A short history of the calendar
AFor thousands of years, people have measured time by observing the sky. Early calendars were often based on the phases of the Moon, but a lunar year does not match the length of the solar year, which caused the seasons to drift out of step over time.
BThe calendar most of the world uses today is the Gregorian calendar, introduced in 1582 to correct small errors that had built up in the earlier Roman calendar. Its careful system of leap years keeps it closely aligned with the seasons, which is why it has been so widely adopted.