Leap Years

Today is February 29th and that means we are in a leap year. Leap Years help keep calendars accurate. What seems like an obvious need has a very complex history.

The Ancient Romans realized the need for extra time. Their calendars were beginning to get ahead of the actual solar cycle because calendar years are 365 days and solar years on Earth are slightly less than 365.25 days. This means every calendar year six hours are lost and every 4 years a day is lost. So every four years was given an extra day to make up the difference.

Problems still existed. Solar years are 365.2422 calendar days (not 365.25) so a difference was growing. In the late 1500s church officials realized that Easter was coming closer and closer to the Spring Equinox and eventually Easter would start occurring in Winter. So Pope Gregory XIII issued Inter Gravissimas which declared every century not divisible by 400 to be a common year (with no leap day) and jumped the calendar 10 days from October 4, 1582 to October 15, 1582. The change allowed for the new, Gregorian, calendar to be neigh accurate.

The close synchronization will keep the Gregorian calendar aligned with solar time for the next 3,100 years.

Tomorrow: Read how the Gregorian calendar took on the world and how other calendars reflect cultural world views.