Happy Leap Day!
What is Leap Day?
February 29th is Leap Day.
But, doesn't February only have 28 days?
Most of the time, but every four years an extra day is added to the calendar.
It has do do with the difference between a calendar year and a solar year.
I know that a calendar year has 365 days. What is a solar year?
A solar year is the time it takes for the earth to make one complete orbit around the sun. This is not exactly 365 days. In fact it is close to 365 and 1/4 days. Because it is difficult to add 1/4 day to the calendar, every four years an extra day (February 29th) is added to the calendar year to make the two equal out.
What would happen if we didn't add the extra day?
Over time the calendar would start to shift and seasons would no longer begin on the same dates. The would become later and later each year. Could you imagine Spring not beginning until June or July? If the calendar were not adjusted, this would eventually happen.
So every four years we can expect the calendar to have an extra day?
Yes and no. A solar year is not exactly 365 and 1/4 days. It is actually 365 days 5 hours 48 minutes and 45 seconds. This means that when a leap day is added to the calendar, it is actually adding back too much time. Over the course of 400 years this adds up to about 3 days too many.
So what happens to the extra days?
They get taken away. Every year that is divisible by 4 (like 2016) is a leap year, unless that date is also divided by 100. Years that are divisible by 100 do not get a leap day added.
But, wait a minute! I remember the year 2000 and it had a leap day!
There is one other quirk to the system. Because the addition of a leap year only adds up to 3 extra day over the course of 400 year, if the year is divisible by 400 it still gets a leap day added. This means that the year 2000 had a leap day, as did the year 1600. The year 2400 will also get one, but the years 2100, 2200, and 2300 will not.
So I guess it is important to look before you leap?