Why is there an extra day in February every 4 years?

Answer 1

See if you can imagine this in your mind as we go. The earth orbits around the sun, and one full orbit gives us one year. See the earth moving in its yearly orbit. Take it as a given fact that we can know very precisely when the earth reaches the very moment when we go from one year to the next. This is hard, but forget about Dec 31, midnight, all that... Just imagine the earth moving around the sun, and CLICK-- for a moment the earth is at the exact spot where it goes from one year to the next. The earth is also rotating on its axis, and the rotational speed is independent of the orbital speed. The exact length of the year cannot be divided into an exact number of full days. So when earth reaches that point in space ending the year, it is NOT, I repeat NOT, always at the very same clock time on Dec 31. You know that there are 365 days in the year, but that is an estimate. There are 365 and 1/4 days in the year, and that isn't quite exact either. So this coming Dec 31, we will actually be shouting "happy new year!" about a quarter of a day short of when we did it last year. Last year, we got all excited about "new year" about a quarter of a day earlier than the year before. It really takes the earth 365 and a quarter days to complete a full year, but we measure time in full days! A quarter of a day per year is actually a big problem, and if we let this continue, before long the months wouldn't match up with the seasons-- not because there would be something wrong with the earth, but because there would be something wrong with the way we are timing events. So, every four years (when the year can be divided by 4 with no remainder) we have to add a day to the calendar. We have to "stretch" a year out now and then, so that the months and the seasons stay in synch. As it turns out, this correction is very good, but it is slightly too much. As a result, full century years are treated a little differently. Full century years, like 1600, 1700, 1800, get a leap year only if they are divisible by 400 without a remainder! Few people realized it, but the leap year that we added in the year 2000 was a very rare event, and an important calendar correction. The next time a century leap year will be given an extra day is the year 2400! The day has to go somewhere, and February is a better choice than the others, since it is the shortest at the usual 28 days.
There is an extra day every leap year because the earth's orbital period is slightly longer than 365 days.