Every four years, we have a U.S. presidential election. In that same year, we also have a Summer Olympics. Not coincidentally, that year also is a leap year, a year with 366 days rather than 365.

Why is this? Can’t we make a better calendar?

To understand why we need leap years, we must first understand a little bit about days and years.

## A Year is not Exactly 365 Days.

There are several kinds of years. Two of the most important are:

** Sidereal Year – **A sidereal year is how long it takes the Earth to revolve one full 360 degree orbit around the sun as measured against the background stars. It takes 365 days, 6 hours, 9 minutes, and 10 seconds.

** Tropical Year – **A tropical year is how long it takes to complete one full seasonal cycle, to go from one vernal (spring) equinox to the next one. It takes 365 days, 5 hours, 48 minutes and 45 seconds.

This is almost 20 minutes shorter than a sidereal year. The difference is caused by the “wobble” of the Earth on its spin axis, which makes the axial tilt slowly shift its orientation and makes the seasons slowly drift with respect to our orbit around the sun.

## A Day is not Exactly 24 Hours.

There are also different kinds of days:

** Sidereal Day – **This is the time it takes the Earth to spin one complete 360 degree rotation on its axis, as measured against the background stars. It is 23 hours, 56 minutes, and 4 seconds long.

** Solar Day – **This is how long it takes the sun to track a full 360-degree circuit in the sky, from one meridian crossing to the next. It is 24 hours long.

The reason for the nearly 4-minute difference between a sidereal day and a solar day is that in one day, the Earth travels about 1.5 million miles along its orbit. So it takes an extra 4 minutes of rotation to bring us back in line with the sun as compared with the day before.

## Why We Have Leap Years.

Since we want to eat lunch when the sun is highest in the sky and we want summer to actually be warm, instead of cold and snowy, we are going to use Tropical Years and Solar Days most of the time, and a Solar Day does not divide evenly into a Tropical Year.

Every year, there are about 6 extra hours needed to complete the orbit. That does not sound like a big deal, but after four years of exactly 365 days each, we are going to be nearly a full day behind where we should be to complete the orbit. After 732 years, we would have a cold, snowy June and flowers blooming in December.

To avoid that problem, we simply add an extra day to the calendar. Since February is somewhat deprived compared to the other months, it receives the extra day.

Because a Tropical Year is 5 hours, 48 minutes, and 45 seconds longer than 365 days, we don’t need a leap year every single four-year period. Every year that ends in ’00 is not a leap year, ** unless** that year also is divisible by the number 400. So 1900 was not a leap year, and 2100 will not be a leap year, but 2000 was that rare year that was a leap year that ended in ’00.