An astrolabe (Greek: στρολάβον astrolabon 'star-taker') 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. Among ancient civilized peoples for many centuries the most important instrument used by astronomers, astrologers and surveyors was the astrolabe, or, more properly, the astrolabe planisphere.
Using an Astrolabe
The Arabs, probably applying the theory of Ptolemy, constructed exceedingly accurate astrolabes. Remains of astrolabes have been found in Assyrian excavations, and the instrument is still of practical value in India and Persia. From Arabia it passed again into Europe, and was used by astronomers and surveyors in Italy, England and other countries until the eighteenth century.
In the medieval Islamic world, they were introduced by Arabsand 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.
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
How to make Astrolabes
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. In addition, Severus Sebokht also wrote a treatise on the astrolabe in Syriac in the mid-seventh century. 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
Astrolabes were further developed in the medieval Islamic world, where Arabic astronomers introduced angular scales to the astrolabe. 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. 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. 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
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.