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November 05, 2008 by Chris Pearce Chris Pearce Published Content: 27 Total Views: 3,786 Favorited By: 0 Sources Full Profile | Follow | Add to Favorites Recommend (1)Single page Font SizePost a comment

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. 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 into the lunar cycle of about 29.5 days.