History of the Calendar
The purpose of the calendar is to reckon past or future time, to show how many days until a certain event takes place – the harvest or a religious festival – or how long since something important happened. The earliest calendars must have been strongly influenced by the geographical location of the people who made them. In colder countries, the concept of the year was determined by the seasons, specifically by the end of winter. But in warmer countries, where the seasons are less pronounced, the Moon became the basic unit for time reckoning; an old Jewish book says that “the Moon was created for the counting of the days.”
Most of the oldest calendars were lunar calendars, based on the time interval from one new moon to the next – a so-called lunation. But even in a warm climate there are annual events that pay no attention to the phases of the Moon. In some areas it was a rainy season; in Egypt it was the annual flooding of the Nile River. The calendar had to account for these yearly events as well.
History of the Lunar Calendar
The lunar calendar became the basis of the calendars of the ancient Chinese, Babylonians, Greeks, and Jews.
During antiquity the lunar calendar that best approximated a solar-year calendar was based on a 19-year period, with 7 of these 19 years having 13 months. In all, the period contained 235 months. Still using the lunation value of 291/2 days, this made a total of 6,9321/2 days, while 19 solar years added up to 6,939.7 days, a difference of just one week per period and about five weeks per century.
Even the 19-year period required adjustment, but it became the basis of the calendars of the ancient Chinese, Babylonians, Greeks, and Jews. This same calendar was also used by the Arabs, but Muhammad later forbade shifting from 12 months to 13 months, so that the Islamic calendar now has a lunar year of about 354 days. As a result, the months of the
Islamic calendar, as well as the Islamic religious festivals, migrate through all the seasons of the year.
History of the Egyptian Calendar
The Egyptian year coincided precisely with the solar year only once every 1,460 years
The ancient Egyptians used a calendar with 12 months of 30 days each, for a total of 360 days per year. About 4000 B. C. they added five extra days at the end of every year to bring it more into line with the solar year.1 These five days became a festival because it was thought to be unlucky to work during that time.
The Egyptians had calculated that the solar year was actually closer to 3651/4 days, but instead of having a single leap day every four years to account for the fractional day (the way we do now), they let the one-quarter day accumulate. After 1,460 solar years, or four periods of 365 years, 1,461 Egyptian years had passed. This means that as the years passed, the Egyptian months fell out of sync with the seasons, so that the summer months eventually fell during winter. Only once every 1,460 years did their calendar year coincide precisely with the solar year.
In addition to the civic calendar, the Egyptians also had a religious calendar that was based on the 291/2-day lunar cycle and was more closely linked with agricultural cycles and the movements of the stars.
The Gregorian Reform
The Julian calendar is phased out.
By the 15th century the Julian calendar had drifted behind the solar calendar by about a week, so that the vernal equinox was falling around March 12 instead of around March 20.