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Oenopides of Chios was an ancient Greek mathematician (geometer) and astronomer, who lived around 450 BC. He was born shortly after 500 BC on the island of Chios, but mostly worked in Athens.
Astronomy
The main accomplishment of Oenopides as an astronomer was his determination of the angle between the plane of the celestial equator, and the zodiac (the yearly path of the sun in the sky). He found this angle to be 24°. In effect this amounted to measuring the inclination of the earth axis. Oenopides's result remained the standard value for two centuries, until Eratosthenes measured it with greater precision.
Oenopides also determined the value of the Great Year, that is, the shortest interval of time that is equal to both an integer number of years and an integer number of months. As the relative positions of the sun and moon repeat themselves after each Great Year, this offers a means to predict solar and lunar eclipses. In actual practice this is only approximately true, because the ratio of the length of the year and that of the month does not exactly match any simple mathematical fraction, and because in addition the lunar orbit varies continuously.
Oenopides put the Great Year at 59 years, corresponding to 730 months. This was a good approximation, but not a perfect one, since 59 (sidereal) years are equal to 21550.1 days, while 730 (synodical) months equal 21557.3 days. The difference therefore amounts to seven days. In addition there are the interfering variations in the lunar orbit. However, a 59 year period had the advantage that it corresponded quite closely to an integer number of orbital revolutions of several planets around the sun, which meant that their relative positions also repeated each Great Year cycle. Before Oenopides a Great Year of eight solar years was in use (= 99 months). Shortly after Oenopides, in 432 BC, Meton and Euctemon discovered the better value of 18 years, equal to 223 months (the socalled Saros period).
Geometry
While Oenopides's innovations as an astronomer mainly concern practical issues, as a geometer he seems to have been rather a theorist and methodologist, who set himself the task to make geometry comply with higher standards of theoretical purity. Thus he introduced the distinction between 'theorems' and 'problems': though both are involved with the solution of an exercise, a theorem is meant to be a theoretical building block to be used as the fundament of further theory, while a problem is only an isolated exercise without further followup or importance.
Oenopides apparently also was the author of the rule that geometrical constructions should use no other means than compass and straightedge. In this context his name adheres to two specific elementary constructions of plane geometry: first, to draw from a given point a straight line perpendicular to a given straight line; and second, on a given straight line and at a given point on it, to construct a rectilineal angle equal to a given rectilineal angle.
Miscellaneous opinions attributed to Oenopides
Several more opinions in various areas are attributed to Oenopides:
 He is said to have given an explanation of the flooding of the Nile each summer. On the basis of observations of the temperature of water in deep wells he seems erroneously to have inferred that underground water is in fact cooler in summer than in winter. In winter, when rain fell and seeped into the ground it would soon evaporate again because of the heat in the soil. However, in summer, when water in the ground was supposedly colder, there would be less evaporation. The surplus of moisture would then have to be carried off otherwise, thereby causing the Nile to overflow.
 To Oenopides was attributed the opinion that formerly the sun had moved along the milky way. However, when it saw how Thyestes, a mythological figure, was served his own son for dinner by his brother Atreus, the sun was so horrified that it left its course and moved to the zodiac instead. This tradition sounds quite unreliable.
 Oenopides was said to have regarded the universe as a living organism, God or the Divine being its soul.
 He is also said to have considered air and fire as being the first principles of the universe.
References
 Ivor BulmerThomas, 'Oenopides of Chios', in: Dictionary of Scientific Biography, Charles Coulston Gillispie, ed. (18 volumes; New York 19701990) volume 10 pp. 179182.
 István M. Bodnár: Oenopides of Chius. A survey of the modern literature with a collection of the ancient testimonia. Berlin 2007 (MaxPlanckInstitut für Wissenschaftsgeschichte, Preprint 327; http://www.mpiwgberlin.mpg.de/Preprints/P327.PDF, 560 KB)
 Kurt von Fritz, 'Oinopides', in: Paulys Realencyclopädie der Classischen Altertumswissenschaft, G. Wissowa, ed. (51 volumes; 18941980) volume 17 (1937) columns 22582272 (in German).

Aglaonice Agrippa Anaximander Andronicus Apollonius Aratus Aristarchus Aristyllus Attalus Autolycus Bion Callippus Cleomedes Cleostratus Conon Eratosthenes Euctemon Eudoxus Geminus Heraclides Hicetas Hipparchus Hippocrates of Chios Hypsicles Menelaus Meton Oenopides Philip of Opus Philolaus Posidonius Ptolemy Pytheas Seleucus Sosigenes of Alexandria Sosigenes the Peripatetic Strabo Thales Theodosius Theon of Alexandria Theon of Smyrna Timocharis
Ancient Greek and Hellenistic mathematics (Euclidean geometry)
Mathematicians
(timeline)
Anaxagoras Anthemius Archytas Aristaeus the Elder Aristarchus Apollonius Archimedes Autolycus Bion Bryson Callippus Carpus Chrysippus Cleomedes Conon Ctesibius Democritus Dicaearchus Diocles Diophantus Dinostratus Dionysodorus Domninus Eratosthenes Eudemus Euclid Eudoxus Eutocius Geminus Heliodorus Heron Hipparchus Hippasus Hippias Hippocrates Hypatia Hypsicles Isidore of Miletus Leon Marinus Menaechmus Menelaus Metrodorus Nicomachus Nicomedes Nicoteles Oenopides Pappus Perseus Philolaus Philon Philonides Porphyry Posidonius Proclus Ptolemy Pythagoras Serenus Simplicius Sosigenes Sporus Thales Theaetetus Theano Theodorus Theodosius Theon of Alexandria Theon of Smyrna Thymaridas Xenocrates Zeno of Elea Zeno of Sidon Zenodorus
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Almagest Archimedes Palimpsest Arithmetica Conics (Apollonius) Catoptrics Data (Euclid) Elements (Euclid) Measurement of a Circle On Conoids and Spheroids On the Sizes and Distances (Aristarchus) On Sizes and Distances (Hipparchus) On the Moving Sphere (Autolycus) Euclid's Optics On Spirals On the Sphere and Cylinder Ostomachion Planisphaerium Sphaerics The Quadrature of the Parabola The Sand Reckoner
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Angle trisection Doubling the cube Squaring the circle Problem of Apollonius
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Results
In Elements
Angle bisector theorem Exterior angle theorem Euclidean algorithm Euclid's theorem Geometric mean theorem Greek geometric algebra Hinge theorem Inscribed angle theorem Intercept theorem Pons asinorum Pythagorean theorem Thales's theorem Theorem of the gnomon
Apollonius
Apollonius's theorem
Other
Aristarchus's inequality Crossbar theorem Heron's formula Irrational numbers Menelaus's theorem Pappus's area theorem Problem II.8 of Arithmetica Ptolemy's inequality Ptolemy's table of chords Ptolemy's theorem Spiral of Theodorus
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