Why Do We Have Leap Years?

Most often we drag through the year but have you ever actually stopped to think, why do we have leap years?

There is actually not a whole number of days in a year. There is 365 days and a 1/4 in a year (or to be precise every 365.2425 days). Every four years we make up for that extra day by adding an extra day on our calendar. We have this problem, because the earths rotation and the earths orbit around the sun have nothing to do with each other. Every time the earth rotates in a full circle, that is one earth day. Every time the earth does a full orbit around the sun, that is an earth year.

The idea of a leap year or leap month was made by the Romans in 46 B.C.E. The Romans kept track off time by using lunar calendars. They did this by tracking the moons cycles. But, the moon goes through a little more then 12 cycles each, in a little more then 29 days. The Romans had the 12 lunar months is too little, and 13 lunar months per year is too many. The Romans decided that they should add a extra lunar month every couple of months to keep things like harvesting, around the same solar date. Instead of having an organised lunar system, the Romans decided to let their leader chose when they should have an extra month. This worked until Julius Caesar came along. The Romans believed that adding on a extra month was bad luck when they were in war. The Romans were in war and dates got thrown off, so Julius decided to scrap the old idea. He proposed that there was a set system that had a consistent time where they would add an extra day. His system is almost identical to ours with 365 days and an extra month every four years. Lunarcalendar

There is a slight issue with this system though; the four year system slightly over-estimates the length of the year by about 10 minutes. They new about this at the time but Julius did not want to complicate things. But 10 minutes a day really adds up, and by the late 1500’s the calendar was about 13 days off from where it was supposed to be. So Pope Gregory XIII made some slight variations and we are actually still using this system. It is called the Gregorian calendar and the change was, “Every 4 years add one extra day, don’t add an extra day if the year is divisible by 100. But do add it if the year is dividable by 400.” If we were still using the Julian calendar, in 2016, we would have been behind by 16 days. The Gregorian calendar is still slightly off and we will have to add a day every 7700 years.

There is slightly more going on in the calendar then it may seem like.



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