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An astrolabe (; ; ) is an
astronomical Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest include ...
instrument dating to ancient times. It serves as a
star chart A star chart is a celestial map of the night sky with astronomical objects laid out on a grid system. They are used to identify and locate constellations, stars, nebulae, galaxies, and planets. They have been used for human navigation since tim ...
and physical model of the visible half-dome of the sky. Its various functions also make it an elaborate
inclinometer An inclinometer or clinometer is an measuring instrument, instrument used for measuring angles of slope, elevation, or depression (geology), depression of an object with respect to gravity's direction. It is also known as a ''tilt indicator'', ' ...
and an analog calculation device capable of working out several kinds of problems in astronomy. In its simplest form it is a metal disc with a pattern of wires, cutouts, and perforations that allows a user to calculate astronomical positions precisely. It is able to measure the
altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...
above the horizon of a celestial body, day or night; it can be used to identify stars or planets, to determine local
latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...
given local time (and vice versa), to survey, or to triangulate. It was used in
classical antiquity Classical antiquity, also known as the classical era, classical period, classical age, or simply antiquity, is the period of cultural History of Europe, European history between the 8th century BC and the 5th century AD comprising the inter ...
, the
Islamic Golden Age The Islamic Golden Age was a period of scientific, economic, and cultural flourishing in the history of Islam, traditionally dated from the 8th century to the 13th century. This period is traditionally understood to have begun during the reign o ...
, the European
Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the 5th to the late 15th centuries, similarly to the post-classical period of global history. It began with the fall of the Western Roman Empire and ...
and the
Age of Discovery The Age of Discovery (), also known as the Age of Exploration, was part of the early modern period and overlapped with the Age of Sail. It was a period from approximately the 15th to the 17th century, during which Seamanship, seafarers fro ...
for all these purposes. The astrolabe, which is a precursor to the sextant, is effective for determining latitude on land or calm seas. Although it is less reliable on the heaving deck of a ship in rough seas, the
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 (declination) or the meridian altitude of a star of known declination. Not an astrolabe ...
was developed to solve that problem.


Applications

The 10th century
astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...
ʿAbd al-Raḥmān al-Ṣūfī wrote a massive text of 386 chapters on the astrolabe, which reportedly described more than 1,000 applications for the astrolabe's various functions. These ranged from the astrological, the astronomical and the religious, to navigation, seasonal and daily time-keeping, and tide tables. At the time of their use, astrology was widely considered as much of a serious science as astronomy, and study of the two went hand-in-hand. The astronomical interest varied between folk astronomy (of the pre-Islamic tradition in Arabia) which was concerned with celestial and seasonal observations, and mathematical astronomy, which would inform intellectual practices and precise calculations based on astronomical observations. In regard to the astrolabe's religious function, the demands of Islamic prayer times were to be astronomically determined to ensure precise daily timings, and the
qibla The qibla () is the direction towards the Kaaba in the Great Mosque of Mecca, Sacred Mosque in Mecca, which is used by Muslims in various religious contexts, particularly the direction of prayer for the salah. In Islam, the Kaaba is believed to ...
, the direction of
Mecca Mecca, officially Makkah al-Mukarramah, is the capital of Mecca Province in the Hejaz region of western Saudi Arabia; it is the Holiest sites in Islam, holiest city in Islam. It is inland from Jeddah on the Red Sea, in a narrow valley above ...
towards which Muslims must pray, could also be determined by this device. In addition to this, the
lunar calendar A lunar calendar is a calendar based on the monthly cycles of the Moon's phases ( synodic months, lunations), in contrast to solar calendars, whose annual cycles are based on the solar year, and lunisolar calendars, whose lunar months are br ...
that was informed by the calculations of the astrolabe was of great significance to the religion of Islam, given that it determines the dates of important religious observances such as
Ramadan Ramadan is the ninth month of the Islamic calendar. It is observed by Muslims worldwide as a month of fasting (''Fasting in Islam, sawm''), communal prayer (salah), reflection, and community. It is also the month in which the Quran is believed ...
.


Etymology

The ''
Oxford English Dictionary The ''Oxford English Dictionary'' (''OED'') is the principal historical dictionary of the English language, published by Oxford University Press (OUP), a University of Oxford publishing house. The dictionary, which published its first editio ...
'' gives the translation "star-taker" for the English word ''astrolabe'' and traces it through medieval Latin to the
Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...
word : , from : "star", and : "to take". In the medieval Islamic world the
Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...
word (i.e., astrolabe) was given various etymologies. In Arabic texts, the word is translated as (, ) – a direct translation of the Greek word.
Al-Biruni Abu Rayhan Muhammad ibn Ahmad al-Biruni (; ; 973after 1050), known as al-Biruni, was a Khwarazmian Iranian scholar and polymath during the Islamic Golden Age. He has been called variously "Father of Comparative Religion", "Father of modern ...
quotes and criticises medieval scientist
Hamza al-Isfahani Ḥamza ibn al-Ḥasan ibn al-Mū'addib al-Iṣbahānī Abū ‘Abd Allāh (; – after 961), commonly known as Ḥamza al-Iṣfahānī or Hamza Esfahani (), was a Persian philologist and historian, who wrote in Arabic during the 'Abbasid and ...
, who stated: : "''asturlab'' is an Arabisation of this Persian phrase" (, meaning "taker of the stars"). In medieval Islamic sources, there is also a
folk etymology Folk etymology – also known as (generative) popular etymology, analogical reformation, (morphological) reanalysis and etymological reinterpretation – is a change in a word or phrase resulting from the replacement of an unfamiliar form by a mo ...
of the word as "lines of lab", where "Lab" refers to a certain son of
Idris Idris may refer to: People * Idris (name), a list of people and fictional characters with the given name or surname * Idris (prophet), Islamic prophet in the Qur'an, traditionally identified with Enoch, an ancestor of Noah in the Bible * Idris ...
(
Enoch Enoch ( ; ''Henṓkh'') is a biblical figure and Patriarchs (Bible), patriarch prior to Noah's flood, and the son of Jared (biblical figure), Jared and father of Methuselah. He was of the Antediluvian period in the Hebrew Bible. The text of t ...
). This etymology is mentioned by a 10th century scientist named al-Qummi but rejected by
al-Khwarizmi Muhammad ibn Musa al-Khwarizmi , or simply al-Khwarizmi, was a mathematician active during the Islamic Golden Age, who produced Arabic-language works in mathematics, astronomy, and geography. Around 820, he worked at the House of Wisdom in B ...
.


History


Ancient era

An astrolabe is essentially a plane (two-dimensional) version of an armillary sphere, which had already been invented in the
Hellenistic period In classical antiquity, the Hellenistic period covers the time in Greek history after Classical Greece, between the death of Alexander the Great in 323 BC and the death of Cleopatra VII in 30 BC, which was followed by the ascendancy of the R ...
and probably been used by
Hipparchus Hipparchus (; , ;  BC) was a Ancient Greek astronomy, Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of the precession of the equinoxes. Hippar ...
to produce his star catalogue.
Theon of Alexandria Theon of Alexandria (; ; ) was a Greek scholar and mathematician who lived in Alexandria, Egypt. He edited and arranged Euclid's '' Elements'' and wrote commentaries on works by Euclid and Ptolemy. His daughter Hypatia also won fame as a mathema ...
() wrote a detailed treatise on the astrolabe. The invention of the plane astrolabe is sometimes wrongly attributed to Theon's daughter
Hypatia Hypatia (born 350–370 – March 415 AD) was a Neoplatonist philosopher, astronomer, and mathematician who lived in Alexandria, Egypt (Roman province), Egypt: at that time a major city of the Eastern Roman Empire. In Alexandria, Hypatia was ...
(born ; died but it is known to have been used much earlier. The misattribution comes from a misinterpretation of a statement in a letter written by Hypatia's pupil
Synesius Synesius of Cyrene (; ; c. 373 – c. 414) was a Greek bishop of Ptolemais in ancient Libya, a part of the Western Pentapolis of Cyrenaica after 410. He was born of wealthy parents at Balagrae (now Bayda, Libya) near Cyrene between 370 and 3 ...
(), which mentions that Hypatia had taught him how to construct a plane astrolabe, but does not say that she invented it. Lewis argues that
Ptolemy Claudius Ptolemy (; , ; ; – 160s/170s AD) was a Greco-Roman mathematician, astronomer, astrologer, geographer, and music theorist who wrote about a dozen scientific treatises, three of which were important to later Byzantine science, Byzant ...
used an astrolabe to make the astronomical observations recorded in the '' Tetrabiblos''. However, Emilie Savage-Smith notes :"there is no convincing evidence that Ptolemy or any of his predecessors knew about the planispheric astrolabe". In chapter 5.1 of the ''
Almagest The ''Almagest'' ( ) is a 2nd-century Greek mathematics, mathematical and Greek astronomy, astronomical treatise on the apparent motions of the stars and planetary paths, written by Ptolemy, Claudius Ptolemy ( ) in Koine Greek. One of the most i ...
'', Ptolemy describes the construction of an armillary sphere, and it is usually assumed that this was the instrument he used. Astrolabes continued to be used in the
Byzantine Empire The Byzantine Empire, also known as the Eastern Roman Empire, was the continuation of the Roman Empire centred on Constantinople during late antiquity and the Middle Ages. Having survived History of the Roman Empire, the events that caused the ...
. Christian philosopher
John Philoponus John Philoponus ( Greek: ; , ''Ioánnis o Philóponos''; c. 490 – c. 570), also known as John the Grammarian or John of Alexandria, was a Coptic Miaphysite philologist, Aristotelian commentator and Christian theologian from Alexandria, Byza ...
wrote a treatise () on the astrolabe in Greek, which is the earliest extant treatise on the instrument.
Mesopotamian Mesopotamia is a historical region of West Asia situated within the Tigris–Euphrates river system, in the northern part of the Fertile Crescent. Today, Mesopotamia is known as present-day Iraq and forms the eastern geographic boundary o ...
bishop Severus Sebokht also wrote a treatise on the astrolabe in the
Syriac language The Syriac language ( ; ), also known natively in its spoken form in early Syriac literature as Edessan (), the Mesopotamian language () and Aramaic (), is an Aramaic#Eastern Middle Aramaic, Eastern Middle Aramaic dialect. Classical Syriac is ...
during the mid-7th century. Sebokht refers to the astrolabe as being made of brass in the introduction of his treatise, indicating that metal astrolabes were known in the Christian East well before they were developed in the Islamic world or in the Latin West.


Medieval era

Astrolabes were further developed in the medieval Islamic world, where Muslim astronomers introduced angular scales to the design, adding circles indicating
azimuth An azimuth (; from ) is the horizontal angle from a cardinal direction, most commonly north, in a local or observer-centric spherical coordinate system. Mathematically, the relative position vector from an observer ( origin) to a point ...
s on the
horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...
. It was widely used throughout the Muslim world, chiefly as an aid to navigation and as a way of finding the
Qibla The qibla () is the direction towards the Kaaba in the Great Mosque of Mecca, Sacred Mosque in Mecca, which is used by Muslims in various religious contexts, particularly the direction of prayer for the salah. In Islam, the Kaaba is believed to ...
, the direction of
Mecca Mecca, officially Makkah al-Mukarramah, is the capital of Mecca Province in the Hejaz region of western Saudi Arabia; it is the Holiest sites in Islam, holiest city in Islam. It is inland from Jeddah on the Red Sea, in a narrow valley above ...
. Eighth-century
mathematician A mathematician is someone who uses an extensive knowledge of mathematics in their work, typically to solve mathematical problems. Mathematicians are concerned with numbers, data, quantity, mathematical structure, structure, space, Mathematica ...
Muhammad al-Fazari is the first person credited with building the astrolabe in the Islamic world. The mathematical background was established by Muslim astronomer Albatenius in his treatise ''Kitab az-Zij'' which was translated into Latin by Plato Tiburtinus (''De Motu Stellarum''). The earliest surviving astrolabe is dated AH 315 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 ''Salah'' (, also spelled ''salat'') is the practice of formal ibadah, worship in Islam, consisting of a series of ritual prayers performed at prescribed times daily. These prayers, which consist of units known as rak'a, ''rak'ah'', include ...
). In the 10th century, al-Sufi first described over 1,000 different uses of an astrolabe, in areas as diverse as
astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...
,
astrology Astrology is a range of Divination, divinatory practices, recognized as pseudoscientific since the 18th century, that propose that information about human affairs and terrestrial events may be discerned by studying the apparent positions ...
,
navigation Navigation is a field of study that focuses on the process of monitoring and controlling the motion, movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navig ...
,
surveying Surveying or land surveying is the technique, profession, art, and science of determining the land, terrestrial Plane (mathematics), two-dimensional or Three-dimensional space#In Euclidean geometry, three-dimensional positions of Point (geom ...
, timekeeping, prayer,
Salat ''Salah'' (, also spelled ''salat'') is the practice of formal ibadah, worship in Islam, consisting of a series of ritual prayers performed at prescribed times daily. These prayers, which consist of units known as rak'a, ''rak'ah'', include ...
,
Qibla The qibla () is the direction towards the Kaaba in the Great Mosque of Mecca, Sacred Mosque in Mecca, which is used by Muslims in various religious contexts, particularly the direction of prayer for the salah. In Islam, the Kaaba is believed to ...
, etc. The spherical astrolabe was a variation of both the astrolabe and the armillary sphere, invented during the
Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the 5th to the late 15th centuries, similarly to the post-classical period of global history. It began with the fall of the Western Roman Empire and ...
by astronomers and
inventors An invention is a unique or novelty (patent), novel machine, device, Method_(patent), method, composition, idea, or process. An invention may be an improvement upon a machine, product, or process for increasing efficiency or lowering cost. It m ...
in the Islamic world. The earliest description of the spherical astrolabe dates to Al-Nayrizi (
fl. ''Floruit'' ( ; usually abbreviated fl. or occasionally flor.; from Latin for 'flourished') denotes a date or period during which a person was known to have been alive or active. In English, the unabbreviated word may also be used as a noun indic ...
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 geared mechanical astrolabe was invented by Abi Bakr of
Isfahan Isfahan or Esfahan ( ) is a city in the Central District (Isfahan County), Central District of Isfahan County, Isfahan province, Iran. It is the capital of the province, the county, and the district. It is located south of Tehran. The city ...
in 1235. The first known metal astrolabe in Western Europe is the Destombes astrolabe made from brass in the eleventh century in Portugal. Metal astrolabes avoided the warping that large wooden ones were prone to, allowing the construction of larger and therefore more accurate instruments. Metal astrolabes were heavier than wooden instruments of the same size, making it difficult to use them in navigation. Herman Contractus of Reichenau Abbey, examined the use of the astrolabe in ''Mensura Astrolai'' during the 11th century. Peter of Maricourt wrote a treatise on the construction and use of a universal astrolabe in the last half of the 13th century entitled ''Nova compositio astrolabii particularis''. Universal astrolabes can be found at the History of Science Museum, Oxford. David A. King, historian of Islamic instrumentation, describes the universal astrolobe designed by Ibn al-Sarraj of
Aleppo Aleppo is a city in Syria, which serves as the capital of the Aleppo Governorate, the most populous Governorates of Syria, governorate of Syria. With an estimated population of 2,098,000 residents it is Syria's largest city by urban area, and ...
(a.k.a. Ahmad bin Abi Bakr; fl. 1328) as "the most sophisticated astronomical instrument from the entire Medieval and Renaissance periods". English author
Geoffrey Chaucer Geoffrey Chaucer ( ; – 25 October 1400) was an English poet, author, and civil servant best known for ''The Canterbury Tales''. He has been called the "father of English literature", or, alternatively, the "father of English poetry". He w ...
() compiled '' A Treatise on the Astrolabe'' for his son, mainly based on a work by Messahalla or Ibn al-Saffar. The same source was translated by French astronomer and astrologer Pélerin de Prusse and others. The first printed book on the astrolabe was ''Composition and Use of Astrolabe'' by
Christian of Prachatice Christian of Prachatice () (1360–1368, Prachatice, Kingdom of Bohemia – 4 September 1439, Prague, Kingdom of Bohemia) was a medieval Bohemian astronomer, mathematician and former priesthood (Catholic Church), Catholic priest who converted to ...
, also using Messahalla, but relatively original. In 1370, the first Indian treatise on the astrolabe was written by the Jain astronomer Mahendra Suri, titled ''Yantrarāja''. A simplified astrolabe, known as a ''balesilha'', was used by sailors to get an accurate reading of latitude while at sea. The use of the ''balesilha'' was promoted by Prince Henry (1394–1460) while navigating for Portugal. The astrolabe was almost certainly first brought north of the Pyrenees by Gerbert of Aurillac (future
Pope Sylvester II Pope Sylvester II (; – 12 May 1003), originally known as Gerbert of Aurillac, was a scholar and teacher who served as the bishop of Rome and ruled the Papal States from 999 to his death. He endorsed and promoted study of Science in the medieva ...
), where it was integrated into the
quadrivium From the time of Plato through the Middle Ages, the ''quadrivium'' (plural: quadrivia) was a grouping of four subjects or arts—arithmetic, geometry, music, and astronomy—that formed a second curricular stage following preparatory work in th ...
at the school in Reims, France, sometime before the turn of the 11th century. In the 15th century, French instrument maker Jean Fusoris () also started remaking and selling astrolabes in his shop in
Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, largest city of France. With an estimated population of 2,048,472 residents in January 2025 in an area of more than , Paris is the List of ci ...
, along with portable sundials and other popular scientific devices of the day. Thirteen of his astrolabes survive to this day. One more special example of craftsmanship in early 15th-century Europe is the astrolabe designed by Antonius de Pacento and made by Dominicus de Lanzano, dated 1420. In the 16th century,
Johannes Stöffler Johannes Stöffler (also ''Stöfler, Stoffler, Stoeffler''; 10 December 1452 – 16 February 1531) was a German mathematician, astronomer, astrologer, priest, maker of astronomical instruments and professor at the University of Tübingen. Life Jo ...
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 Batch production is a method of manufacturing in which products are made as specified groups or amounts, within a time frame. A batch can go through a series of steps in a large manufacturing process to make the final desired product. Batch produ ...
by
division of labor The division of labour is the separation of the tasks in any economic system or organisation so that participants may specialise (Departmentalization, specialisation). Individuals, organisations, and nations are endowed with or acquire specialis ...
. Greek painter Ieremias Palladas incorporated a sophisticated astrolabe in his 1612 painting depicting
Catherine of Alexandria Catherine of Alexandria, also spelled Katherine, was, according to tradition, a Christian saint and Virginity, virgin, who was martyred in the early 4th century at the hands of the emperor Maxentius. According to her hagiography, she was both a ...
. The painting, entitled ''Catherine of Alexandria''; in addition to the saint, showed a device labelled the 'system of the universe' (). The device featured the classical planets with their Greek names:
Helios In ancient Greek religion and Greek mythology, mythology, Helios (; ; Homeric Greek: ) is the god who personification, personifies the Sun. His name is also Latinized as Helius, and he is often given the epithets Hyperion ("the one above") an ...
(Sun),
Selene In ancient Greek mythology and Ancient Greek religion, religion, Selene (; , meaning "Moon")''A Greek–English Lexicon's.v. σελήνη is the goddess and personification of the Moon. Also known as Mene (), she is traditionally the daughter ...
(Moon),
Hermes Hermes (; ) is an Olympian deity in ancient Greek religion and mythology considered the herald of the gods. He is also widely considered the protector of human heralds, travelers, thieves, merchants, and orators. He is able to move quic ...
(Mercury),
Aphrodite Aphrodite (, ) is an Greek mythology, ancient Greek goddess associated with love, lust, beauty, pleasure, passion, procreation, and as her syncretism, syncretised Roman counterpart , desire, Sexual intercourse, sex, fertility, prosperity, and ...
(Venus),
Ares Ares (; , ''Árēs'' ) is the List of Greek deities, Greek god of war god, war and courage. He is one of the Twelve Olympians, and the son of Zeus and Hera. The Greeks were ambivalent towards him. He embodies the physical valor necessary for ...
(Mars),
Zeus Zeus (, ) is the chief deity of the List of Greek deities, Greek pantheon. He is a sky father, sky and thunder god in ancient Greek religion and Greek mythology, mythology, who rules as king of the gods on Mount Olympus. Zeus is the child ...
(Jupiter), and Cronos (Saturn). The depicted device also had celestial spheres, following the Ptolemaic model, and Earth was shown as a blue sphere with circles of geographic coordinates. A complicated line representing the axis of the Earth covered the entire instrument.


Astrolabes and clocks

Mechanical
astronomical clock An astronomical clock, horologium, or orloj 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 initially influenced by the astrolabe; they could be seen in many ways 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 (1292–1336) was an English mathematician, astronomer, horologist, and cleric who made major contributions to astronomy and horology while serving as abbot of St Albans Abbey in Hertfordshire. Biography Richard was b ...
's clock () consisted essentially of a star map rotating behind a fixed rete, similar to that of an astrolabe. Many astronomical clocks use an astrolabe-style display, such as the famous clock at Prague, adopting a stereographic projection (see below) of the ecliptic plane. In recent times, astrolabe watches have become popular. For example, Swiss watchmaker Ludwig Oechslin designed and built an astrolabe wristwatch in conjunction with Ulysse Nardin in 1985. Dutch watchmaker Christaan van der Klauuw also manufactures astrolabe watches today.


Construction

An astrolabe consists of a disk with a wide, raised rim, 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 In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...
and is engraved with a
stereographic projection In mathematics, a stereographic projection is a perspective transform, perspective projection of the sphere, through a specific point (geometry), point on the sphere (the ''pole'' or ''center of projection''), onto a plane (geometry), plane (th ...
of
circle A circle is a shape consisting of all point (geometry), points in a plane (mathematics), plane that are at a given distance from a given point, the Centre (geometry), centre. The distance between any point of the circle and the centre is cal ...
s denoting
azimuth An azimuth (; from ) is the horizontal angle from a cardinal direction, most commonly north, in a local or observer-centric spherical coordinate system. Mathematically, the relative position vector from an observer ( origin) to a point ...
and
altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...
and representing the portion of the
celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...
above the local horizon. The rim of the mater is typically graduated into hours of time, degrees of arc, or both. Above the mater and tympan, the ''rete'', a framework bearing a projection of the
ecliptic The ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...
plane and several
pointers Pointer may refer to: People with the name * Pointer (surname), a surname (including a list of people with the name) * Pointer Williams (born 1974), American former basketball player Arts, entertainment, and media * ''Pointer'' (journal), the ...
indicating the positions of the brightest
star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...
s, is free to rotate. These pointers are often just simple points, but depending on the skill of the craftsman can be very elaborate and artistic. There are examples of astrolabes with artistic pointers in the shape of balls, stars, snakes, hands, dogs' heads, and leaves, among others. The names of the indicated stars were often engraved on the pointers in Arabic or Latin. Some astrolabes have a narrow '' rule'' or ''label'' which rotates over the rete, and may be marked with a scale of
declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. The declination angle is measured north (positive) or ...
s. The rete, representing the
sky The sky is an unobstructed view upward from the planetary surface, surface of the Earth. It includes the atmosphere of Earth, atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from ...
, functions as a
star chart A star chart is a celestial map of the night sky with astronomical objects laid out on a grid system. They are used to identify and locate constellations, stars, nebulae, galaxies, and planets. They have been used for human navigation since tim ...
. When it is rotated, the stars and the
ecliptic The ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...
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. 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 A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single and specific day within such a system. A calendar is ...
for converting the day of the month to the sun's position on the ecliptic, trigonometric scales, and graduation of 360 degrees around the back edge. The ''
alidade An alidade () (archaic forms include alhidade, alhidad, alidad) or a turning board 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 Triangulation (surveying), tr ...
'' 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. The alidade had vertical and horizontal cross-hairs which plots locations on an azimuthal ring called an almucantar (altitude-distance circle). An arm called a radius connects from the center of the astrolabe to the optical axis which is parallel with another arm also called a radius. The other radius contains graduations of altitude and distance measurements. A shadow square also appears on the back of some astrolabes, developed by Muslim astrologists in the 9th Century, whereas devices of the Ancient Greek tradition featured only altitude scales on the back of the devices. This was used to convert shadow lengths and the altitude of the sun, the uses of which were various from surveying to measuring inaccessible heights. Devices were usually signed by their maker with an inscription appearing on the back of the astrolabe, and if there was a patron of the object, their name would appear inscribed on the front, or in some cases, the name of the reigning sultan or the teacher of the astrolabist has also been found to appear inscribed in this place. The date of the astrolabe's construction was often also signed, which has allowed historians to determine that these devices are the second oldest scientific instrument in the world. The inscriptions on astrolabes also allowed historians to conclude that astronomers tended to make their own astrolabes, but that many were also made to order and kept in stock to sell, suggesting there was some contemporary market for the devices.


Mathematical basis

The construction and design of astrolabes are based on the application of the
stereographic projection In mathematics, a stereographic projection is a perspective transform, perspective projection of the sphere, through a specific point (geometry), point on the sphere (the ''pole'' or ''center of projection''), onto a plane (geometry), plane (th ...
of the
celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...
. The point from which the projection is usually made is the
South Pole The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is the point in the Southern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True South Pole to distinguish ...
. The plane onto which the projection is made is that of the
Equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...
.


Designing a tympanum through stereographic projection

The tympanum captures the celestial coordinate axes upon which the ''rete'' will rotate. It is the component that will enable the precise determination of a star's position at a specific time of
day A day is the time rotation period, period of a full Earth's rotation, rotation of the Earth with respect to the Sun. On average, this is 24 hours (86,400 seconds). As a day passes at a given location it experiences morning, afternoon, evening, ...
and
year A year is a unit of time based on how long it takes the Earth to orbit the Sun. In scientific use, the tropical year (approximately 365 Synodic day, solar days, 5 hours, 48 minutes, 45 seconds) and the sidereal year (about 20 minutes longer) ...
. Therefore, it should project: # The
zenith The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (Vertical and horizontal, plumb line) opposite to the gravity direction at that location (nadir). The z ...
, which will vary depending on the
latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...
of the astrolabe user. # The horizon line and almucantar or circles parallel to the horizon, which will allow for the determination of a celestial body's
altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...
(from the horizon to the zenith). # The celestial meridian (north-south meridian, passing through the zenith) and secondary meridians (circles intersecting the north-south meridian at the zenith), which will enable the measurement of
azimuth An azimuth (; from ) is the horizontal angle from a cardinal direction, most commonly north, in a local or observer-centric spherical coordinate system. Mathematically, the relative position vector from an observer ( origin) to a point ...
for a celestial body. # The three main circles of latitude ( Capricorn,
Equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...
, and
Cancer Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Po ...
) to determine the exact moments of
solstice A solstice is the time when the Sun reaches its most northerly or southerly sun path, excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around 20–22 June and 20–22 December. In many countries ...
s and
equinox A solar equinox is a moment in time when the Sun appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, arou ...
es throughout the year.


The tropics and the equator define the tympanum

On the right side of the image above: # The blue sphere represents the
celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...
. # The blue arrow indicates the direction of true north (the North Star). # The central blue point represents Earth (the observer's location). # The geographic south of the celestial sphere acts as the projection pole. # The celestial equatorial plane serves as the projection plane. # Three parallel circles represent the projection on the celestial sphere of Earth's main circles of latitude: #* In orange, the celestial
Tropic of Cancer The Tropic of Cancer, also known as the Northern Tropic, is the Earth's northernmost circle of latitude where the Sun can be seen directly overhead. This occurs on the June solstice, when the Northern Hemisphere is tilted toward the Sun ...
. #* In purple, the celestial
equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...
. #* In green, the celestial
Tropic of Capricorn The Tropic of Capricorn (or the Southern Tropic) is the circle of latitude that contains the subsolar point at the December (or southern) solstice. It is thus the southernmost latitude where the Sun can be seen directly overhead. It also reach ...
. When projecting onto the celestial equatorial plane, three concentric circles correspond to the celestial sphere's three circles of latitude (left side of the image). The largest of these, the projection on the celestial equatorial plane of the celestial
Tropic of Capricorn The Tropic of Capricorn (or the Southern Tropic) is the circle of latitude that contains the subsolar point at the December (or southern) solstice. It is thus the southernmost latitude where the Sun can be seen directly overhead. It also reach ...
, defines the size of the astrolabe's tympanum. The center of the tympanum (and the center of the three circles) is actually the north-south axis around which Earth rotates, and therefore, the ''rete'' of the astrolabe will rotate around this point as the hours of the day pass (due to Earth's rotational motion). The three concentric circles on the tympanum are useful for determining the exact moments of
solstice A solstice is the time when the Sun reaches its most northerly or southerly sun path, excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around 20–22 June and 20–22 December. In many countries ...
s and
equinox A solar equinox is a moment in time when the Sun appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, arou ...
es throughout the year: if the sun's altitude at noon on the ''rete'' is known and coincides with the outer circle of the tympanum (Tropic of Capricorn), it signifies the
winter solstice The winter solstice, or hibernal solstice, occurs when either of Earth's geographical pole, poles reaches its maximum axial tilt, tilt away from the Sun. This happens twice yearly, once in each hemisphere (Northern Hemisphere, Northern and So ...
(the sun will be at the
zenith The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (Vertical and horizontal, plumb line) opposite to the gravity direction at that location (nadir). The z ...
for an observer at the Tropic of Capricorn, meaning summer in the southern hemisphere and winter in the northern hemisphere). If, on the other hand, its altitude coincides with the inner circle (Tropic of Cancer), it indicates the
summer solstice The summer solstice or estival solstice occurs when one of Earth's poles has its maximum tilt toward the Sun. It happens twice yearly, once in each hemisphere ( Northern and Southern). The summer solstice is the day with the longest peri ...
. If its altitude is on the middle circle (equator), it corresponds to one of the two
equinox A solar equinox is a moment in time when the Sun appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, arou ...
es.


The horizon and the measurement of altitude

On the right side of the image above: # The blue arrow indicates the direction of true north (the North Star). # The central blue point represents Earth (the observer's location). # The black arrow represents the
zenith The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (Vertical and horizontal, plumb line) opposite to the gravity direction at that location (nadir). The z ...
direction for the observer (which would vary depending on the observer's
latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...
). # The two black circles represent the
horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...
surrounding the observer, which is perpendicular to the zenith vector and defines the portion of the
celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...
visible to the observer, and its projection on the celestial equatorial plane. # The geographic south of the celestial sphere acts as the projection pole. # The celestial equatorial plane serves as the projection plane. When projecting the
horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...
onto the celestial equatorial plane, it transforms into an ellipse upward-shifted relatively to the center of the tympanum (both the observer and the projection of the north-south axis). This implies that a portion of the celestial sphere will fall outside the outer circle of the tympanum (the projection of the celestial
Tropic of Capricorn The Tropic of Capricorn (or the Southern Tropic) is the circle of latitude that contains the subsolar point at the December (or southern) solstice. It is thus the southernmost latitude where the Sun can be seen directly overhead. It also reach ...
) and, therefore, won't be represented. Additionally, when drawing circles parallel to the horizon up to the zenith ( almucantar), and projecting them on the celestial equatorial plane, as in the image above, a grid of consecutive ellipses is constructed, allowing for the determination of a star's altitude when its ''rete'' overlaps with the designed tympanum.


The meridians and the measurement of azimuth

On the right side of the image above: # The blue arrow indicates the direction of true north (the North Star). # The central blue point represents Earth (the observer's location). # The black arrow represents the
zenith The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (Vertical and horizontal, plumb line) opposite to the gravity direction at that location (nadir). The z ...
direction for the observer (which would vary depending on the observer's
latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...
). # The two black circles represent the
horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...
surrounding the observer, which is perpendicular to the zenith vector and defines the portion of the
celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...
visible to the observer, and its projection on the celestial equatorial plane. # The five red dots represent the
zenith The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (Vertical and horizontal, plumb line) opposite to the gravity direction at that location (nadir). The z ...
, the
nadir The nadir is the direction pointing directly ''below'' a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface. The direction opposite of the nadir is the zenith. Et ...
(the point on the
celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...
opposite the zenith with respect to the observer), their projections on the celestial equatorial plane, and the center (with no physical meaning attached) of the circle obtained by projecting the secondary meridian (see below) on the celestial equatorial plane. # The orange circle represents the celestial meridian (or meridian that goes, for the observer, from the north of the horizon to the south of the horizon passing through the zenith). # The two red circles represent a secondary meridian with an
azimuth An azimuth (; from ) is the horizontal angle from a cardinal direction, most commonly north, in a local or observer-centric spherical coordinate system. Mathematically, the relative position vector from an observer ( origin) to a point ...
of 40° East relative to the observer's horizon (which, like all secondary meridians, intersects the principal meridian at the zenith and nadir), and its projection on the celestial equatorial plane. # The geographic south of the celestial sphere acts as the projection pole. # The celestial equatorial plane serves as the projection plane. When projecting the celestial meridian, it results in a straight line that overlaps with the vertical axis of the tympanum, where the
zenith The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (Vertical and horizontal, plumb line) opposite to the gravity direction at that location (nadir). The z ...
and
nadir The nadir is the direction pointing directly ''below'' a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface. The direction opposite of the nadir is the zenith. Et ...
are located. However, when projecting the 40° E meridian, another circle is obtained that passes through both the zenith and nadir projections, so its center is located on the perpendicular
bisection In geometry, bisection is the division of something into two equal or congruent parts (having the same shape and size). Usually it involves a bisecting line, also called a ''bisector''. The most often considered types of bisectors are the ''s ...
of the segment connecting both points. In deed, the projection of the celestial meridian can be considered as a circle with an infinite radius (a straight line) whose center is on this bisection and at an infinite distance from these two points. If successive meridians that divide the celestial sphere into equal sectors (like "orange slices" radiating from the zenith) are projected, a family of curves passing through the zenith projection on the tympanum is obtained. These curves, once overlaid with the ''rete'' containing the major stars, allow for determining the
azimuth An azimuth (; from ) is the horizontal angle from a cardinal direction, most commonly north, in a local or observer-centric spherical coordinate system. Mathematically, the relative position vector from an observer ( origin) to a point ...
of a star located on the ''rete'' and rotated for a specific time of day.


See also

*
astronomy in the medieval Islamic world Medieval Islamic astronomy comprises the astronomical developments made in the Islamic world, particularly during the Islamic Golden Age (9th–13th centuries), and mostly written in the Arabic language. These developments mostly took place in th ...
*
equatorium An equatorium (plural, equatoria) is an astronomy, astronomical Mechanical calculator, calculating instrument. It can be used for finding the positions of the Moon, Sun, and planets without arithmetic operations, using a geometrical model to re ...
* Hamburg Planetarium *
list of astronomical instruments An astronomical instrument is a device for observing, measuring or recording astronomical data. They are used in the scientific field of astronomy, a natural science that studies celestial objects and the phenomena that occur in the cosmos, with ...
* Philippe Danfrie, designer and maker of
mathematical instrument A mathematical instrument is a tool or device used in the study or practice of mathematics. In geometry, construction of various proofs was done using only a compass and straightedge; arguments in these proofs relied only on idealized properti ...
s, globes and astrolabes * planisphere * planetarium * Volvelle * Yantraraja * Zeiss-Planetarium Jena


Footnotes


References


Bibliography

* * * * * * * * * * *


External links


Interactive digital astrolabe by Alex Boxer



Astrolabe Tech Made ... Not So Easy

paper astrolabe generator, from the ESO

"Hello World!" for the Astrolabe: The First Computer
Video of Howard Covitz's Presentation at Ignite Phoenix, June 2009
Slides for Presentation
Licensed as Creative Commons by-nc-nd.
Video of Tom Wujec demonstrating an astrolabe.
Taken at TEDGlobal 2009. Includes clickable transcript. Licensed as Creative Commons by-nc-nd.
Archive of James E. Morrison's extensive website on Astrolabes

A working model of the Dr. Ludwig Oechslin's Astrolabium Galileo Galilei watch

Ulysse Nardin Astrolabium Galilei Galileo: A Detailed Explanation

Fully illustrated online catalogue of world's largest collection of astrolabes

Mobile astrolabe and horologium
* Medieval equal hou
horary quadrant
* {{Authority control Analog computers Ancient Greek astronomy Ancient Greek technology Astrometry Astronomical instruments Astronomy in the medieval Islamic world Greek inventions Historical scientific instruments Inclinometers Marine navigation Mechanical calculators Navigational equipment Technology in the medieval Islamic world Scientific equipment