Greek astronomy
4385475
224044379
2008-07-07T01:51:02Z
Gwernol
266416
Reverted edits by [[Special:Contributions/Gguidelas|Gguidelas]] ([[User talk:Gguidelas|talk]]) to last version by 79.33.5.74
'''Greek astronomy''' is the [[astronomy]] of those who wrote in the [[Greek language]] in [[classical antiquity]]. It is understood to include the [[Ancient Greece|ancient Greek]], [[Hellenistic civilization|Hellenistic]], [[Greco-Roman]], and [[Late Antiquity]] eras. It is not limited [[geographic]]ally to [[Greece]] or to ethnic [[Greek people|Greeks]], as the Greek language had become the language of scholarship throughout the [[Hellenistic world]] following the conquests of [[Alexander the Great|Alexander]]. This phase of Greek astronomy is also known as '''Hellenistic astronomy''', while the pre-Hellenistic phase is known as '''Classical Greek astronomy'''. During the Hellenistic and Roman periods, much of the Greek and non-Greek [[astronomers]] working in the Greek tradition studied at the [[Musaeum]] and the [[Library of Alexandria]] in [[Ptolemaic Egypt]]. The development of astronomy by the Greek and Hellenistic astronomers is considered by historians to be a major phase in the [[history of astronomy]] in [[Western culture]]. It was influenced by [[Babylonian astronomy]]; in turn, it influenced [[Islamic astronomy]] and [[Western Europe]]an astronomy.
== Archaic Greek astronomy ==
References to identifiable [[stars]] and [[constellation]]s appear in the writings of [[Homer]] and [[Hesiod]], the earliest surviving examples of Greek literature. In the ''[[Iliad]]'' and the ''[[Odyssey]]'', Homer refers to the following celestial objects:
* the constellation [[Boötes]]
* the [[star cluster]] [[Hyades (star cluster)|Hyades]]
* the constellation [[Orion (constellation)|Orion]]
* the star cluster [[Pleiades (star cluster)|Pleiades]]
* [[Sirius]], the '''Dog Star'''
* the constellation [[Ursa Major]]
Hesiod, who wrote in the early 7th century BCE, adds the star [[Arcturus]] to this list in his poetic calendar ''Works and Days''. Though neither Homer nor Hesiod set out to write a scientific work, they hint at a rudimentary [[cosmology]] of a [[flat earth]] surrounded by an "[[Oceanus|Ocean River]]." Some stars rise and set (disappear into the ocean, from the viewpoint of the Greeks); others are [[Circumpolar constellation|ever-visible]]. At certain times of the year, certain stars will rise or set at sunrise or sunset.
[[Image:Anaximander.jpg|left|thumb|Anaximander]]
Speculation about the [[cosmos]] was common in [[Pre-Socratic philosophy]] in the 6th and 5th centuries BCE. [[Anaximander]] (c.610 BC–c. 546 BC) described a cylindrical earth suspended in the center of the cosmos, surrounded by rings of fire. [[Philolaus]] (c. 480 BC–c. 405 BC) the [[Pythagoreanism|Pythagorean]] described a cosmos with the stars, planets, [[Sun]], [[Moon]], [[Earth]], and a counter-Earth (''[[Antichthon]]'')--ten bodies in all--circling an unseen central fire. Such reports show that Greeks of the 6th and 5th centuries were aware of the planets and speculated about the structure of the cosmos.
=== The planets in early Greek astronomy ===
The name "planet" comes from the Greek term πλανήτης, ''planētēs'', meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relation to the other stars. Five planets can be seen with the naked eye: [[Mercury (planet)|Mercury]], [[Venus]], [[Mars]], [[Jupiter]], and [[Saturn]]. Sometimes the [[luminary|luminaries]], the Sun and Moon, are added to the list of [[naked eye planets]] to make a total of seven. Since the planets disappear from time to time when they approach the Sun, careful attention is required to identify all five. [[Observations and explorations of Venus|Observations of Venus]] are not straightforward. Early Greeks thought that the evening and morning appearances of Venus represented two different objects, calling it ''[[Hesperus]]'' ("evening star") when it appeared in the western evening sky and ''Phosphorus'' ("light-bringer") when it appeared in the eastern morning sky. They eventually came to recognize that both objects were the same planet. [[Pythagoras]] is given credit for this realization.
The planets eventually received names drawn from [[Greek mythology]]. The equivalent names in [[Roman mythology]] are the basis for the modern English [[Astronomical naming conventions#Names of planets|names of the planets]].
== Calendars ==
Many ancient [[calendars]] are based on the cycles of the Sun or Moon. The [[Hellenic calendar]] incorporated these cycles. A [[lunisolar calendar]] based on both cycles is difficult. Some Greek astronomers worked out calendars based on the [[eclipse cycle]].
== Eudoxan astronomy ==
In classical Greece, astronomy was a branch of [[mathematics]]; astronomers sought to create geometrical models that could imitate the appearances of celestial motions. This tradition began with the Pythagoreans, who placed astronomy among the four mathematical arts (along with [[arithmetic]], [[geometry]], and [[music]]). The study of [[number]] comprising the four arts was later called the ''[[quadrivium]]''.
Although he was not a creative mathematician, [[Plato]] (427-347 BCE) included the ''quadrivium'' as the basis for philosophical education in the ''Republic''. He encouraged a younger mathematician, [[Eudoxus of Cnidus]] (c. 410 BCE-c. 347 BCE), to develop a system of Greek astronomy. According to a modern historian of science, [[David C. Lindberg|David Lindberg]]:
:''In their work we find (1) a shift from stellar to planetary concerns, (2) the creation of a geometrical model, the "two-sphere model," for the representation of stellar and planetary phenomena, and (3) the establishment of criteria governing theories designed to account for planetary observations''. (Lindberg 1992, p. 90)
The '''two-sphere model''' is a [[geocentric model]]. It divides the [[cosmos]] into two regions:
* A spherical Earth, central and motionless (the [[sublunary sphere]]).
* A spherical heavenly realm centered on the Earth, which may contain multiple rotating spheres made of [[Aether (classical element)|aether]]
[[Image:Ptolemaicsystem-small.png|thumb|Renaissance woodcut illustrating the two-sphere model.]]
Plato's main books on cosmology are the ''[[Timaeus (dialogue)|Timaeus]]'' and the ''[[Republic]]''. In them he described the two-sphere model and said there were eight circles or spheres carrying the seven planets and the fixed stars. He put the celestial objects in the following order, beginning with the one closest to Earth:
# Moon
# Sun
# Venus
# Mercury
# Mars
# Jupiter
# Saturn
# Fixed stars
According to the "[[Myth of Er]]" in the ''Republic'', the cosmos is the [[Spindle of Necessity]], attended by [[Sirens]] and spun by the three daughters of the Goddess Necessity known collectively as the [[Moirae]] or Fates.
According to a story reported by [[Simplicius of Cilicia]] (6th century CE), Plato posed a question for the Greek mathematicians of his day: "By the assumption of what uniform and orderly motions can the apparent motions of the planets be accounted for?" (quoted in Lloyd 1970, p. 84). Plato proposed that the seemingly chaotic wandering motions of the planets could be explained by combinations of uniform circular motions centered on a spherical Earth, apparently a novel idea in the 4th century.
Eudoxus rose to the challenge by assigning to each planet a set of [[concentric]] spheres. By tilting the axes of the spheres, and by assigning each a different period of revolution, he was able to approximate the celestial "appearances." Thus, he was the first to attempt a mathematical description of the motions of the planets. A general idea of the content of ''On Speeds'', his book on the planets, can be gleaned from [[Aristotle]]'s ''[[Metaphysics (Aristotle)|Metaphysics]]'' XII, 8, and a commentary by Simplicius on ''De caelo'', another work by Aristotle. Since all his own works are lost, our knowledge of Eudoxus is obtained from secondary sources. [[Aratus]]'s poem on [[astronomy]] is based on a work of Eudoxus, and possibly also [[Theodosius of Bithynia|Theodosius of Bithynia's]] ''Sphaerics''. They give us an indication of his work in [[spherical astronomy]] as well as planetary motions.
[[Callippus]], a Greek astronomer of the 4th century, added seven spheres to Eudoxus' original 27 (in addition to the planetary spheres, Eudoxus included a sphere for the fixed stars). Aristotle described both systems, but insisted on adding "unrolling" spheres between each set of spheres to cancel the motions of the outer set. Aristotle was concerned about the physical nature of the system; without unrollers, the outer motions would be transferred to the inner planets.
== Hellenistic astronomy ==
=== Planetary models and observational astronomy ===
The Eudoxan system had several critical flaws. One was its inability to predict motions exactly. Callippus' work may have been an attempt to correct this flaw. A related problem is the inability of his models to explain why planets appear to change speed. A third flaw is its inability to explain changes in the brightness of planets as seen from Earth. Because the spheres are concentric, planets will always remain at the same distance from Earth. This problem was pointed out in Antiquity by [[Autolycus of Pitane]] (c. 310 BCE).
[[Apollonius of Perga]] (c. 262 BC–c. 190 BCE) responded by introducing two new mechanisms that allowed a planet to vary its distance and speed: the [[Eccentric (mechanism)|eccentric]] deferent and the [[deferent and epicycle]]. The '''deferent''' is a circle carrying the planet around the Earth. (The word ''deferent'' comes from the Latin ''ferro, ferre'', meaning "to carry.") An '''eccentric deferent''' is slightly off-center from Earth. In a '''deferent and epicycle''' model, the deferent carries a small circle, the '''epicycle''', which carries the planet. The deferent-and-epicycle model can mimic the eccentric model, as shown by '''Apollonius' theorem'''. It can also explain [[Prograde and retrograde motion#Apparent retrograde motion|retrogradation]], which happens when planets appear to reverse their motion through the [[zodiac]] for a short time. Modern historians of astronomy have determined that Eudoxus' models could only have approximated retrogradation crudely for some planets, and not at all for others.
In the 2nd century BCE, [[Hipparchus]], aware of the extraordinary accuracy with which [[Babylonian]] astronomers could predict the planets' motions, insisted that Greek astronomers achieve similar levels of accuracy. Somehow he had access to Babylonian observations or predictions, and used them to create better geometrical models. For the Sun, he used a simple eccentric model, based on observations of the [[equinoxes]], which explained both changes in the speed of the Sun and differences in the lengths of the [[seasons]]. For the Moon, he used a [[deferent and epicycle]] model. He could not create accurate models for the remaining planets, and criticized other Greek astronomers for creating inaccurate models.
Hipparchus also compiled a [[star catalogue]]. According to [[Pliny the Elder]], he observed a ''[[nova]]'' (new star). So that later generations could tell whether other stars came to be, perished, moved, or changed in brightness, he recorded the position and brightness of the stars. [[Ptolemy]] mentioned the catalogue in connection with Hipparchus' [[discovery of precession]]. ([[Precession]] of the equinoxes is a slow motion of the place of the equinoxes through the zodiac, caused by the shifting of the Earth's axis). Hipparchus thought it was caused by the motion of the sphere of fixed stars.
=== Heliocentrism and cosmic scales ===
In the 3rd century BCE, [[Aristarchus of Samos]] proposed an alternate [[cosmology]] (arrangement of the universe): a [[Heliocentrism|heliocentric]] model of the [[solar system]], placing the Sun, not the Earth, at the center of the known universe (hence he is sometimes known as the "Greek [[Copernicus]]"). His astronomical ideas were not well-received, however, and only a few brief references to them are preserved. We know the name of one follower of Aristarchus: [[Seleucus of Seleucia]].
Aristarchus also wrote a book ''[[Aristarchus On the Sizes and Distances|On the Sizes and Distances of the Sun and Moon]]'', which is his only work to have survived. In this work, he calculated the sizes of the Sun and Moon, as well as their distances from the Earth in [[Earth radius|Earth radii]]. Shortly afterwards, [[Eratosthenes]] calculated the size of the Earth, providing a value for the Earth radii which could be plugged into Aristarchus' calculations. Hipparchus wrote another book ''[[Hipparchus_On_Sizes_and_Distances|On the Sizes and Distances of the Sun and Moon]]'', which has not survived. Both Aristarchus and Hipparchus drastically underestimated the distance of the Sun from the Earth.
== Astronomy in the Greco-Roman and Late Antique eras ==
[[Hipparchus]] is considered to have been among the most important Greek astronomers, because he introduced the concept of exact prediction into astronomy. He was also the last innovative astronomer before Claudius [[Ptolemy]], a mathematician who worked at [[Alexandria]] in [[Aegyptus (Roman province)|Roman Egypt]] in the 2nd century CE. Ptolemy's works on astronomy and [[astrology]] include the ''[[Almagest]]'', the ''Planetary Hypotheses'', and the ''[[Tetrabiblos]]'', as well as the ''Handy Tables'', the ''Canobic Inscription'', and other minor works.
===Ptolemaic astronomy===
The ''Almagest'' is one of the most influential books in the history of Western astronomy. In this book, Ptolemy explained how to predict the behavior of the planets, as Hipparchus could not, with the introduction of a new mathematical tool, the [[equant]]. The ''Almagest'' gave a comprehensive treatment of astronomy, incorporating theorems, models, and observations from many previous mathematicians. This fact may explain its survival, in contrast to more specialized works that were neglected and lost. Ptolemy placed the planets in the order that would remain standard until it was displaced by the [[heliocentric]] system and the [[Tychonic system]]:
# Moon
# Mercury
# Venus
# Sun
# Mars
# Jupiter
# Saturn
# Fixed stars
The extent of Ptolemy's reliance on the work of other mathematicians, in particular his use of Hipparchus' star catalogue, has been debated since the 19th century. A controversial claim was made by Robert R. Newton in the 1970s. in ''The Crime of Claudius Ptolemy'', he argued that Ptolemy faked his observations and falsely claimed the catalogue of Hipparchus as his own work. Newton's theories have not been adopted by most historians of astronomy.
A few mathematicians of Late Antiquity wrote commentaries on the ''Almagest'', including [[Pappus of Alexandria]] as well as [[Theon of Alexandria]] and his daughter [[Hypatia]]. Ptolemaic astronomy became standard in medieval western European and [[Islamic astronomy]] until it was displaced by [[Maragheh observatory|Maraghan]], [[Heliocentrism|heliocentric]] and [[Tychonic system]]s by the 16th century. However, recently discovered manuscripts reveal that Greek [[astrologers]] of Antiquity continued using pre-Ptolemaic methods for their calculations (Aaboe, 2001).
==Interactions with Indian astronomy==
{{see|Indian astronomy}}
[[Image:AiKhanoumSunDial.jpg|thumb|250px|Greek equatorial [[sun dial]], [[Ai-Khanoum]], [[Afghanistan]] 3rd-2nd century BCE.]]
[[Hellenistic civilization|Hellenistic]] astronomy is known to have been practiced near India in the [[Greco-Bactrian Kingdom|Greco-Bactrian]] city of [[Ai-Khanoum]] from the [[3rd century BCE]]. Various sun-dials, including an equatorial sundial adjusted to the latitude of [[Ujjain]] have been found in archaeological excavations there.<ref>"Afghanistan, les trésors retrouvés", p269</ref> Numerous interactions with the [[Mauryan Empire]], and the later expansion of the [[Indo-Greek Kingdom|Indo-Greeks]] into India suggest that some transmission may have happened during that period.<ref>"Les influences de l'astronomie grecques sur l'astronomie indienne auraient pu commencer de se manifester plus tot qu'on ne le pensait, des l'epoque Hellenistique en fait, par l'intermediaire des colonies grecques des Greco-Bactriens et Indo-Grecs" (French) Afghanistan, les trésors retrouvés", p269. Translation: "The influence of Greek astronomy on Indian astronomy may have taken place earlier than thought, as soon as the Hellenistic period, through the agency of the Greek colonies of the Greco-Bactrians and the Indo-Greeks.</ref>
Several Greco-Roman astrological treatises are also known to have been imported into India during the first few centuries of our era. The ''[[Yavanajataka]]'' ("Sayings of the Greeks") was translated from Greek to Sanskrit by [[Yavanesvara]] during the 2nd century CE, under the patronage of the [[Western Satrap]] [[Saka]] king [[Rudradaman I]].
Later in the 6th century, the ''[[Romaka Siddhanta]]'' ("Doctrine of the Romans"), and the ''[[Paulisa Siddhanta]]'' ("Doctrine of [[Paulus Alexandrinus|Paul]]") were considered as two of the five main astrological treatises, which were compiled by [[Varahamihira]] in his ''Pañca-siddhāntikā'' ("Five Treatises").<ref>"the Pañca-siddhāntikā ("Five Treatises"), a compendium of Greek, Egyptian, Roman and Indian astronomy. Varāhamihira's knowledge of Western astronomy was thorough. In 5 sections, his monumental work progresses through native Indian astronomy and culminates in 2 treatises on Western astronomy, showing calculations based on Greek and Alexandrian reckoning and even giving complete Ptolemaic mathematical charts and tables. Encyclopedia Britanica Vol12, p269 [http://www.iranchamber.com/podium/history/030812_varahamihira_iranic_astronomer.php Source]</ref> Varahamihira wrote in the [[Brihat-Samhita]]: "The Greeks, though impure, must be honored since they were trained in sciences and therein, excelled others....."<ref>":Mleccha hi yavanah tesu samyak shastram idam sthitam
:Rsivat te api pujyante kim punar daivavid dvijah
:-(Brhatsamhita 2.15)</ref> The [[Garga Samhita]] also says: "The [[Yona|Yavanas]] are [[barbarian]]s, yet the science of astronomy originated with them and for this they must be reverenced like gods."
Others have suggested mutual influence between Hellenistic and Indian astronomers, as it is known that Brahmins and Yogis were active in the Mediterranean.<ref name=Thompson>Richard L. Thompson (2003). ''Vedic Cosmography and Astronomy'', p. 15-17, 181-198. [[Motilal Banarsidass]] Publ. ISBN 8120819543.</ref><ref name=PA>J. S. Phillimore (1912). ''Philostratus in Honour of Appolonius of Tiana''. Clarenden Press, [[Oxford]].</ref>
== Sources for Greek astronomy ==
Many Greek astronomical texts are known only by name, and perhaps by a description or quotations. Some elementary works have survived because they were largely non-mathematical and suitable for use in schools. Books in this class include the ''Phaenomena'' of [[Euclid]] and two works by [[Autolycus of Pitane]]. Three important textbooks, written shortly before Ptolemy's time, were written by [[Cleomedes]], [[Geminus]], and [[Theon of Smyrna]]. Books by Roman authors like Pliny the Elder and [[Vitruvius]] contain some information on Greek astronomy. The most important primary source is the ''Almagest'', since Ptolemy refers to the work of many of his predecessors (Evans 1998, p. 24).
== Famous astronomers of antiquity ==
In addition to the authors named in the article, the following list of people who worked on mathematical astronomy or cosmology may be of interest.
[[Image:Pythagoras-Münz.JPG|thumb|Pythagoras]]
* [[Anaxagoras]]
* [[Apollonius of Tyana]]
* [[Archimedes]]
* [[Archytas]]
* [[Aristaeus the Elder|Aristaeus]]
* [[Aristillus]]
* [[Conon of Samos]]
* [[Democritus]]
* [[Empedocles]]
* [[Heraclides Ponticus]]
* [[Hicetas]]
* [[Hippocrates of Chios]]
* [[Ambrosius Theodosius Macrobius|Macrobius]]
* [[Martianus Capella]]
* [[Menelaus of Alexandria]] ([[Menelaus theorem]])
* [[Meton of Athens]]
* [[Parmenides]]
* [[Porphyry (philosopher)|Porphyry]]
* [[Posidonius]]
* [[Proclus]]
* [[Pythagoras]]
* [[Thales]]
* [[Theodosius of Bithynia]]
== Notes ==
{{reflist}}
== References ==
* Aaboe, Asger. ''Episodes from the Early History of Astronomy''. Springer, 2001.
* [[J. L. E. Dreyer|Dreyer, J. L. E. ]] ''A History of Astronomy from Thales to Kepler''. New York: Dover Publications, 1953.
* Evans, James. ''The History and Practice of Ancient Astronomy''. New York: Oxford University Press, 1998.
* [[Thomas Heath|Heath, Thomas]]. ''Aristarchus of Samos''. Oxford: Clarendon Press, 1913.
* [[Lindberg, David C.]] ''The Beginnings of Western Science: The European Scientific Tradition in Philosophical, Religious, and Institutional Context, 600 B.C. to A.D. 1450''. Chicago: University of Chicago Press, 1992.
* Lloyd, G. E. R. ''Early Greek Science: Thales to Aristotle''. New York: W.W. Norton & Co., 1970.
* [[Otto E. Neugebauer|Neugebauer, Otto]]. ''A History of Ancient Mathematical Astronomy''. 3 vols. Berlin: Springer, 1975. (Commonly abbreviated as ''HAMA'')
* Newton, Robert R. ''The Crime of Claudius Ptolemy''. Baltimore: Johns Hopkins University Press, 1977.
* Pedersen, Olaf. ''Early Physics and Astronomy: A Historical Introduction''. 2nd edition. Cambridge: Cambridge University Press, 1993.
* {{it}} [http://www.giovannipastore.it/ISTRUZIONI.htm Pastore Giovanni, ''ANTIKYTHERA E I REGOLI CALCOLATORI'', Rome, 2006, privately published]
== See also ==
*[[Antikythera Mechanism]]
*[[Greek mathematics]]
*[[History of astronomy]]
*[[Babylonian influence on Greek astronomy]]
== External links ==
* [http://www.csit.fsu.edu/~dduke/models Almagest Planetary Model Animations]
* [http://www-history.mcs.st-andrews.ac.uk/history/index.html MacTutor History of Mathematics Archive]
*[http://www.giovannipastore.it/ANTIKYTHERA.htm The Antikythera Calculator (Italian and English versions)]
{{Greek astronomy}}
[[Category:Ancient astronomy]]
[[Category:Ancient Greek astronomy]]
[[Category:Ancient Greek society]]
[[Category:History of astronomy]]
[[Category:History of astrology]]
[[Category:Early scientific cosmologies]]
[[it:Astronomia greca]]
[[ro:Astronomia în Grecia antică]]