Why a Leap Year in 2024?

In 2024 we add February 29 to the calendar. Leap years, which debuted in the Gregorian calendar in the 16th Century, occur every four years, with exceptions, to keep our human time-keeping devices in sync with the natural rhythm of the earth.

The earth rotates 365 times in about one revolution around the sun. Actually, after 365 rotations the earth comes up just a smidgen short of its January 1 starting line. It needs six additional hours of travel time to reach the starting line. After four years, the earth is 24 hours shy of the starting line on January 1, so we add a day on the calendar to allow it to catch up to its starting line in space. Since February has room to grow, we tack the extra day onto that month.

Here's my simple demonstratin of why we have leap years: http://youtu.be/e10PwkScF-I.

Analogy: Track Practice

A perspective different from the video above is to imagine you are a runner at a track practice. The coach sets a pace for you to sprint around the track. "Go!", she says, and you're off. However, one stride before you get back to the starting point she says "Go!" again. You're behind, but you start the next split. Maintaining your pace, when you approach the starting point the second time you're now two paces behind when the coach shouts "Go!" After the third lap, three strides behind. After the fourth lap, four strides behind.

This time, with you four strides behind, the coach allows you to catch up. She waits until you've reached the starting line before barking "Go!" again.

As a runner, you're moving straight ahead, but as a planet you'd have to rotate as you revolve around the track--a quarter turn for each stride. At the start, you (the running rotating planet) are facing forward. After the first lap (when you're short of the finish and when coach says "Go!") you finish facing right, a quarter turn short of that final rotation. After the second split, you're facing backward, two quarter rotations behind. After the third split, you're facing left, three fourths short of your final rotation. After the fourth split, you're facing forward again, but four strides short of the line.

When the coach allows you four strides to get back to the starting line, with each stride you rotate the quarter turn, so after four strides you're back at the starting line and facing straight ahead again. With each stride equivalent to 6 hours, those four quarter-turn strides are your leap day.