We got hit with a blizzard in Duluth, Minn. Wednesday and my planned blog on Leap Day was delayed – to say the least. So let’s get to it before February 29 comes to a quick end.

While we normally think of Earth revolving around the sun in 365 days, it actually does so in 365 1/4 days. Those extra quarter days add up to **one full day** in four years, which is added into the calendar every fourth year as February 29. The Leap Day was introduced by none other than **Julius Caesar **in 46 B.C.

If we ignored the quarter day and carried merrily along, we’d soon discover the calendar would fall behind by one day after four years. No big deal, right? But after only 100 years the slip would amount to about 24 days. Instead of the first day of spring starting on March 21, it wouldn’t begin until mid-April.

To keep everything tidy, so that our calendar year, which is based on a whole number of days, matches up with the true time it takes Earth to orbit the sun, we add a Leap Day every four years. Problem solved, right? Well not quite. Earth’s precise orbital period is 365 days, 5 hours, 48 minutes and 46 seconds. That’s 11 minutes and 14 seconds shy of a quarter day. So that we don’t *overcompensate* with Leap Days, we omit them three times every 400 years based on this simple formula:

* Leap year occurs in years divisible by four, except for those divisible by 100 and not divisible by 400. This means that 2000 and 2400 are leap years, while 1800, 1900, 2100, 2200, 2300 and 2500 are *not* leap years. That last adjustment to the calendar was made by **Pope Gregory XIII** in 1582. Our calendar and Earth’s orbital period will now be congruent for a very long time – 3,300 years to be exact – when they’ll diverge again by one full day. Come that time, who knows if we’ll still even be using calendars. Something to think about as March tiptoes in after February’s long-winded exit.

Hi Bob,

We in Moorhead got hit with the same blizzard as you. When we try to apply whole and simple number numbers to a cosmos that doesn’t always allow for it reminds of what Carl Sagan once wrote.

“The universe is not required to be in perfect harmony with human ambition.”

Thanks for the nice explanation of leap day,

Travis

Hi Travis,

I like how you put that, and the Sagan quote is most fitting.