Ethan Siegel in Forbes:
Once approximately every four years, the elusive entity that occurs this Monday — February 29th — comes along. The historical origins and urban legends associated with it are incredibly interesting, but the reason there’s any such thing asLeap Day at all is because of the physics of planet Earth. The Earth, of course, is rotating on its axis while simultaneously revolving around the Sun. That rotation is responsible for sunrise, sunset, moonrise, moonset, the Coriolis effect, and the rotation of all the stars in the night sky about the poles. Revolution, on the other hand, is responsible for the seasons; when your hemisphere tilted away from the Sun, that’s when you have your winter (and minimum daylight), and when your hemisphere is tilted towards the Sun, that’s when you have your luminous summer.
And you probably learned that a day is 24 hours, due to the rotation, while a year is 365 days (with an occasional 366 for leap years), taking care of the revolution. It turns out it’s a little more complicated than that!