The modern Gregorian calendar is modeled very closely on a Roman version that dates back more than 2,000 years. Early Roman calendars were likely cribbed from Greek models that operated around the lunar cycle. But because the Romans considered even numbers unlucky, they eventually altered their calendar to ensure that each month had an odd number of days. This practice continued until 46 B.C., when Julius Caesar and the astronomer Sosigenes instituted the Julian system to align the calendar with the solar year. Caesar lengthened the number of days in a year from 355 to the now-familiar 365 and eventually included the 12 months as we know them today. The Julian calendar was almost perfect, but it miscalculated the solar year by 11 minutes. These few minutes ultimately threw the calendar off by several days. This led to the adoption of the nearly identical Gregorian calendar in 1582, which fixed the discrepancy by altering the schedule of leap years.
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