In ancient Rome, Julius Caesar established the 365-day calendar to match the changing seasons created by the earth's movement around the sun. Caesar recognized a problem, though: the earth actually takes 365¼ days to circle the sun. The extra one-quarter day would gradually shift the calendar away from the seasons. Caesar's solution was to add an extra day every four years, in what are now called leap years. Caesar's math wasn't quite right, though. His approach added too many days. So, in 1582, Pope Gregory XIII improved on Caesar's work. He proposed that century years should not be leap years unless they can be evenly divided by four hundred. Thus, 1600 would be a leap year, while 1700 would not. Pope Gregory's calendar, called the Gregorian calendar, is still in use today.