History of the Calendar
|Artifacts from the Paleolithic suggest that the moon
was used to calculate time as early as 12,000, and possibly
even 30,000 BP. Lunar calendars were among the first to
appear, with all years having twelve lunar months
(approximately 354 days). Without intercalation to add days
or months to some years, seasons quickly drift in a calendar
based solely on twelve lunar months. Lunisolar calendars
have a thirteenth month added to some years to make up for
the difference between a full year (now known to be about
365.24 days) and a year of just twelve lunar months. The
numbers twelve and thirteen came to feature prominently in
many cultures, at least partly due to this relationship of
months to years.
The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar. This Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per year. Pope Gregory XIII introduced a correction in 1582; the Gregorian calendar was only slowly adopted by different nations over a period of centuries, but is today by far the one in most common use around the world.