Astronomy Fun Fact #40
Leap year exists to account for the fact that a day on Earth is actually 24 hours and 4 minutes long.
A "day" is defined as the amount of time it takes the Sun to return to the same spot in the sky as the Earth rotates. But the Earth isn't just rotating, it's also orbiting around the Sun.
Every day, as the Earth moves farther along its orbit, it has to rotate just a bit more than 360 degrees to make up for the extra distance before the Sun appears in the same spot in the sky. And it takes the Earth ~4 minutes to do that extra bit of rotation.