“Egyptians were probably the first people to adopt a predominantly solar calendar. Every year the Nile flooded and left behind rich black soil. They depended upon these annual deposits for growing many of their crops.
They observed that the Dog Star, Sirius, reappeared in the eastern sky just before sunrise after several months of invisibility, and that the annual flood of the Nile River occurred soon after this. Using this event to fix their calendar, they came to recognize a year 365 days long, with an extra dividend of five days added at the end. But they did not allow for the necessary extra fourth a day, and their calendar slowly drifted into error.
The Romans borrowed their first calendar from the Greeks, who got it from the Egyptians. In 46 B.C., Julius Caesar asked the astronomer Sosigenes to review teh calendar and improbé it. He solved the problem by adding a day to February every fourth year. The Julian calendar was widely used for more than 1500 years but it, too, was not completely correct- after a while the equinox was not occurring on the correct date.
In 1582, on the advice of astronomers, Pope Gregory XIII corrected the discrepancy between the Sun and the calendar by ordering ten days dropped from October that year only. This procedure restored the equinox to its proper date. To correct the Julian calendar’s error regularly, the Pope decreed that February would have an extra day in century years that could be divided by 400, such as 1600 and 2000, but not in other century years, such as 1700, 1800, and 1900. We still use this Gregorian calendar today.”
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