It’s Leap Day today, and that means people born on February 29th finally get to have a birthday. But why do we have Leap Years? What’s the purpose? Well, it ensures that our seasons don’t drift.
You see, it takes the Earth 365.256363004 days to orbit the sun (this is known as the sidereal year). The calendar year is 365 days. But every 4 years, we have an extra day. So, we add a day to the years that can be divided by 4. This year is 2016, and is divisible by 4.
Sounds simple, doesn’t it? But it gets a bit complicated. This accounts for 365.25 days. But what about the extra 0.006363004 days? This accumulates, and after 400 years, it equals about 3 days extra. So, we have 3 too many Leap Days every 400 years. So what do we do? We just drop the Leap Days that happen on century years, but keep the ones that can be divided by 400. As a result, the year 2000 was a Leap Year, but 1900 and 2100 are not.
Leap years fall on years that can be divided by 4, except if they can be divided by 100 (not including the ones that can be divided by 400). This balances the calendar and seasons, and we don’t have any drift.
But why is it called Leap Year? Well, in normal years, let’s say that March 31st is a Monday, then the next year it’s a Tuesday, and then it’s a Wednesday the following year. But then, on a Leap Year, we leap over the next day (Thursday) so it’s on a Friday. And that is why we call it a Leap Year.