Astrolabe

Astrolabe

Overview

An astrolabe is an elaborate inclinometer
Inclinometer
An inclinometer or clinometer is an instrument for measuring angles of slope , elevation or depression of an object with respect to gravity...

, historically used by astronomers
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

, navigators, and astrologers. Its many uses include locating and predicting the positions of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, planets, and star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s, determining local time given local latitude and longitude, surveying, triangulation
Triangulation
In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...

, and to cast horoscope
Horoscope
In astrology, a horoscope is a chart or diagram representing the positions of the Sun, Moon, planets, the astrological aspects, and sensitive angles at the time of an event, such as the moment of a person's birth. The word horoscope is derived from Greek words meaning "a look at the hours" In...

s. It was used in classical antiquity
Classical antiquity
Classical antiquity is a broad term for a long period of cultural history centered on the Mediterranean Sea, comprising the interlocking civilizations of ancient Greece and ancient Rome, collectively known as the Greco-Roman world...

, through the Islamic Golden Age
Islamic Golden Age
During the Islamic Golden Age philosophers, scientists and engineers of the Islamic world contributed enormously to technology and culture, both by preserving earlier traditions and by adding their own inventions and innovations...

, the European Middle Ages
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

 and Renaissance
Renaissance
The Renaissance was a cultural movement that spanned roughly the 14th to the 17th century, beginning in Italy in the Late Middle Ages and later spreading to the rest of Europe. The term is also used more loosely to refer to the historical era, but since the changes of the Renaissance were not...

 for all these purposes. In the Islamic world, it was also used to calculate the Qibla
Qibla
The Qiblah , also transliterated as Qibla, Kiblah or Kibla, is the direction that should be faced when a Muslim prays during salah...

 and to find the times for Salah, prayers.

There is often confusion between the astrolabe and the mariner's astrolabe
Mariner's astrolabe
The mariner's astrolabe, also called sea astrolabe, was an inclinometer used to determine the latitude of a ship at sea by measuring the sun's noon altitude or the meridian altitude of a star of known declination. Not an astrolabe proper, the mariner's astrolabe was rather a graduated circle with...

.
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Encyclopedia

An astrolabe is an elaborate inclinometer
Inclinometer
An inclinometer or clinometer is an instrument for measuring angles of slope , elevation or depression of an object with respect to gravity...

, historically used by astronomers
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

, navigators, and astrologers. Its many uses include locating and predicting the positions of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, planets, and star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s, determining local time given local latitude and longitude, surveying, triangulation
Triangulation
In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...

, and to cast horoscope
Horoscope
In astrology, a horoscope is a chart or diagram representing the positions of the Sun, Moon, planets, the astrological aspects, and sensitive angles at the time of an event, such as the moment of a person's birth. The word horoscope is derived from Greek words meaning "a look at the hours" In...

s. It was used in classical antiquity
Classical antiquity
Classical antiquity is a broad term for a long period of cultural history centered on the Mediterranean Sea, comprising the interlocking civilizations of ancient Greece and ancient Rome, collectively known as the Greco-Roman world...

, through the Islamic Golden Age
Islamic Golden Age
During the Islamic Golden Age philosophers, scientists and engineers of the Islamic world contributed enormously to technology and culture, both by preserving earlier traditions and by adding their own inventions and innovations...

, the European Middle Ages
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

 and Renaissance
Renaissance
The Renaissance was a cultural movement that spanned roughly the 14th to the 17th century, beginning in Italy in the Late Middle Ages and later spreading to the rest of Europe. The term is also used more loosely to refer to the historical era, but since the changes of the Renaissance were not...

 for all these purposes. In the Islamic world, it was also used to calculate the Qibla
Qibla
The Qiblah , also transliterated as Qibla, Kiblah or Kibla, is the direction that should be faced when a Muslim prays during salah...

 and to find the times for Salah, prayers.

There is often confusion between the astrolabe and the mariner's astrolabe
Mariner's astrolabe
The mariner's astrolabe, also called sea astrolabe, was an inclinometer used to determine the latitude of a ship at sea by measuring the sun's noon altitude or the meridian altitude of a star of known declination. Not an astrolabe proper, the mariner's astrolabe was rather a graduated circle with...

. 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.

Etymology


OED gives the Dustin is the best translation "star-taker" for the English word "astrolabe" and traces it, through medieval Latin, to a Greek word astrolabon. In the medieval Islamic world the word "asturlab" (i.e. astrolabe) was given various etymologies. In Arabic texts the word is translated as "akhdh al-kawakib" (lit. "taking the stars") which corresponds to an interpretation of the Greek word . al-Biruni
Al-Biruni
Abū al-Rayḥān Muḥammad ibn Aḥmad al-BīrūnīArabic spelling. . The intermediate form Abū Rayḥān al-Bīrūnī is often used in academic literature...

 quotes and criticizes the medieval scientist Hamza al-Isfahani who had stated " asturlab is an arabization of this Persian phrase" : sitara yab meaning "taker of the stars" In medieval Islamic sources there is also a "fictional" and popular etymology of the words as "lines of lab". In this popular etymology "Lab" is a certain son of Idris (=Enoch). This etymology is mentioned by a 10th century scientist called al-Qummi but rejected by al-Khwarizmi .

Ancient world


An early astrolabe was invented in the Hellenistic world
Hellenistic civilization
Hellenistic civilization represents the zenith of Greek influence in the ancient world from 323 BCE to about 146 BCE...

 in 150 BC and is often attributed to Hipparchus
Hipparchus
Hipparchus, the common Latinization of the Greek Hipparkhos, can mean:* Hipparchus, the ancient Greek astronomer** Hipparchic cycle, an astronomical cycle he created** Hipparchus , a lunar crater named in his honour...

. A marriage of the planisphere
Planisphere
A planisphere is a star chart analog computing instrument in the form of two adjustable disks that rotate on a common pivot. It can be adjusted to display the visible stars for any time and date. It is an instrument to assist in learning how to recognize stars and constellations...

 and dioptra
Dioptra
A dioptra is a classical astronomical and surveying instrument, dating from the 3rd century BCE. The dioptra was a sighting tube or, alternatively, a rod with a sight at both ends, attached to a stand...

, the astrolabe was effectively an analog calculator capable of working out several different kinds of problems in spherical astronomy. Theon of Alexandria
Theon of Alexandria
Theon was a Greek scholar and mathematician who lived in Alexandria, Egypt. He edited and arranged Euclid's Elements and Ptolemy's Handy Tables, as well as writing various commentaries...

 wrote a detailed treatise on the astrolabe, and argues that Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

 used an astrolabe to make the astronomical observations recorded in the Tetrabiblos
Tetrabiblos
The Tetrabiblos , also known under the Latin title Quadripartitum , is a text on the philosophy and practice of astrology, written in the second century AD by the Alexandrian scholar Claudius Ptolemy ....

.

Astrolabes continued in use in the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

-speaking world throughout the Byzantine
Byzantine Empire
The Byzantine Empire was the Eastern Roman Empire during the periods of Late Antiquity and the Middle Ages, centred on the capital of Constantinople. Known simply as the Roman Empire or Romania to its inhabitants and neighbours, the Empire was the direct continuation of the Ancient Roman State...

 period. About 550 AD the Christian philosopher John Philoponus
John Philoponus
John Philoponus , also known as John the Grammarian or John of Alexandria, was a Christian and Aristotelian commentator and the author of a considerable number of philosophical treatises and theological works...

 wrote a treatise on the astrolabe in Greek, which is the earliest extant Greek treatise on the instrument. In addition, Severus Sebokht, a bishop who lived in Mesopotamia, also wrote a treatise on the astrolabe in Syriac in the mid-7th century. 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 Christian East well before they were developed in the Islamic world or the Latin West.

Medieval era




Astrolabes were further developed in the medieval Islamic world
Islamic Golden Age
During the Islamic Golden Age philosophers, scientists and engineers of the Islamic world contributed enormously to technology and culture, both by preserving earlier traditions and by adding their own inventions and innovations...

, where Muslim astronomers introduced angular scales to the astrolabe, adding circles indicating azimuths on the horizon
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...

. It was widely used throughout the Muslim world
Muslim world
The term Muslim world has several meanings. In a religious sense, it refers to those who adhere to the teachings of Islam, referred to as Muslims. In a cultural sense, it refers to Islamic civilization, inclusive of non-Muslims living in that civilization...

, chiefly as an aid to navigation and as a way of finding the Qibla
Qibla
The Qiblah , also transliterated as Qibla, Kiblah or Kibla, is the direction that should be faced when a Muslim prays during salah...

, the direction of Mecca
Mecca
Mecca is a city in the Hijaz and the capital of Makkah province in Saudi Arabia. The city is located inland from Jeddah in a narrow valley at a height of above sea level...

. The first person credited with building the astrolabe in the Islamic world is reportedly the 8th century mathematician Muhammad al-Fazari
Muhammad al-Fazari
Abu abdallah Muhammad ibn Ibrahim al-Fazari was a Muslim philosopher, mathematician and astronomer. He is not to be confused with his father Ibrāhīm al-Fazārī, also an astronomer and mathematician....

. The mathematical background was established by the Muslim 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
Plato Tiburtinus
Plato Tiburtinus was a 12th century Italian mathematician, astronomer and translator who lived in Barcelona from 1116 to 1138. He is best known for translating Hebrew and Arabic documents into Latin, and was apparently the first to translate information on the astrolabe from Arabic.Plato of...

 (De Motu Stellarum). The earliest surviving dated astrolabe is dated AH
Islamic calendar
The Hijri calendar , also known as the Muslim calendar or Islamic calendar , is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to date events in many Muslim countries , and used by Muslims everywhere to determine the proper day on which to celebrate Islamic...

 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
Salat
Salah is the practice of formal prayer in Islam. Its importance for Muslims is indicated by its status as one of the Five Pillars of Sunni Islam, of the Ten Practices of the Religion of Twelver Islam and of the 7 pillars of Musta'lī Ismailis...

). In the 10th century, al-Sufi first described over 1,000 different uses of an astrolabe, in areas as diverse as astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

, astrology, horoscopes, navigation
Mariner's astrolabe
The mariner's astrolabe, also called sea astrolabe, was an inclinometer used to determine the latitude of a ship at sea by measuring the sun's noon altitude or the meridian altitude of a star of known declination. Not an astrolabe proper, the mariner's astrolabe was rather a graduated circle with...

, surveying
Surveying
See Also: Public Land Survey SystemSurveying or land surveying is the technique, profession, and science of accurately determining the terrestrial or three-dimensional position of points and the distances and angles between them...

, time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

keeping, prayer
Prayer
Prayer is a form of religious practice that seeks to activate a volitional rapport to a deity through deliberate practice. Prayer may be either individual or communal and take place in public or in private. It may involve the use of words or song. When language is used, prayer may take the form of...

, Salah, Qibla
Qibla
The Qiblah , also transliterated as Qibla, Kiblah or Kibla, is the direction that should be faced when a Muslim prays during salah...

, etc.

The spherical astrolabe, a variation of both the astrolabe and the armillary sphere
Armillary sphere
An armillary sphere is a model of objects in the sky , consisting of a spherical framework of rings, centred on Earth, that represent lines of celestial longitude and latitude and other astronomically important features such as the ecliptic...

, was invented during the Middle Ages
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

 by astronomers and inventors in the Islamic world
Islamic Golden Age
During the Islamic Golden Age philosophers, scientists and engineers of the Islamic world contributed enormously to technology and culture, both by preserving earlier traditions and by adding their own inventions and innovations...

. The earliest description of the spherical astrolabe dates back to Al-Nayrizi
Al-Nayrizi
Abū’l-‘Abbās al-Faḍl ibn Ḥātim al-Nairīzī was a 9th-10th century Persian mathematician and astronomer from Nayriz, Fars, Iran.He flourished under al-Mu'tadid, Caliph from 892 to 902, and compiled astronomical tables, writing a book for al-Mu'tadid on atmospheric phenomena.Nayrizi wrote...

 (fl.
Floruit
Floruit , abbreviated fl. , is a Latin verb meaning "flourished", denoting the period of time during which something was active...

 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." The first gear
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....

ed mechanical astrolabe was later invented by Abi Bakr of Isfahan in 1235.

Peter of Maricourt
Peter of Maricourt
Pierre Pelerin de Maricourt , Petrus Peregrinus de Maricourt or Peter Peregrinus of Maricourt; was a 13th century French scholar who conducted experiments on magnetism and wrote the first extant treatise describing the properties of magnets...

, in the last half of the 13th century, also wrote a treatise on the construction and use of a universal astrolabe (Nova compositio astrolabii particularis). Universal astrolabes can be found at the History of Science Museum in Oxford.

The English author Geoffrey Chaucer
Geoffrey Chaucer
Geoffrey Chaucer , known as the Father of English literature, is widely considered the greatest English poet of the Middle Ages and was the first poet to have been buried in Poet's Corner of Westminster Abbey...

 (ca. 1343–1400) compiled a treatise on the astrolabe
Treatise on the Astrolabe
A Treatise on the Astrolabe is a medieval essay on the astrolabe by Geoffrey Chaucer. It begins:or, in a more modern English spelling,According to the introduction, the work was to have five parts:#A description of 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 India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

n treatise on the astrolabe was written by the Jain
Jainism
Jainism is an Indian religion that prescribes a path of non-violence towards all living beings. Its philosophy and practice emphasize the necessity of self-effort to move the soul towards divine consciousness and liberation. Any soul that has conquered its own inner enemies and achieved the state...

 astronomer Mahendra Suri
Mahendra Suri
Mahendra Sūri is the 14th century Jain astronomer who wrote the Yantraraja, the first Indian treatise on the astrolabe. He was a pupil of Madana Suri.-See also:*Indian mathematicians...

.

The first known metal astrolabe known in Western Europe was developed in the 15th century by Rabbi
Rabbi
In Judaism, a rabbi is a teacher of Torah. This title derives from the Hebrew word רבי , meaning "My Master" , which is the way a student would address a master of Torah...

 Abraham Zacuto
Abraham Zacuto
Abraham Zacuto was a Sephardi Jewish astronomer, astrologer, mathematician and historian who served as Royal Astronomer in the 15th century to King John II of Portugal. The crater Zagut on the Moon is named after him....

 in Lisbon
Lisbon
Lisbon is the capital city and largest city of Portugal with a population of 545,245 within its administrative limits on a land area of . The urban area of Lisbon extends beyond the administrative city limits with a population of 3 million on an area of , making it the 9th most populous urban...

. 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
Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...

, along with portable sundial
Sundial
A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a...

s and other popular scientific gadget
Gadget
A gadget is a small technological object that has a particular function, but is often thought of as a novelty. Gadgets are invariably considered to be more unusually or cleverly designed than normal technological objects at the time of their invention...

s of the day. Finally, one more special example of craftsmanship in the early 15th century Europe is the astrolabe dated 1420, designed by Antonius de Pacento and made by Dominicus de Lanzano.

In the 16th century, Johannes Stöffler
Johannes Stöffler
Johannes Stöffler was a German mathematician, astronomer, astrologer, priest, maker of astronomical instruments and professor at the University of Tübingen.-Life:...

 published Elucidatio fabricae ususque astrolabii, a manual of the construction and use of the astrolabe. Four identical 16th century astrolabes made by Georg Hartmann
Georg Hartmann
Georg Hartmann was a German engineer, instrument maker, author, printer, humanist, churchman, and astronomer....

 provide some of the earliest evidence for batch production
Batch production
Batch production is a technique used in manufacturing, in which the object in question is created stage by stage over a series of workstations. Batch production is common in bakeries and in the manufacture of sports shoes, pharmaceutical ingredients , inks, paints and adhesives. In the manufacture...

 by division of labor.

Astrolabes and clocks



At first mechanical astronomical clock
Astronomical clock
An astronomical clock is a clock with special mechanisms and dials to display astronomical information, such as the relative positions of the sun, moon, zodiacal constellations, and sometimes major planets.-Definition:...

s 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
Richard of Wallingford
Richard of Wallingford was an English mathematician who made major contributions to astronomy/astrology and horology while serving as abbot of St Albans Abbey in Hertfordshire.-Biography:...

's clock (c. 1330) consisted essentially of a star map rotating behind a fixed rete, similar to that of an astrolabe.

Many astronomical clocks, such as the famous clock at Prague
Prague Orloj
The Prague Astronomical Clock or Prague Orloj is a medieval astronomical clock located in Prague, the capital of the Czech Republic, at . The clock was first installed in 1410, making it the third-oldest astronomical clock in the world and the oldest one still working.-Description:The Orloj is...

, 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
Ulysse Nardin
Ulysse Nardin is a watch manufacturer founded in 1846 in Le Locle, Switzerland. Historically Ulysse Nardin was best known for being a manufacturer of marine chronometers, but today Ulysse Nardin produces complicated mechanical watches.-History:...

.

Construction




An astrolabe consists of a disk, called the mater
Mater
*Mater is a formal term for mother, from Latin.*Mater : The dura mater and pia mater are membranes surrounding the brain and spinal cord in humans.Mater may also refer to:-Australia:*Mater Hospital, Sydney, New South Wales...

(mother), which is deep enough to hold one or more flat plates called tympans, or climate
Clime
The seven climes was a notion of dividing the Earth into zones in Classical Antiquity....

s
. A tympan
Tympan
In hand-operated letterpress printing, the bruzer tympan is the taut cloth or paper mounted in a frame which is placed over the sheet of paper immediately prior to lowering the platen to make the impression....

 is made for a specific latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

 and is engraved with a stereographic projection
Stereographic projection
The stereographic projection, in geometry, is a particular mapping that projects a sphere onto a plane. The projection is defined on the entire sphere, except at one point — the projection point. Where it is defined, the mapping is smooth and bijective. It is conformal, meaning that it...

 of circle
Circle
A circle is a simple shape of Euclidean geometry consisting of those points in a plane that are a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius....

s denoting azimuth and altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...

 and representing the portion of the celestial sphere
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...

 above the local horizon. The rim of the mater is typically graduated into hours of time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

, degrees of arc
Arc
Arc may refer to:-Mathematics:*Arc , a segment of a differentiable curve*Arc , a particular type of set of points of a projective plane*Arcminute, a measure used for angles, equal to 1/60th of a degree...

, or both. Above the mater and tympan, the rete
Rete
Rete may refer to: An interwoven network of nerves, blood vessels or passageways.-Italy:*Net , in Latin*La Rete , the common name given to Movimento per la Democrazia , an Italian political party-Technology:...

, a framework bearing a projection of the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...

 plane and several pointer
Pointer (rod)
A pointer or pointing stick is a solid rod used to point manually, in the form of a stick, but always finished off or artificially produced....

s indicating the positions of the brightest star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s, is free to rotate. Some astrolabes have a narrow rule
Rule
Rule, ruler, ruling usually refers to standards for activities. They may refer to:- Human activity :* Business rule, a rule pertaining to the structure or behavior internal to an organization* Game rules, rules that define how a game is played...

or label which rotates over the rete, and may be marked with a scale of declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...

s.

The rete, representing the sky
Sky
The sky is the part of the atmosphere or outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a pale blue surface because the air scatters the sunlight. The sky is sometimes...

, functions as a star chart
Star chart
A star chart is a map of the night sky. Astronomers divide these into grids to use them more easily. They are used to identify and locate astronomical objects such as stars, constellations and galaxies. They have been used for human navigation since time immemorial...

. When it is rotated, the stars and the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...

 move over the projection of the coordinates on the tympan. One complete rotation corresponds to the passage of a day. The astrolabe is therefore a predecessor of the modern planisphere
Planisphere
A planisphere is a star chart analog computing instrument in the form of two adjustable disks that rotate on a common pivot. It can be adjusted to display the visible stars for any time and date. It is an instrument to assist in learning how to recognize stars and constellations...

.

On the back of the mater there is often engraved a number of scales that are useful in the astrolabe's various applications; these vary from designer to designer, but might include curves for time conversions, a calendar
Calendar
A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. The name given to each day is known as a date. Periods in a calendar are usually, though not...

 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
Alhidade
An alidade is a device that allows one to sight a distant object and use the line of sight to perform a task. This task can be, for example, to draw a line on a plane table in the direction of the object or to measure the angle to the object from some reference point...

is attached to the back face. An alidade can be seen in the lower right illustration of the Persian astrolabe above. When the astrolabe is held vertically, the alidade can be rotated and the sun or a star sighted along its length, so that its 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.

See also

  • Antikythera mechanism
    Antikythera mechanism
    The Antikythera mechanism is an ancient mechanical computer designed to calculate astronomical positions. It was recovered in 1900–1901 from the Antikythera wreck. Its significance and complexity were not understood until decades later. Its time of construction is now estimated between 150 and 100...

  • Armillary sphere
    Armillary sphere
    An armillary sphere is a model of objects in the sky , consisting of a spherical framework of rings, centred on Earth, that represent lines of celestial longitude and latitude and other astronomically important features such as the ecliptic...

  • Astrarium
    Astrarium
    An astrarium, also called a planetarium, is the mechanical representation of the cyclic nature of astronomical objects in one timepiece. It is an astronomical clock.-Greek and Roman World:...

  • Astronomical clock
    Astronomical clock
    An astronomical clock is a clock with special mechanisms and dials to display astronomical information, such as the relative positions of the sun, moon, zodiacal constellations, and sometimes major planets.-Definition:...

  • Cosmolabe
    Cosmolabe
    The cosmolabe was an ancient astronomical instrument resembling the astrolabe, formerly used for measuring the angles between heavenly bodies. It is also called pantacosm. Jacques Besson also uses this name, or universal instrument, for his invention described in Le cosmolabe , which could be used...

  • Equatorium
  • Islamic astronomy
    Islamic astronomy
    Islamic astronomy or Arabic astronomy comprises the astronomical developments made in the Islamic world, particularly during the Islamic Golden Age , and mostly written in the Arabic language. These developments mostly took place in the Middle East, Central Asia, Al-Andalus, and North Africa, and...

  • Orrery
    Orrery
    An orrery is a mechanical device that illustrates the relative positions and motions of the planets and moons in the Solar System in a heliocentric model. Though the Greeks had working planetaria, the first orrery that was a planetarium of the modern era was produced in 1704, and one was presented...

  • Philippe Danfrie
    Philippe Danfrie
    Philippe Danfrie the elder , was a designer and maker of mathematical instruments in metal and paper, as well as a type-cutter, engraver, minter of coins and medals, publisher and author. Much is known about Danfrie's life and activities...

    , designer and maker of mathematical instrument
    Mathematical instrument
    A mathematical instrument is a tool or device used in the study or practice of mathematics.Most instruments are used within the field of geometry, including the ruler, dividers, protractor, set square, compass, ellipsograph and opisometer...

    s, globes and astrolabes
  • Planetarium
    Planetarium
    A planetarium is a theatre built primarily for presenting educational and entertaining shows about astronomy and the night sky, or for training in celestial navigation...

  • Planisphere
    Planisphere
    A planisphere is a star chart analog computing instrument in the form of two adjustable disks that rotate on a common pivot. It can be adjusted to display the visible stars for any time and date. It is an instrument to assist in learning how to recognize stars and constellations...

  • Prague Orloj
    Prague Orloj
    The Prague Astronomical Clock or Prague Orloj is a medieval astronomical clock located in Prague, the capital of the Czech Republic, at . The clock was first installed in 1410, making it the third-oldest astronomical clock in the world and the oldest one still working.-Description:The Orloj is...

  • Sextant (astronomical)
    Sextant (astronomical)
    Sextants for astronomical observations were used primarily for measuring the positions of stars. They are little used today, having been replaced over time by transit telescopes, astrometry techniques, and satellites such as Hipparcos....

  • Sharafeddin Tusi
    Sharafeddin Tusi
    ' was a Persian mathematician and astronomer of the Islamic Golden Age .- Biography :...

    , the inventor of the linear astrolabe
  • Torquetum
    Torquetum
    The torquetum or turquet is a medieval astronomical instrument designed to take and convert measurements made in three sets of coordinates: Horizon, equatorial, and ecliptic...

  • Canterbury Astrolabe Quadrant
    Canterbury Astrolabe Quadrant
    The Canterbury Astrolabe Quadrant is a medieval astrolabe believed to date from 1388, and which was found in an archeological dig at the House of Agnes in Canterbury, Kent, England in 2005....

  • Hypatia
  • Marshall Islands stick chart
    Marshall Islands stick chart
    Stick charts were made and used by the Marshallese to navigate the Pacific Ocean by canoe off the coast of the Marshall Islands. The charts represented major ocean swell patterns and the ways the islands disrupted those patterns, typically determined by sensing disruptions in ocean swells by...


External links