Acoreus was a wise man consulted by Julius Caesar
Julius Caesar
Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....

, according to the Roman
Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

 writer Lucan, asking him many questions about ancient Egypt
Ancient Egypt
Ancient Egypt was an ancient civilization of Northeastern Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. Egyptian civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh...

’s history and its calendar
Egyptian calendar
The ancient civil Egyptian calendar had a year that was 360 days long and was divided into 12 months of 30 days each, plus five extra days at the end of the year. The months were divided into three weeks of ten days each...

. Caesar learned that the Egyptians based their year on the solar year, that is, on the apparent motion of the Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 through all of the zodiac
In astronomy, the zodiac is a circle of twelve 30° divisions of celestial longitude which are centred upon the ecliptic: the apparent path of the Sun across the celestial sphere over the course of the year...

al constellations, and that the Egyptians knew that such a year averaged 365 1/4 days.

The Egyptians
Egyptians are nation an ethnic group made up of Mediterranean North Africans, the indigenous people of Egypt.Egyptian identity is closely tied to geography. The population of Egypt is concentrated in the lower Nile Valley, the small strip of cultivable land stretching from the First Cataract to...

 were the first people to base their calendar on the solar year, though in a form that caused it to wander through all the seasons, based on the annual flooding of the Nile
The Nile is a major north-flowing river in North Africa, generally regarded as the longest river in the world. It is long. It runs through the ten countries of Sudan, South Sudan, Burundi, Rwanda, Democratic Republic of the Congo, Tanzania, Kenya, Ethiopia, Uganda and Egypt.The Nile has two major...

. Their calendar year had just 365 days, organized into 12 months of 30 days each with five epagomenal days added to the end of each year. Although they realized that their year wandered, they did not attempt to stop it until 238 BC
238 BC
Year 238 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Gracchus and Falto...

, when the pharaoh Ptolemy III attempted to add an extra day to every fourth year, but these leap days were apparently never implemented. He may have based his intercalation
Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.- Solar calendars :...

 frequency on the Callipic cycle, which consisted of 76 solar years averaging 365¼ days each, invented about a century earlier by the Greek astronomer Callippus
Callippus or Calippus was a Greek astronomer and mathematician.Callippus was born at Cyzicus, and studied under Eudoxus of Cnidus at the Academy of Plato. He also worked with Aristotle at the Lyceum, which means that he was active in Athens prior to Aristotle's death in 322...

. Although that cycle was never used in any calendar, knowledge of it would have resided in the great library at Alexandria, established by Ptolemy II several decades earlier.

Julius Caesar based his calendar on this Egyptian knowledge of the average solar year supplied by Acoreus and Sosigenes
Sosigenes of Alexandria
Sosigenes of Alexandria was named by Pliny the Elder as the astronomer consulted by Julius Caesar for the design of the Julian calendar. Little is known about him apart from Pliny's Natural History...

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