'Leap Year Hopscotch': Why We Sometimes Skip 'Leap Day'

MINNEAPOLIS (WCCO) -- We will add one day to our calendar in 2020 because it's a leap year.

But, why? Let's start with the basics.

A year is defined as one trip around the sun, so 365 days -- right?

Wrong. One revolution actually takes about 356-and-a-quarter days.

Do the math, and you will find it takes 5.8 hours more than a calendar year for earth to go around the sun.

Over time, that would add up. In fact, if we did away with leap days, 100 years from now our calendar would be off by 24 days.

So, we add one full day to the calendar every four years. Problem solved, right? Not quite, because 365-and-a-quarter is not quite 365.24.

Even that little difference adds up to an error of about 11 minutes every year.

In 100 years, that would mean our calendar would be off 18 hours. Not bad, but still too much over the long run.

That's where "Leap Year Hopscotch" comes in. To help bring the calendar back into alignment, we skip Leap Day if it falls on the start of a century, unless that century is divisible by 400.  In those years -- 2000 was the last one -- Leap Day is observed.

Even with this crazy rule, the calendar doesn't quite match the earth's revolution, but it's close enough.

It will take over three millennia before our calendar is one day off. At that point, it's someone else's problem.