Happy Leap Day!

This year we got an extra day because a solar year lasts 365 days and 6 hours. Every four years, these extra hours add up to another day, a day we have decided to insert into our calendar at the end of February. (Developed in ancient Rome by Julius Caesar and therefore named after him.)

Without the leap year correction, every four years the calendar year falls behind the seasons by one day. After 754 years, the seasons would be 180 degrees out of phase with the calendar. The first day of autumn in the northern hemisphere would be March 21st instead of September 21st. After 1508 years, the calendar would go all the way around the seasons and would once again be synchronized with the sun.

Now things are back in order, or are they?

Not quite. Even 365.25 days is not the exact duration of a solar year - it's a fraction shorter.




The Gregorian calendar, named after calendar reformer Pope Gregory XIII, solves this problem by leaving out three Julian leap-days every 400 years. Algorithmically, if the year is divisible by 100, avoid the leap-day UNLESS the year is divisible by 400.

Are we done yet? Not quite! There is still some drift.

The modern calendar does one final tweak on this: if the year is divisible by 4000, leave out the leap-day despite the Gregorian recommendation above.

And to account for variation beyond this, we insert leap seconds. However, I won't really concern myself with this in my lifetime.



Rather, I will go out and enjoy the beginning of spring (in the northern hemisphere...)