The next leap year is on February 29, 2020. Almost every 4 years we experience a leap year where one extra day is added to February, the shortest month of the year. So instead of 28 days in February, we get 29. But why is there a leap year?
Leap years are actually key in ensuring our calendar works properly.
making up for lost time
Most of the world now uses the Gregorian calendar, a solar calendar, which is based on the revolutions of the Earth around the sun. We know how long a year is from tracking the sun’s position and the seasons. The calendar has 365 days. But it actually takes the Earth about 365 days, 5 hours, 48 minutes and 45 second to revolve around the sun once. Without adding an extra day to February almost every 4 years, about 6 hours would be lost every year. That means it would only take 100 years for the calendar to become inaccurate by about 24 days.
Keep in mind that a leap year is not every 4 years. For every 400 years, 3 leap years need to be skipped. This is to make up for slight time discrepancies. So for example, 1700, 1800 and 1900 were not leap years. As you can see, these years are not evenly divisible by 400. In fact, any century year that is not evenly divisible by 400 can be a leap year.
The Gregorian calendar
Leap years first came into use more than 2000 years ago. The Roman general Julius Caesar adopted leap years into the Julian calendar which began in 45 BC. But at that time the only requirement for a year to become a leap year was for it to be divisible by four. This meant too many leap years occurred. This is a problem because of the effect on the seasons as the time adds up. By the 16th century, the vernal equinox had moved up from March 21 to March 11.
The Julian calendar was adjusted in 1528 by Pope Gregory XIII to rectify the time discrepancies. According to the Gregorian calendar, a leap year is identified as being evenly divided by 4 and also evenly divided by both 100 and 400. If a year can be evenly divided by 100 but not also evenly by 400, it is not a leap year.
You might be wondering how February became the default month for a leap year. The reason goes back to Julius Caesar. In the Julian Calendar, February had 30 days, July had 31 days and August only had 29. When Augustus became the new roman emperor he wanted August to have 31 days. To do this he added two extra days to August. One of those days came from February leaving it with 28 and not 29 days.