The (Edited from Wikipedia)

SUMMARY

An astrolabe is an elaborate inclinometer, historically used by astronomers, navigators, and astrologers. Its many uses include locating and predicting the positions of the , , , and , determining local time given local and vice versa, surveying, and . It was used in classical antiquity, the , the European and for all these purposes. In the Islamic world, it was also used to calculate the and to find the times for Salat prayers.

There is often confusion between the astrolabe and the mariner's astrolabe. While the astrolabe could be useful for determining latitude on , it was an awkward instrument for use on the heaving deck of a ship or in wind. The mariner's astrolabe was developed to solve these problems.

HISTORY

The Influence of Ancient

Old Babylonian refers to the astronomy that was practiced during and after the First Babylonian Dynasty (ca. 1830 BC) and before the Neo-Babylonian Empire (ca. 626 BC).

The Babylonians were the first to recognize that astronomical phenomena are periodic and apply mathematics to their predictions. Tablets dating back to the Old Babylonian period document the application of mathematics to the variation in the length of daylight over a solar year. Centuries of Babylonian observations of celestial phenomena are recorded in the series of cuneiform tablets known as the Enûma Anu Enlil—the oldest significant astronomical text that we possess is Tablet 63 of the Enûma Anu Enlil, the Venus tablet of Ammisaduqa, which lists the first and last visible risings of Venus over a period of about 21 years. It is the earliest evidence that planetary phenomena were recognized as periodic.

The MUL.APIN contains catalogues of stars and as well as schemes for predicting heliacal risings and settings of the planets, and lengths of daylight as measured by a clock, , shadows, and intercalations. The Babylonian GU text arranges stars in 'strings' that lie along declination circles and thus measure right-

1 ascensions or time intervals, and also employs the stars of the zenith, which are also separated by given right-ascensional differences. There are dozens of cuneiform Mesopotamian texts with real observations of eclipses, mainly from .

Babylonian Planetary Theory

The Babylonians were the first civilization known to possess a functional theory of the planets. The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th-century BC copy of a list of observations of the motions of the Venus that probably dates as early as the second millennium BC.

The Babylonian astrologers also laid the foundations of what would eventually become Western astrology. The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC, comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.

In Babylonian cosmology, the and the heavens were depicted as a "spatial whole, even one of round shape" with references to "the of heaven and earth" and "the totality of heaven and earth". Their worldview was not exactly geocentric either. The idea of geocentrism, where the center of the Earth is the exact center of the universe, did not yet exist in Babylonian cosmology, but was established later by the Greek philosopher 's . In contrast, Babylonian cosmology suggested that the cosmos revolved around circularly with the heavens and the earth being equal and joined as a whole.

Arithmetic and Geometry in

Though there is a lack of surviving material on Babylonian planetary theory, it appears most of the Chaldean astronomers were concerned mainly with ephemerides ( tables) and not with theory. It had been thought that most of the predictive Babylonian planetary models that have survived were usually strictly empirical and arithmetical, and usually did not involve geometry, cosmology, or speculative philosophy like that of the later Hellenistic (Greek) models, though the Babylonian astronomers were concerned with the philosophy dealing with the ideal nature of the early universe. Babylonian procedure texts describe, and ephemerides employ, arithmetical procedures to compute the time and place of significant astronomical events. Babylonian astronomers sometimes used geometrical methods, prefiguring the methods to describe the motion of Jupiter over time in an abstract mathematical space.

In contrast to Greek astronomy which was dependent upon cosmology (the study of the origin, evolution, and eventual fate of the universe), Babylonian astronomy was

2 independent from cosmology. Whereas Greek astronomers expressed prejudice in favor of circles or rotating with uniform motion , such a preference did not exist for Babylonian astronomers. For the Babylonians, uniform circular motion was never a requirement for planetary orbits. There is no evidence that the celestial bodies moved in uniform circular motion, or along , in Babylonian astronomy.

Contributions made by the Chaldean astronomers during this period include the discovery of eclipse cycles and saros cycles, and many accurate astronomical observations. For example, they observed that the Sun's motion along the was not uniform, though they were unaware of why this was; it is today known that this is due to the Earth moving in an elliptic orbit around the Sun, with the Earth moving swifter when it is nearer to the Sun at perihelion and moving slower when it is farther away at aphelion.

Chaldean astronomers known to have followed this model include Naburimannu (fl. 6th–3rd century BC), Kidinnu (d. 330 BC), Berossus (3rd century BCE), and (fl. 240 BCE). They are known to have had a significant influence on the Greek astronomer and the Egyptian astronomer , as well as other Hellenistic astronomers.

Transmission from the Babylonians to the Greeks

Many of the works of ancient Greek and Hellenistic writers (including mathematicians, astronomers, and geographers) have been preserved up to the present time, or some aspects of their work and thought are still known through later references. However, achievements in these fields by earlier ancient Near Eastern civilizations, notably those in Babylonia, were forgotten for a long time. Since the discovery of key archaeological sites in the 19th century, many cuneiform writings on clay tablets have been found, some of them related to astronomy.

Since the rediscovery of the Babylonian civilization, it has become apparent that Hellenistic astronomy was strongly influenced by the Chaldeans. The best documented borrowings are those of Hipparchus (2nd century BC) and Claudius Ptolemy (2nd century AD).

In 1900, Franz Xaver Kugler demonstrated that Ptolemy had stated in his IV.2 that Hipparchus improved the values for the Moon's periods known to him from "even more ancient astronomers" by comparing eclipse observations made earlier by "the Chaldeans", and by himself.

3 It is clear that Hipparchus (and Ptolemy after him) had an essentially complete list of eclipse observations covering many centuries. Most likely these had been compiled from the "diary" tablets: these are clay tablets recording all relevant observations that the Chaldeans routinely made.

All this knowledge was transferred to the Greeks probably shortly after the conquest by (331 BC). According to the late classical philosopher Simplicius (early 6th century), Alexander ordered the translation of the historical Babylonian astronomical records under supervision of his chronicler Callisthenes of Olynthus, who sent it to his uncle Aristotle. The translation of the astronomical records required profound knowledge of the cuneiform script, the language, and the procedures, so it seems likely that it was done by some unidentified Chaldeans.

Astrolabe Development

An early astrolabe was invented in the Hellenistic world by , around 220 BC or in 150 BC and is often attributed to Hipparchus. A marriage of the and , the astrolabe was effectively an analog calculator capable of working out several different kinds of problems in spherical astronomy.

Theon of wrote a detailed treatise on the astrolabe, and Lewis argues that Ptolemy used an astrolabe to make the astronomical observations recorded in the Tetrabiblos. It is generally accepted that Greek astrologers, in either the first or second centuries BC, invented the astrolabe, an instrument that measures the altitude of stars and planets above the . Some historians attribute its invention to Hypatia, the daughter of the mathematician Theon Alexandricus (c. 335 – c. 405), and others note that Synesius, a student of Hypatia, credits her for the invention in his letters.

Astrolabes continued in use in the Greek-speaking world throughout the Byzantine period. About 550 AD the Christian philosopher John Philoponus wrote a treatise on the astrolabe in Greek, which is the earliest extant Greek treatise on the instrument. In addition, Severus Sebokht, a bishop who lived in , 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 were known in the Christian East well before they were developed in the Islamic world or the West.

STEREOGRAPHIC PROJECTION

In geometry, the stereographic projection is a particular mapping (function) that projects a onto a plane. The projection is defined on the entire sphere, except at

4 one point: the projection point. Where it is defined, the mapping is smooth and bijective. It is conformal, meaning that it preserves angles. It is neither isometric nor area-preserving: that is, it preserves neither distances nor the areas of figures.

Intuitively, then, the stereographic projection is a way of picturing the sphere as the plane, with some inevitable compromises. Because the sphere and the plane appear in many areas of mathematics and its applications, so does the stereographic projection; it finds use in diverse fields including complex analysis, cartography, geology, and photography.

The stereographic projection was known to Hipparchus, Ptolemy and probably earlier to the Egyptians. It was originally known as the planisphere projection. Planisphaerium by Ptolemy is the oldest surviving document that describes it. One of its most important uses was the representation of celestial charts. The term planisphere is still used to refer to such charts.

Map-making

The fundamental problem of cartography is that no map from the sphere to the plane can accurately represent both angles and areas. In general, area-preserving map projections are preferred for statistical applications, while angle-preserving (conformal) map projections are preferred for navigation.

Stereographic projection falls into the second category. When the projection is centered at the Earth's north or , it has additional desirable properties: It sends meridians to rays emanating from the origin and parallels to circles centered at the origin.

The stereographic is the only projection that maps all circles of a sphere to circles. This property is valuable in planetary mapping when craters are typical features. The set of circles passing through the point of projection have unbounded radius, and therefore degenerate into lines.

HIPPARCHUS

Hipparchus of Nicaea was a Greek astronomer, geographer, and mathematician. He is considered the founder of but is most famous for his incidental discovery of precession of the equinoxes.

5 Hipparchus was born in Nicaea, Bithynia (now Iznik, Turkey), and probably died on the island of . He is known to have been a working astronomer at least from 162 to 127 bc.

Hipparchus is considered the greatest ancient astronomical observer and, by some, the greatest overall astronomer of antiquity. He was the first whose quantitative and accurate models for the motion of the Sun and Moon survive. For this he certainly made use of the observations and perhaps the mathematical techniques accumulated over centuries by the Babylonians and other people from Mesopotamia. He developed trigonometry and constructed trigonometric tables, and he solved several problems of .

With his solar and lunar theories and his trigonometry, he may have been the first to develop a reliable method to predict solar eclipses. His other reputed achievements include the discovery and measurement of Earth's precession, the compilation of the first comprehensive star catalog of the western world, and possibly the invention of the astrolabe, also of the , which he used during the creation of much of the star catalogue. It would be three centuries before Claudius Ptolemaeus' synthesis of astronomy would supersede the work of Hipparchus.

ASTROLABE CONSTRUCTION

An astrolabe consists of a disk, called the mater (mother), which is deep enough to hold one or more flat plates called tympans, or climates. A tympan is made for a specific latitude and is engraved with a stereographic projection of circles denoting azimuth and altitude and representing the portion of the 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 plane and several pointers indicating the positions of the brightest stars, 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. Some astrolabes have a narrow rule or label which rotates over the rete, and may be marked with a of declinations.

The rete, representing the sky, functions as a star chart. When it is rotated, the stars and the ecliptic 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.

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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 for converting the day of the month to the sun's position on the ecliptic, trigonometric scales, and a graduation of 360 degrees around the back edge.

The alidade is attached to the back face. An alidade can be seen in the lower right illustration of the 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.

THE HEAVENLY SPHERES

On the Heavens is Aristotle's chief cosmological treatise: written in 350 BC it contains his astronomical theory and his ideas on the concrete workings of the terrestrial world.

According to Aristotle in On the Heavens , the heavenly bodies are the most perfect realities, (or "substances"), whose motions are ruled by principles other than those of bodies in the . The latter are composed of one or all of the four classical elements (earth, water, , ) and are perishable; but the matter of which the heavens are made is imperishable aether, so they are not subject to generation and corruption. Hence their motions are eternal and perfect, and the perfect motion is the circular one, which, unlike the earthly up-and down-ward locomotions, can last eternally selfsame. As substances, celestial bodies have matter (aether) and form (a given period of uniform rotation). Sometimes Aristotle seems to regard them as living beings with a rational soul as their form.

This work is significant as one of the defining pillars of the Aristotelian worldview, a school of philosophy that dominated intellectual thinking for almost two millennia. Similarly, this work and others by Aristotle were important seminal works by which much of was derived.

In Aristotle's fully developed celestial model, the spherical Earth is at the centre of the universe and the planets are moved by either 47 or 55 interconnected spheres that form a unified planetary system. Aristotle says the exact number of spheres, and hence the number of movers, is to be determined by astronomical investigation, but he added additional spheres to counteract the motion of the outer spheres. Aristotle considers that these spheres are made of an unchanging fifth element, the aether. Each of these concentric spheres is moved by its own god — an unchanging divine unmoved mover, and who moves its sphere simply by virtue of being loved by it.

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In his Almagest , the astronomer Ptolemy (fl. ca. 150 AD) developed geometrical predictive models of the motions of the stars and planets and extended them to a unified physical model of the cosmos in his Planetary hypotheses. By using eccentrics and epicycles, his geometrical model achieved greater mathematical detail and predictive accuracy than had been exhibited by earlier concentric spherical models of the cosmos.

Aristotelian philosophy and cosmology was influential in the Islamic world, where his ideas were taken up by the Falsafa school of philosophy throughout the later half of the first millennia AD. Of these, philosophers and Avicenna are especially notable. Averroes in particular wrote extensively about On The Heavens , trying for some time to reconcile the various themes of Aristotelian philosophy, such as natural movement of the elements and the concept of planetary spheres centered on the Earth, with the mathematics of Ptolemy. These ideas would remain central to the philosophical thinking of that culture until the rise to prominence of Al-Ghazali, a philosopher and theologian who argued against Aristotelianism and during the 12th century.

Athens and Jerusalem

European philosophers had a similarly complex relationship with De Caelo, attempting to reconcile church doctrine with the mathematics of Ptolemy and the structure of Aristotle. A particularly cogent example of this is in the work of Thomas Aquinas, theologian, philosopher and writer of the 13th century. Known today as St. Thomas of the Catholic Church, Aquinas worked to synthesize Aristotle's cosmology with Christian doctrine, an endeavor that led him to reclassify Aristotle's unmoved movers as angels and attributing the 'first cause' of motion in the celestial spheres to them. Otherwise, Aquinas accepted Aristotle's explanation of the physical world, including his cosmology and .

ISLAMIC GOLDEN AGE

Bakhtshooa Gondishapoori were Persian or Assyrian Nestorian Christian physicians from the 7th, 8th, and 9th centuries, spanning 6 generations and 250 years. Some of the members of the family served as the personal physicians of Caliphs. Jurjis son of Bukht- Yishu was awarded 10,000 dinars by al-Mansur after attending to his malady in 765 AD. It is even said that one of the members of this family was received as physician to Imam Sajjad (the 4th Shia Imam) during his illness in the events of Karbala.

Like most physicians in the early Abbasid courts, they came from the Academy of Jundi Shapur in Persia (in modern-day southwestern Iran). They were well versed in the Greek

8 and Hindi sciences, including those of , Aristotle, , and Galen, which they aided in translating while working in Jundi Shapur (Gondeshapur).

In the course of their integration into the changing society after the Islamic invasion of Persia, the family acquired Arabic while preserving Persian as oral language for about 200 years.

The family was originally from Ahvaz, near Jundi Shapur, however they eventually moved to the city of Baghdad and later on to Nsibin in Northern Syria, which was part of the Persian Empire in the Sassanid era. Yahya al-Barmaki, the vizier and mentor to Harun al-Rashid, provided patronage to the Hospital and Academy of Jundi Shapur and helped assure the promotion and growth of astronomy, medicine and philosophy, not only in Persia but also in the Abbasid Empire in general.

Al-Mansur

Al-Mansur or Abu Ja'far Abdallah ibn Muhammad al-Mansur (714 AD – 775 AD) was the second Abbasid Caliph reigning from 754 AD – 775 AD and succeeding his brother. Al-Mansur is generally regarded as the real founder of the Abbasid , one of the largest polities in world history, for his role in stabilizing and institutionalizing the dynasty.

During his reign, Islamic literature and scholarship in the Islamic world began to emerge in full force, supported by the Abbasid promotion of scholarly research, best exemplified by the Abbasid-sponsored Translation Movement.

It was under al-Mansur that a committee, mostly made up of Syriac-speaking Christians from Jundi Shapur, was set up in Baghdad with the purpose of translating extant Greek works into Arabic. Due to the Abbasid's orientation toward the East, many Persians came to play a crucial role in the Empire, both culturally as well as politically. This was in contrast to the preceding Umayyad era, in which non-Arabs were kept out of these affairs. Shu'ubiya emerged at this time, due to the rising of Iranian autonomy; it was a literary movement among Persians which expressed their belief in the superiority of Persian art and culture, and catalyzed the emergence of Arab-Persian dialogues in the 9th century AD.

In 756, al-Mansur sent over 4,000 Arab mercenaries to assist the Chinese in the An Shi Rebellion against An Lushan; after the war, they remained in . Al-Mansur was referred to as "A-p'u-ch'a-fo" in the Chinese T'ang Annals.

9 Al-Mansur’s investment in scholarship ignited the Islamic Golden Age. During this period, the Muslims showed a strong interest in assimilating the scientific knowledge of the civilizations that had been conquered. Many classic works of antiquity that might otherwise have been lost were translated from Greek, Roman, Persian, Indian, Chinese, Egyptian, and Phoenician civilizations into Arabic and Persian, and later in turn translated into Turkish, Hebrew, and Latin.

Christians (particularly Nestorian Christians) contributed to the Arab Islamic Civilization during the Ummayad and the Abbasid periods by translating works of Greek philosophers to Syriac and afterwards to Arabic. During the 4th through the 7th centuries, scholarly work in the Syriac and Greek languages was either newly initiated, or carried on from the .

Centers of learning and of transmission of classical wisdom included colleges such as the School of Nisibis, and later the School of Edessa, and the renowned hospital and medical academy of Jundishapur; libraries included the Library of Alexandria and the Imperial Library of Constantinople; other centers of translation and learning functioned at Merv, Salonika, Nishapur and Ctesiphon, situated just south of what later became Baghdad. Nestorians played a prominent role in the formation of Arab culture, especially at Jundishapur school.

This period is traditionally said to have ended with the collapse of the Abbasid caliphate due to Mongol invasions and their Sacking of Baghdad in 1258 AD.

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