Biography:Sosigenes of Alexandria

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Sosigenes of Alexandria
Σωσιγένης ὁ Ἀλεξανδρεύς
Known forConsulted by Julius Caesar for the design of the Julian calendar
Scientific career

Sosigenes of Alexandria (Greek: Σωσιγένης ὁ Ἀλεξανδρεύς) was a Greek astronomer from Ptolemaic Egypt who, according to Roman historian Pliny the Elder, was consulted by Julius Caesar for the definition of the Julian calendar.[1][2] This year format existed prior to its renaming and was already in use by astronomers, having been found in the Antikythera mechanism, which predates Caesar's life.[3]


Little is known about him apart from Pliny's Natural History. Sosigenes appears in Book 18, 210-212:

... There were three main schools, the Chaldaean, the Egyptian, and the Greek; and to these a fourth was added in our country by Caesar during his dictatorship, who with the assistance of the learned astronomer Sosigenes (Sosigene perito scientiae eius adhibito) brought the separate years back into conformity with the course of the sun.[citation needed]

Sosigenes is credited with work on the orbit of Mercury, which is described by Pliny in Book 2, chapter 6, of his Natural History:

The star next to Venus is Mercury, by some called Apollo; it has a similar orbit, but is by no means similar in magnitude or power. It travels in a lower circle, with a revolution nine days quicker, shining sometimes before sunrise and sometimes after sunset, but according to Cidenas and Sosigenes never more than 22 degrees away from the sun.[4]

The introduction of the Julian year occurred in 46 BC. This particular year lasted 445 days in Rome to correct the erroneous old Roman calendar.[citation needed]

Some sources state that the Julian calendar was designed by Aristarchus of Samos[citation needed], although it is not clear where this conclusion originates. Ptolemy III Euergetes, Aristarchus' contemporary, decreed a reform of the Egyptian calendar in 238 BC, but it was never implemented.[citation needed] The reform, however, would have added an extra day (leap day) to the 365-day Egyptian calendar every four years, a feature shared by the Julian calendar.[citation needed]

Sosigenes wrote several books, although only the titles of three of them survive: Essay on Aristotle (Υπόμνημα επί Αριστοτέλη), The Sky (Περί Ουρανού), and Optics (Περί όψεως).[citation needed]

Cultural depiction

Sosigenes was portrayed by Hume Cronyn in the 1963 movie Cleopatra. This portrayal is heavily fictionalized: he serves as Cleopatra's tutor/adviser and later her envoy to Rome. He is ultimately murdered in the Forum by Octavian, commencing his war against Egypt. None of these events are present in historical record and were invented for the film.[citation needed]

See also


  1. "Sosigenes of Alexandria | Greek astronomer mathematician". 
  2. Dialetis, Dimitris (2007). "Sosigenes of Alexandria". in Hockey, Thomas. Biographical dictionary of astronomers. vol. II, M-Z. Springer. p. 1074. 
  3. Freeth, Tony, Yanis Bitsakis, Xenophon Moussas, John H. Seiradakis, Agamemnon Tselikas, Helen Mangou, Mary Zafeiropoulou et al. "Decoding the ancient Greek astronomical calculator known as the Antikythera Mechanism." Nature 444, no. 7119 (2006): 587-591.; Moussas, Xenophon. The Antikythera Mechanism: The Oldest Computer and Mechanical Cosmos. School of Physics and Astronomy, University of Birmingham, 2014; Moussas, Xenophon. "Antikythera Mechanism as evidence for Hellenistic technology excellence." In Hellenistic Alexandria: Celebrating 24 Centuries–Papers presented at the conference held on December 13–15 2017 at Acropolis Museum, Athens, p. 209. Archaeopress Publishing Ltd, 2019.
  4. Book II, chapter 6, 36-41 in Pliny the Elder, Natural History I. Loeb Classical Library 330. Translated by H. Rackman, 1938, p.193. In a very old translation from C. Plinius Secundus, The Historie of the World, translated by Philemon Holland (1601), it is book 2 chapter 8, and reads: "Next upon it, but nothing of that bignesse and powerful efficacie, is the starre Mercurie, of some cleped Apollo: in an inferiour circle hee goeth, after the like manner, a swifter course by nine daies: shining sometimes before the sunne rising, otherwhiles after his setting, never farther distant from him than 23 degrees, as both the same Timæus and Sosigenes doe shew."