ballast. The instrument measures 207 mm in diameter, its ballast is 14 mm thick at the top and 28 mm at the bottom, and it weighs 3288 grams. A Spanish escudo of about 1520 found at the same site suggests that the Mounts Bay astrolabe may have been made in Spain, though an illustration in William Bourne's A Regiment for Sea of 1574 is similar.

The Khalili Collection holds a ninth-century North African astrolabe of cast brass, whose star pointers are inscribed with Babylonian-Hellenistic star lists alongside individual stars bearing pre-Islamic Beduin names ^{5Planispheric Astrolabe (SCI 430) · Khalili Collections ↗}. The Museum of the History of Science catalogue records an astrolabe dated 1647/8 (A.H. 1057), made by Muhammad Muqim al-Yazdi in Persia and presented by Lewis Evans in 1924 as part of the Museum collection ^{11Astrolabe by Muḥammad Muqīm al-Yazdī (catalogue entry 45747) · Museum of the History of Science, Oxford ↗}.

Chaucer's Treatise on the Astrolabe is divided into five parts and written in plain English under full light rules and naked words, because the boy knew but little Latin. The treatise instructs the reader to put the ring of the astrolabe upon the right thumb, turn the left side against the sunlight, and move the rule until the sunbeams shine through both holes.

The astrolabe was a key astronomical instrument during the Middle Ages, and it is classed among the early scientific instruments. Iranica describes it as an instrument used in astronomy for demonstration and solution of problems in spherics, measuring altitudes, and telling time. It also served as a cultural resource shared between Islam and the West.

The Oxford exhibition covers a thousand years of astrolabes, from the ninth century to the nineteenth. The Museum of the History of Science in Oxford houses the world's largest and most important collection of astrolabes, ranging from India and the Middle East to Europe and spanning two thousand years of history.

• 374/984-85
  Oldest surviving Persian astrolabe
  A Persian astrolabe made by Hamed ibn Khezr Khojandi is dated to this year in the Islamic era.^{9Asṭorlāb · Encyclopaedia Iranica ↗}
• around 1000 CE
  Transmission to Europe
  Knowledge of the astrolabe reached Europe through Islamic Spain.^{2Astrolabe · World History Encyclopedia ↗}
• around 1270
  Alphonsine Tables
  The Alphonsine Tables appeared in Castile, superseding the Toledan Tables in European use.^{7Al-Zarqali · MacTutor History of Mathematics Archive, University of St Andrews ↗}