The astronomer behind the Julian calendar is known from only a few lines of Pliny
When Julius Caesar reformed Rome's chaotic calendar, he consulted a Greek astronomer named Sosigenes. Almost everything known about him comes from Pliny the Elder's Natural History. Even the tag of Alexandria, attached to his name for centuries, turns out to be an 18th-century invention with no ancient source behind it.
Sosigenes worked in the first century BC. Pliny, writing about the rival calendar traditions of the Chaldeans, Egyptians and Greeks, says a fourth was added at Rome by Caesar during his dictatorship, who with the help of Sosigenes brought the drifting civil year back into step with the sun. The Julian year was introduced in 46 BC. To correct the accumulated error of the old Roman calendar, that single year ran to 445 days, earning it a reputation as the longest year in history.
Pliny credits him with one more piece of astronomy. Describing Mercury, the planet next to Venus, Pliny notes that it moves in a lower circle, completes its revolution nine days faster, and shows itself sometimes before sunrise and sometimes after sunset. Citing Cidenas and Sosigenes, he adds that it never strays more than 22 degrees from the sun, a limit on how far the planet appears from our star in the sky.
The label Sosigenes of Alexandria appears to be a later mix-up. Classicists began adding it in the 18th century, and its oldest known appearance is in a 1739 book on Roman customs by Le Fèvre de Morsan. No ancient writer calls him Alexandrian. The likely cause was a merger of two separate reports: Appian, Cassius Dio and Macrobius say Caesar's reform drew on Egyptian learning, while Pliny names Sosigenes among the experts consulted.
Popular culture filled the gaps freely. In the 1963 film Cleopatra, Hume Cronyn played him as the queen's tutor and adviser, later her envoy to Rome, who is killed in the Forum by Octavian, sparking war with Egypt. None of that has any basis in the historical record; the scenes were written for the film.
Source: Sosigenes (astronomer)
