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Calendar, method of reckoning days and months of the year. The earliest calendars were based upon the phases of the moon. The first day of each month was the day of the new moon. When the moon had gone through its cycle of phases 12 times, a year was said to have passed. Unfortunately, the moon takes about 29 days to go through its phases, and consequently 12 lunar months (354 days, 8 hrs, 48 min) are 11 days short of a full year. Only by adding an extra month every 3 years or so could the calendar be kept roughly in line with the seasons. The Jewish and Muslim calendars are lunar calendars. The ancient Egyptians used a solar calendar with 12 months of 30 days each and added 5 feast days at the end of the year, making it one of the most accurate early calendars. But the length of the solar year is about 365 days, so that the Egyptian calendar got 1 day out of step with the seasons every 4 years. The Roman calendar, originally contained only 10 months and began with March. Later January and February were added to the end of the year, and the months received their odd numbers of days in an attempt to fit every day of the year into months of roughly equal length. In 46 B.C. Julius Caesar reformed the calendar, making January the first month and adding a leap year day to February in every fourth year to prevent the calendar from getting out of step. This system, the Julian calendar, continued to be used in Europe for over 1,500 years. Because a year is not exactly 365 days long, but slightly shorter (365 days, 5 hrs, 48 min, 46 sec), adding an extra day every 4 years proved too much. By the 16th century the calendar was 10 days out of step with the seasons. Pope Gregory VIII decided that Thursday, Oct. 4, 1582, would be followed by Friday, Oct. 15, thus wiping out the error. To prevent a recurrence, he ruled that a century year could be a leap year only if divisible by 400. The Gregorian, or New Style, calendar, though not perfect, is adequate for most practical purposes and is the calendar in widest use today.

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