Egyptian calendar

The ancient Egyptian calendar, devised more than five thousand years ago, was built in several stages. Originally, the calendar was lunar, or based on the cycles of the moon. The Egyptians grouped the calendar into three four-month seasons; each of these seasons corresponded with the ebb and flow of the Nile River. They related this calendar with the rise of Sirius, the brightest star in the sky, but that created a problem because their moon-based calendar did not coordinate well with the rise of a star, which is a sun-based event. Therefore, the Egyptians eventually designed a standard, or civic, calendar that was more practical to use for administrative or economic purposes. In that standard calendar, a year was 365 days long, consisting of twelve thirty-day months and an additional five special days added at the end of the year. The Romans ultimately adopted a variation of the Egyptian standard calendar.

Brief History

The year in which the Egyptian calendar was created is debatable. Scholars place it anywhere after 4231 BCE, although society was not literate at that time. The development of the calendar was gradual. As the ancient Egyptians recognized different patterns of the Nile and the sun, they incorporated those patterns into their calendar.

The Egyptian calendar developed as a way of predicting approximately when the Nile would rise. The initial ancient calendar began with the Egyptians’ observations of the climate of their land. Egypt is an arid, dry region, so the Egyptians were heavily reliant on the Nile River as a source of water for agriculture and daily needs. The rise and lowering of the Nile formed the basis of the calendar.

Noting that the movement of the Nile followed the stages of the moon, they developed a lunar calendar that was divided into three Nile-based seasons: the season of the rise of the river, the season of the fall of the river, and the season in which the river is at its lowest.

Eventually, they added a sun-based event to their calendar. They pinned the start of the year to the initial appearance of Sothis, the bright dawn star, which heralded the coming rise of the river.

By 2773, the Egyptians recognized their need for a more reliable, standardized calendar. By counting the days between the risings of Sothis, they created a 365-day long civic calendar, similar to the calendar of modern times. The calendar comprised twelve 30-day long months, with an extra five days tacked on at the end of each year for religious festivities.

Although the civic calendar corresponded closely to the number of days in the solar year, it failed to account for the extra quarter day it takes for the earth to rotate around the sun. Centuries later, when Julius Caesar adopted the Egyptian calendar, he adjusted it by adding a leap-day every four years.

Overview

The ancient Egyptians worked in agriculture, so they were highly attuned to changes in the Nile River, their main source of water. Typically, the Nile begins to rise at the end of May, reaches its peak and floods at around the beginning of September, and begins to fall by the middle or end of October. The Egyptians called the time of the rising of the Nile, when they had to move to higher lands, "Akhet," which means "flood." They called the period in which the level of the river fell, when they planted crops, "Proyet," which means "the coming forth." Last, they called the time period in which the water was lowest, when they reaped their crops, "Shomu," which means "harvest time."

The Egyptians noted that the moon passed through roughly four cycles during each of the Nile’s stages. In each lunar cycle, the moon starts off as a tiny sliver, then grows to a full sphere within fifteen days, and then spends the next fifteen days shrinking down to a small sliver again. On that foundation, they developed a lunar calendar of twelve months, which they grouped into three Nile-based seasons. The new year began with the rise of the Nile.

Although they could not identify any link between the sun and the Nile, the Egyptians decided to add a sun-based source as the start of the new year. Their decision stemmed from their recognition of the need for a more stable method of measuring the passage of time than the erratic rising of the Nile because flooding could occur within any time of an eighty-day period.

They chose Sothis, or Sirius, the brightest of the pre-dawn stars as their marker. Sothis is not visible for part of the year. The Egyptians realized that when Sothis reappears, the flooding of the Nile will follow soon thereafter, so they marked the rise of Sothis as the start of the new year.

They soon realized that correlating the rise of Sothis with the lunar cycle was complicated because Sothis is sun-based, so it rises every 365 ¼ days. The lunar calendar lasts about 354 days, so the rising of the star quickly became out of sync with their moon-based calendar. Therefore, every two or three years, the Egyptians had to add an extra month to their calendar in order to correct the lack of coordination between Sothis’s rise and the moon’s cycles.

Since the lunar calendar was difficult to use, in 2773, the Egyptians introduced a new, standardized civic calendar, developed by counting the number of days between each rise of Sothis. The civic calendar was three seasons long as well, each lasting four months. There were twelve months, each thirty days long, and the months were divided into weeks of ten days apiece, resulting in a 360-day long year.

To make the year long enough to account for the times between the rise of Sothis, they added a five-day month at the end of each year. This month, called "Epagomenal," was demarcated as a period of religious festivities for celebrating the gods’ birthdays.

Their 365-day year was still not completely accurate, because each solar year is actually 365-1/4 days long. The Egyptians did not incorporate leap years into their calendar to compensate for the shortfall, so their calendar ended up slipping at a rate of one quarter of a day each year.

Finally, in 238 BCE, Ptolemy III, the third king of the Egyptian Ptolemaic dynasty, solved the problem by adding leap years. With his system, every four years, there were six epagomenal days instead of five. However, the Egyptians did not embrace this change.

In 46 BCE, Julius Caesar revised the calendar, adding a leap-year day every four years. The modern Western calendar is based on that Egyptian-Julian calendar. Still, the Egyptians were not willing to adopt Caesar’s calendar until they converted to Christianity, around the fourth century CE.

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