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An early Baroque artist's rendition of Claudius Ptolemaeus.
Born c. AD 90
Died c. AD 168
Alexandria, Egypt
Occupation mathematician, geographer, astronomer, astrologer

Claudius Ptolemaeus (Greek: Κλαύδιος Πτολεμαῖος Klaúdios Ptolemaîos; c. AD 90 – c. 168), known in English as Ptolemy (pronounced /ˈtɒləmɪ/), was a Roman citizen of Greek or Egyptian ancestry.[1] He was a mathematician, astronomer, geographer, astrologer and a poet of a single epigram in the Greek Anthology[2][3]. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the Thebaid. He died in Alexandria around AD 168.[4]

Ptolemy was the author of several scientific treatises, at least three of which were of continuing importance to later Islamic and European science. The first is the astronomical treatise now known as the Almagest (in Greek, Ἡ Μεγάλη Σύνταξις, "The Great Treatise", originally Μαθηματικὴ Σύνταξις, "Mathematical Treatise"). The second is the Geography, which is a thorough discussion of the geographic knowledge of the Greco-Roman world. The third is the astrological treatise known sometimes in Greek as the Apotelesmatika (Ἀποτελεσματικά), more commonly in Greek as the Tetrabiblos (Τετράβιβλος "Four books"), and in Latin as the Quadripartitum (or four books) in which he attempted to adapt horoscopic astrology to the Aristotelian natural philosophy of his day.



The name Claudius is a Roman nomen; the fact that Ptolemy bore it proves that he was a Roman citizen. It would have suited custom if the first of Ptolemy's family who became a citizen (whether it was he or an ancestor) took the nomen from a Roman called Claudius, who was in some sense responsible for granting citizenship. If, as was not uncommon, this Roman was the emperor, the citizenship would have been granted between AD 41 and 68 (when Claudius, and then Nero, were emperors). The astronomer would also have had a praenomen, which remains unknown. However, it may have been Tiberius, as that praenomen was very common among those whose families had been granted citizenship by these emperors.

Ptolemaeus (Ptolemy) is a Greek name. It occurs once in Greek mythology, and is of Homeric form. It was quite common among the Macedonian upper class at the time of Alexander the Great, and there were several among Alexander's army, one of whom in 323 BC made himself King of Egypt: Ptolemy I Soter; all the kings after him, until Egypt became a Roman province in 30 BC, were also Ptolemies. There is little evidence on the subject of Ptolemy's ancestry (though see above on his family's Roman citizenship), but most scholars and historians consider it unlikely that Ptolemy was related to the royal dynasty of the Ptolemies.[citation needed]

Beyond his being considered a member of Alexandria's Greek society, few details of Ptolemy's life are known. He wrote in Ancient Greek and is known to have utilised Babylonian astronomical data.[5][6] Although a Roman citizen, many scholars have concluded that ethnically, Ptolemy was a Greek,[7][8][9] while some suggest that he was ethnically an Egyptian, though Hellenized.[8][10][11][12] He was often known in later Arabic sources as "the Upper Egyptian",[13] suggesting that he may have had origins in southern Egypt.[12] Later Arabic astronomers, geographers and physicists referred to him by his name in Arabic: بطليموسBatlaymus.[14]


The Almagest is the only surviving comprehensive ancient treatise on astronomy. Babylonian astronomers had developed arithmetical techniques for calculating astronomical phenomena; Greek astronomers such as Hipparchus had produced geometric models for calculating celestial motions; Ptolemy, however, claimed to have derived his geometrical models from selected astronomical observations by his predecessors spanning more than 800 years, though astronomers have for centuries suspected that his models' parameters were adopted independently of observations.[15] Ptolemy presented his astronomical models in convenient tables, which could be used to compute the future or past position of the planets.[16] The Almagest also contains a star catalogue, which is an appropriated version of a catalogue created by Hipparchus. Its list of forty-eight constellations is ancestral to the modern system of constellations, but unlike the modern system they did not cover the whole sky (only the sky Hipparchus could see). Through the Middle Ages it was spoken of as the authoritative text on astronomy, with its author becoming an almost mythical figure, called Ptolemy, King of Alexandria.[17] The Almagest was preserved, like most of Classical Greek science, in Arabic manuscripts (hence its familiar name). Because of its reputation, it was widely sought and was translated twice into Latin in the 12th century, once in Sicily and again in Spain.[18] Ptolemy's model, like those of his predecessors, was geocentric and was almost universally accepted until the appearance of simpler heliocentric models during the scientific revolution.

His Planetary Hypotheses went beyond the mathematical model of the Almagest to present a physical realization of the universe as a set of nested spheres,[19] in which he used the epicycles of his planetary model to compute the dimensions of the universe. He estimated the Sun was at an average distance of 1210 Earth radii while the radius of the sphere of the fixed stars was 20,000 times the radius of the Earth.[20]

Ptolemy presented a useful tool for astronomical calculations in his Handy Tables, which tabulated all the data needed to compute the positions of the Sun, Moon and planets, the rising and setting of the stars, and eclipses of the Sun and Moon. Ptolemy's Handy Tables provided the model for later astronomical tables or zījes. In the Phaseis (Risings of the Fixed Stars) Ptolemy gave a parapegma, a star calendar or almanac based on the hands and disappearances of stars over the course of the solar year.


Ptolemy's other main work is his Geographia. This also is a compilation of what was known about the world's geography in the Roman Empire during his time. He relied somewhat on the work of an earlier geographer, Marinos of Tyre, and on gazetteers of the Roman and ancient Persian Empire, but most of his sources beyond the perimeter of the Empire were unreliable.[citation needed]

The first part of the Geographia is a discussion of the data and of the methods he used. As with the model of the solar system in the Almagest, Ptolemy put all this information into a grand scheme. Following Marinos, he assigned coordinates to all the places and geographic features he knew, in a grid that spanned the globe. Latitude was measured from the equator, as it is today, but Ptolemy preferred in book 8 to express it as the length of the longest day rather than degrees of arc (the length of the midsummer day increases from 12h to 24h as one goes from the equator to the polar circle). In books 2 through 7, he used degrees and put the meridian of 0 longitude at the most western land he knew, the "Blessed Islands", probably the Cape Verde islands (not the Canary Islands, as long accepted) as suggested by the location of the six dots labelled the "FORTUNATA" islands near the left extreme of the blue sea of Ptolemy's map here reproduced.

A 15th century manuscript copy of the Ptolemy world map, reconstituted from Ptolemy's Geographia (circa 150), indicating the countries of "Serica" and "Sinae" (China) at the extreme east, beyond the island of "Taprobane" (Sri Lanka, oversized) and the "Aurea Chersonesus" (Malay Peninsula).

Ptolemy also devised and provided instructions on how to create maps both of the whole inhabited world (oikoumenè) and of the Roman provinces. In the second part of the Geographia he provided the necessary topographic lists, and captions for the maps. His oikoumenè spanned 180 degrees of longitude from the Blessed Islands in the Atlantic Ocean to the middle of China, and about 80 degrees of latitude from The Shetlands to anti-Meroe (east coast of Africa); Ptolemy was well aware that he knew about only a quarter of the globe, and an erroneous extension of China southward suggests his sources did not reach all the way to the Pacific Ocean.

The maps in surviving manuscripts of Ptolemy's Geographia, however, date only from about 1300, after the text was rediscovered by Maximus Planudes. It seems likely that the topographical tables in books 2-7 are cumulative texts - texts which were altered and added to as new knowledge became available in the centuries after Ptolemy (Bagrow 1945). This means that information contained in different parts of the Geography is likely to be of different date.

A printed map from the 15th century depicting Ptolemy's description of the Ecumene, (1482, Johannes Schnitzer, engraver).

Maps based on scientific principles had been made since the time of Eratosthenes (3rd century BC), but Ptolemy improved projections. It is known that a world map based on the Geographia was on display in Autun, France in late Roman times. In the 15th century Ptolemy's Geographia began to be printed with engraved maps; the earliest printed edition with engraved maps was produced in Bologna in 1477, followed quickly by a Roman edition in 1478 (Campbell, 1987). An edition printed at Ulm in 1482, including woodcut maps, was the first one printed north of the Alps. The maps look distorted as compared to modern maps, because Ptolemy's data were inaccurate. One reason is that Ptolemy estimated the size of the Earth as too small: while Eratosthenes found 700 stadia for a great circle degree on the globe, in the Geographia Ptolemy uses 500 stadia. It is highly probable that these were the same stadion since Ptolemy switched from the former scale to the latter, between the Syntaxis and the Geographia and severely readjusted longitude degrees accordingly. If they both used the Attic stadion of about 185 meters, then the older estimate is 1/6 too large, and Ptolemy's value is 1/6 too small, a difference explained as due to ancient scientists' use of simple methods of measuring the earth, which were corrupted either high or low by a factor of 5/6, due to air's bending of horizontal light rays by 1/6 of the Earth's curvature.[citation needed] See also Ancient Greek units of measurement and History of geodesy.

Because Ptolemy derived many of his key latitudes from crude longest day values, his latitudes are erroneous on average by roughly a degree (2 degrees for Byzantium, 4 degrees for Carthage), though capable ancient astronomers knew their latitudes to more like a minute. (Ptolemy's own latitude was in error by 14'.) He agreed (Geographia 1.4) that longitude was best determined by simultaneous observation of lunar eclipses, yet he was so out of touch with the scientists of his day that he knew of no such data more recent than 500 years ago (Arbela eclipse). When switching from 700 stadia per degree to 500, he (or Marinos) expanded longitude differences between cities accordingly (a point 1st realized by P.Gosselin in 1790), resulting in serious over-stretching of the Earth's east-west scale in degrees, though not distance. Achieving highly precise longitude remained a problem in geography until the invention of the marine chronometer at the end of the 18th century. It must be added that his original topographic list cannot be reconstructed: the long tables with numbers were transmitted to posterity through copies containing many scribal errors, and people have always been adding or improving the topographic data: this is a testimony to the persistent popularity of this influential work in the history of cartography.


The mathematician Claudius Ptolemy 'the Alexandrian' as imagined by a 16th century artist

Ptolemy's treatise on astrology, known in Greek as both the Apotelesmatika ("Astrological Outcomes" or "Effects") and "Tetrabiblios" ("Four Books"), and in Latin as the Quadripartitum ("Four books"), was the most popular astrological work of antiquity and also had great influence in the Islamic world and the medieval Latin West. It was first translated from Arabic into Latin by Plato of Tivoli (Tiburtinus), while he was in Spain (FA Robbins, 1940; Thorndike 1923). The Tetrabiblos is an extensive and continually reprinted treatise on the ancient principles of horoscopic astrology in four books (Greek tetra means "four", biblos is "book"). That it did not quite attain the unrivaled status of the Almagest was perhaps because it did not cover some popular areas of the subject, particularly electional astrology (interpreting astrological charts for a particular moment to determine the outcome of a course of action to be initiated at that time), and medical astrology, which were later adoptions.

The great popularity that the Tetrabiblos did possess might be attributed to its nature as an exposition of the art of astrology and as a compendium of astrological lore, rather than as a manual. It speaks in general terms, avoiding illustrations and details of practice. Ptolemy was concerned to defend astrology by defining its limits, compiling astronomical data that he believed was reliable and dismissing practices (such as considering the numerological significance of names) that he believed to be without sound basis.

Much of the content of the Tetrabiblos was collected from earlier sources; Ptolemy's achievement was to order his material in a systematic way, showing how the subject could, in his view, be rationalized. It is, indeed, presented as the second part of the study of astronomy of which the Almagest was the first, concerned with the influences of the celestial bodies in the sublunar sphere. Thus explanations of a sort are provided for the astrological effects of the planets, based upon their combined effects of heating, cooling, moistening, and drying.

Ptolemy's astrological outlook was quite practical: he thought that astrology was like medicine, that is conjectural, because of the many variable factors to be taken into account: the race, country, and upbringing of a person affects an individual's personality as much if not more than the positions of the Sun, Moon, and planets at the precise moment of their birth, so Ptolemy saw astrology as something to be used in life but in no way relied on entirely.


Ptolemy also wrote an influential work, Harmonics, on music theory and the mathematics of music. After criticizing the approaches of his predecessors, Ptolemy argued for basing musical intervals on mathematical ratios (in contrast to the followers of Aristoxenus and in agreement with the followers of Pythagoras) backed up by empirical observation (in contrast to the overly theoretical approach of the Pythagoreans). Ptolemy wrote about how musical notes could be translated into mathematical equations and vice versa in Harmonics. This is called Pythagorean tuning because it was first discovered by Pythagoras. However, Pythagoras believed that the mathematics of music should be based on the specific ratio of 3:2 whereas Ptolemy merely believed that it should just generally involve tetrachords and octaves. He presented his own divisions of the tetrachord and the octave, which he derived with the help of a monochord. Ptolemy's astronomical interests also appeared in a discussion of the "music of the spheres."


His Optics is a work that survives only in a poor Arabic translation and in about twenty manuscripts of a Latin version of the Arabic, which was translated by Eugene of Palermo (circa 1154). In it Ptolemy writes about properties of light, including reflection, refraction, and colour. The work is a significant part of the early history of optics.[21]

Named after Ptolemy

There are several characters or items named after Ptolemy, including:

See also


  1. ^ See 'Background' section on his status as a Roman citizen
  2. ^ Select Epigrams from the Greek Anthology By John William Mackail Page 246 ISBN 1406922943 2007
  3. ^ Mortal am I, the creature of a day..
  4. ^ Jean Claude Pecker (2001), Understanding the Heavens: Thirty Centuries of Astronomical Ideas from Ancient Thinking to Modern Cosmology, p. 311, Springer, ISBN 3540631984.
  5. ^ Asger Aaboe, Episodes from the Early History of Astronomy, New York: Springer, 2001), p. 62-65.
  6. ^ Alexander Jones, "The Adaptation of Babylonian Methods in Greek Numerical Astronomy," in The Scientific Enterprise in Antiquity and the Middle Ages, p. 99.
  7. ^ Enc. Britannica 2007, "Claudius Ptolemaeus"
  8. ^ a b Victor J. Katz (1998). A History of Mathematics: An Introduction, p. 184. Addison Wesley, ISBN 0321016181.
  9. ^ "Ptolemy." Britannica Concise Encyclopedia. Encyclopædia Britannica, Inc., 2006. 20 Jul. 2008.
  10. ^ George Sarton (1936). "The Unity and Diversity of the Mediterranean World", Osiris 2, p. 406-463 [429].
  11. ^ John Horace Parry (1981). The Age of Reconnaissance, p. 10. University of California Press. ISBN 0520042352.
  12. ^ a b Martin Bernal (1992). "Animadversions on the Origins of Western Science", Isis 83 (4), p. 596-607 [602, 606].
  13. ^ J. F. Weidler (1741). Historia astronomiae, p. 177. Wittenberg: Gottlieb. (cf. Martin Bernal (1992). "Animadversions on the Origins of Western Science", Isis 83 (4), p. 596-607 [606].)
  14. ^ edited by Shahid Rahman, Tony Street, Hassan Tahiri. (2008). "The Birth of Scientific Controversies, The Dynamics of the Arabic Tradition and Its Impact on the Development of Science: Ibn al-Haytham’s Challenge of Ptolemy’s Almagest". The Unity of Science in the Arabic Tradition. 11. Springer Netherlandsdoi=10.1007/978-1-4020-8405-8. pp. 183–225 [183]. doi:10.1007/978-1-4020-8405-8. ISBN 978-1-4020-8404-1. 
  15. ^ "Dennis Rawlins". The International Journal of Scientific History. Retrieved 2009-10-07. 
  16. ^ Bernard R. Goldstein, "Saving the Phenomena: The Background to Ptolemy's Planetary Theory", Journal for the History of Astronomy, 28 (1997): 1-12
  17. ^ S. C. McCluskey, Astronomies and Cultures in Early Medieval Europe, Cambridge: Cambridge Univ. Pr. 1998, pp. 20-21.
  18. ^ Charles Homer Haskins, Studies in the History of Mediaeval Science, New York: Frederick Ungar Publishing, 1967, reprint of the Cambridge, Mass., 1927 edition
  19. ^ Dennis Duke, Ptolemy's Cosmology
  20. ^ Bernard R. Goldstein, ed., The Arabic Version of Ptolemy's Planetary Hypotheses, Transactions of the American Philosophical Society, 57, 4 (1967), pp. 9-12.
  21. ^ Smith, A. Mark (1996). Ptolemy's Theory of Visual Perception– An English translation of the Optics. The American Philosophical Society. ISBN 0-87169-862-5. Retrieved 27 June 2009. 
  22. ^ Mars Labs. Google Maps.


Texts and translations

  • Bagrow, L. (January 1, 1945). "The Origin of Ptolemy's Geographia". Geografiska Annaler 27: 318–387. doi:10.2307/520071. ISSN 16513215. 
  • Berggren, J. Lennart, and Alexander Jones. 2000. Ptolemy's Geography: An Annotated Translation of the Theoretical Chapters. Princeton and Oxford: Princeton University Press. ISBN 0-691-01042-0.
  • Campbell, T. (1987). The Earliest Printed Maps. British Museum Press. 
  • Hübner, Wolfgang, ed. 1998. Claudius Ptolemaeus, Opera quae exstant omnia Vol III/Fasc 1: ΑΠΟΤΕΛΕΣΜΑΤΙΚΑ (= Tetrabiblos). De Gruyter. ISBN 978-3-598-71746-8 (Bibliotheca scriptorum Graecorum et Romanorum Teubneriana). (The most recent edition of the Greek text of Ptolemy's astrological work, based on earlier editions by F. Boll and E. Boer.)
  • Neugebauer, Otto (1975). A History of Ancient Mathematical Astronomy. I-III. Berlin and New York: Sprnger Verlag. 
  • Nobbe, C. F. A., ed. 1843. Claudii Ptolemaei Geographia. 3 vols. Leipzig: Carolus Tauchnitus. (The most recent edition of the complete Greek text)
  • Ptolemy. 1930. Die Harmonielehre des Klaudios Ptolemaios, edited by Ingemar Düring. Göteborgs högskolas årsskrift 36, 1930:1. Göteborg: Elanders boktr. aktiebolag. Reprint, New York: Garland Publishing, 1980.
  • Ptolemy. 2000. Harmonics, translated and commentary by Jon Solomon. Mnemosyne, Bibliotheca Classica Batava, Supplementum, 0169-8958, 203. Leiden and Boston: Brill Publishers. ISBN 9004115919
  • Stevenson, Edward Luther (trans. and ed.). 1932. Claudius Ptolemy: The Geography. New York: New York Public Library. Reprint, New York: Dover, 1991. (This is the only complete English translation of Ptolemy's most famous work. Unfortunately, it is marred by numerous mistakes and the placenames are given in Latinised forms, rather than in the original Greek).
  • Stückelberger, Alfred, and Gerd Graßhoff (eds). 2006. Ptolemaios, Handbuch der Geographie, Griechisch-Deutsch. 2 vols. Basel: Schwabe Verlag. ISBN 978-3-7965-2148-5. (Massive 1018 pp. scholarly edition by a team of a dozen scholars that takes account of all known manuscripts, with facing Greek and German text, footnotes on manuscript variations, color maps, and a CD with the geographical data)
  • Taub, Liba Chia (1993). Ptolemy's Universe: The Natural Philosophical and Ethical Foundations of Ptolemy's Astronomy. Chicago: Open Court Press. ISBN 0-8126-9229-2. 

External links

Primary sources

Secondary material

Animated Illustrations


Up to date as of January 14, 2010

From Wikiquote

Claudius Ptolemaeus (Greek: Κλαύδιος Πτολεμαῖος; ca. 100 – ca. 178), known in English as Ptolemy, was an ancient Greek geographer, astronomer, and astrologer who probably lived and worked in Alexandria, off the coast of Egypt.




  • I know that I am mortal by nature and ephemeral, but when I trace at my pleasure the windings to and fro of the heavenly bodies, I no longer touch earth with my feet. I stand in the presence of Zeus himself and take my fill of ambrosia.
    • Penned in the margins.


  • Everything that is hard to attain is easily assailed by the generality of men.
    • Book I, sec. 1
  • The length of life takes the leading place among inquiries about events following birth.
    • Book III, sec. 10
  • As material fortune is associated with the properties of the body, so honor belongs to those of the soul.
    • Book IV, sec. 1
  • There are three classes of friendship and enmity, since men are so disposed to one another either by preference or by need or through pleasure and pain.
    • Book IV, sec. 7

External links

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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

There is more than one meaning of Ptolemy discussed in the 1911 Encyclopedia. We are planning to let all links go to the correct meaning directly, but for now you will have to search it out from the list below by yourself. If you want to change the link that led you here yourself, it would be appreciated.


Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary



Wikipedia has an article on:



From Ancient Greek Πτολεμαῖος (Ptolemaios)


  • (UK) IPA: /ˈtɒləmi/, SAMPA: /"tQl@mi/
  • (US) enPR: tälʹəm-ē, IPA: /ˈtɑləmi/, SAMPA: /"tAl@mi/

Proper noun




  1. A male given name of Greek origin, borne by historical persons in ancient Egypt.

Derived terms


Bible wiki

Up to date as of January 23, 2010

From BibleWiki


Prince (tetrarch) of Iturea and Chalcis from about 85 to 40 B.C., in which year he died; son of Mennæus. He tried to extend his kingdom by warlike expeditions (Strabo, xvi. 2, § 10); and ruled the Lebanon, threatened Damascus, subjugated several districts on the Phenician coast, and once had Paneas in his hands (Josephus, "Ant." xv. 10, §§ 1-3). In fact, the whole of Galilee had formerly been in the possession of the Itureans, and had been taken away from them in 103 by Aristobulus I. (ib. xiii. 11, § 3).

The Jews thought themselves oppressed by Ptolemy, and hence Aristobulus II., at that time stillprince and sent by his mother, Alexandra, undertook an expedition against Damascus to protect it against Ptolemy (ib. 16, § 3; idem, "B. J." i. 5, § 3). Pompey destroyed Ptolemy's strongholds in the Lebanon and doubtless took away from him the Hellenic cities, as he did in Judea. When Aristobulus II. was murdered by Pompey's party in Judea (49 B.C.), his sons and daughters found protection with Ptolemy ("Ant." xiv. 7, § 4; "B. J." i. 9, § 2). It may be that the national Jewish party at that time depended for support on the Itureans in Chalcis, and perhaps the following statement has reference to that fact: "On the 17th of Adar danger threatened the rest of the 'Soferim' in the city of Chalcis, and it was salvation for Israel" (Meg. Ta'an. xii.).

Antigonus, son of Aristobulus, also supported Ptolemy in his effort to establish himself as king in Judea ("Ant." xiv. 12, § 1). Ptolemy died just as the Parthians were invading Judea (ib. xiv. 13, § 3; "B. J." i. 13, § 1). He was succeeded by his son Lysanias.

Bibliography: Grätz, Gesch. 4th ed., iii. 148, 174, 186; Schürer, Gesch. 3d ed., i. 712-713.


Strategus of Jericho; son of Abubus (= (missing hebrew text) ), son-in-law of Simon Maccabeus. He wished to gain possession of the rulership over Judea, and hence when his father-in-law was visiting him at the fortress of Dok, near Jericho, in the month of Shebaṭ, in the 177th year of the Seleucid era (= 135 B.C.). Ptolemy gave a banquet at which he caused Simon and his two sons Mattathias and Judas to be murdered (I Mace. xvi. 11-17; Josephus, "Ant." xiii. 7, § 4). Moreover, he seat men to murder the third son, John Hyrcanus, who was in Gazara; but the latter, having been warned in time, killed the men, and took possession of Jerusalem, so that Ptolemy was obliged to retire to Dagon (doubtless identical with Dok). Here he was besieged by John; but as he threatened to kill John's mother, who was in his power, and as the Sabbatical year was approaching, the siege was unsuccessful. Although Ptolemy was now able to withdraw without opposition, he nevertheless caused John's mother to be killed before he left ("Ant." xiii. 8, § 1; "B. J." i. 2, §§ 3, 4).

Bibliography: Grätz, Gesch. 4th ed. iii. 62-65; Schürer, Gesch. 3d ed., i. 255-258.

This entry includes text from the Jewish Encyclopedia, 1906.

Simple English

A medieval (ideal) portrait of Claudius Ptolemaeus

Ptolemy is the English name for Claudius Ptolemaeus. He was a Greek who probably lived and worked in Alexandria, in Egypt. He lived from about 85 to 165 A.D. He is famous for his work on astronomy and geography.

Very little is known about his personal life.

He was an astronomer, mathematician, and geographer. He described in his writing the Greek or geocentric view of the universe. The Greeks thought that the Earth was the center of the universe. He also thought out and described the apparent motions of the planets as they were known in his time.

Ptolemy explained and extended Hipparchus's system of epicycles and eccentric circles to explain his geocentric (Earth-centered) theory of the solar system. Ptolemy's system involved at least 80 epicycles to explain the motions of the Sun, the Moon, and the five planets known in his time. He believed the planets and sun moved around the Earth in this order: Mercury, Venus,Sun, Mars, Jupiter, Saturn.

This system became known as the Ptolemaic system. It predicts the positions of the planets accurately enough for naked-eye observations, so it seemed accurate at the time. This is described in his book Mathematical Syntaxis (widely called the Almagest), a thirteen-book mathematical treatment of the phenomena of astronomy. It contains a wide variety of information ranging from earth conceptions to sun, moon, and star movement as well as eclipses and an explanation of the length of months. The Almagest also included a star catalog containing 48 constellations, using the names we still use today.

In addition to his well known works in astronomy, Claudius Ptolemy was very important in the history of geography and cartography (Making maps). He was influential up to the 16th century when his ideas were disproved by Nicolaus Copernicus. Ptolemy of course knew that the Earth is a sphere. Ptolemy's is the first known projection of the sphere onto a plane. His Geography remained the main work on the subject until the time of Christopher Columbus. But he had Asia extending much too far east, which may have been a factor in Columbus's decision to try to reach India by sailing west from Europe.

The Ptolemaic explanation of the motions of the planets remained the accepted wisdom until the Polish scholar Copernicus proposed a heliocentric view, or sun-centered view in 1543. It should be noted, too, that Ptolemy's system is actually more accurate than Copernicus's. The heliocentric formulation does not improve on Ptolemy's until Kepler's Laws are also added.

Ptolemy may not actually have believed in the reality of his system. He may have thought of it only as a method of calculating positions.

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