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(originally published to Helium writing site, now gone)

Today we take our solar calendar for granted. But it was the ancient Egyptians who were the first to develop a solar calendar. Before the unification of Lower and Upper Egypt around 3150 BCE into what we call the ancient Egyptian civilization, the two countries developed their own calendars. In Lower Egypt, the winter solstice was regarded as the birthplace of their sun god Ra. Around 4500 BCE, they counted the time elapsed between Ra’s visits to his birthplace as 365 days. To keep track of his birthday, they introduced a lunisolar calendar of this length. It had 12 moons or months of 29 or 30 days each and an additional or intercalary month every two or three years as the first month. This meant the celebration of the birth of Ra could always be in the last month.

In Upper Egypt, the year was measured as the time elapsed between floodings of the Nile River. This was a very important event for the farming communities living along its banks and they wanted a way of determining the actual time of the flood. They noticed Sirius, or the Star of Isis or the Nile Star as they called it, rising next to the sun every 365 days, a few days before the Nile’s inundation. This coincided with the summer solstice. Priests declared the start of a new year as soon as they saw Sirius in this position. This was the first sidereal calendar, or one based on star movements.

When the two Egypts unified, so did their calendars. This was relatively easy as the period between the winter solstice and the rising of Sirius just before the summer solstice is about six months. In an otherwise lunar calendar, the rising of Sirius became the dominant marker, with the interval of its successive appearances next to the sun being just 12 minutes shorter than the solar year. But ancient Egypt soon ran into problems with its new calendar, basically because the solar year of about 365 days doesn’t divide equally into the lunar cycle of about 29.5 days.

The new Egyptian calendar soon failed to serve all purposes and they introduced additional calendars. While keeping their lunar-sidereal calendar for agriculture, religion and everyday life, they developed a civil calendar of 12 months of 30 days plus five extra days, for government and administration. They also brought in three seasons of four months: inundation from late June, growth from late October and harvest from late February. But the civil calendar of 365 days was just short of the solar year and progressively fell out of alignment with the seasons. As a result, the Egyptians brought in a second lunar calendar for religious events. But it was based on the civil year and, to keep the two aligned, an intercalary month was added when necessary. The original lunar-sidereal calendar was retained for agricultural purposes as it aligned with the seasons.

So by 2500 BCE, ancient Egypt had three calendars, and they ran side by side for over 2,000 years. Despite the Egyptians calculating a solar year as 365.25 days by around 2050 BCE or earlier, the priests wouldn’t change their sacred calendar and it drifted from the solar year and the seasons at nearly a quarter of a day per year, or by a whole year over a period of about 1,500 years.

It was not until 238 BCE that Ptolemy III rectified the situation by introducing an extra day every fourth year. This calendar became the basis of the Julian calendar, which had as its impetus none other than Cleopatra VII (69-30 BCE). She and younger brother Ptolemy XIII had fallen out and were vying for the throne. During the Roman civil war, Julius Caesar’s rival Pompey had fled to Alexandria. In 48 BCE, knowing that Caesar was about to visit and that Egypt was in debt to Rome, Ptolemy arranged the assassination of Pompey to please Caesar.

Two days later, Caesar arrived in the Egyptian capital and Ptolemy presented him with Pompey’s severed, pickled head. But Caesar wasn’t impressed, probably because Pompey was still Consul of Rome and was widower of Caesar’s only daughter, who had died in childbirth with their son. Caesar took the city and installed himself as arbiter between Ptolemy and Cleopatra. Taking advantage of Caesar’s anger with her brother, Cleopatra got inside a roll of carpet or bed coverings and had servants present it to Caesar. It was unrolled and out tumbled the young queen, naked. This time, Caesar was impressed and the two became lovers, despite a 30 year age difference. He abandoned his plans of annexing Egypt and supported Cleopatra’s claim to the throne.

She discussed with him how the Egyptians’ set their time by the sun rather than the moon, and introduced him to the astronomer Sosigenes. He and Cleopatra told Caesar about her ancestor Ptolemy III’s calendar, the current Egyptian calendar. Caesar liked it, dropped in the Roman months, and he and Cleopatra negotiated the fine details such as the date of New Year’s day. The Julian calendar was introduced into ancient Egypt before its defeat by the Roman Empire in 31 BCE, although its uptake varied between regions.