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Welcome to the Upside-Down: Here’s what happens without leap years

Thursday, February 27th 2020, 9:00 am - Leap years are vitally important to keep our seasons from drifting

Saturday is a special day, when we're going to see something that only happens once every four years or so. We're going to have a Leap Day.

A leap day is a little 'hiccup' - if you will - in the normal progression of our yearly 'Gregorian' calendar. Every four years, give or take, we tack an extra day onto the end of the month of February, thus lengthening that specific year from 365 days to 366 days. We've been following this schedule of 'leap years', so far, for the past 437 years.

Given that this practice was introduced in the late 14th Century, it could be seen as archaic. Still, the addition of leap years to our calendar has been vitally important for us. Without them, we wouldn't be able to keep our calendar and the seasons lined up.

This video, made by Dr. James O'Donoghue, a planetary scientist at JAXA, the Japanese space agency, shows exactly what would happen if we didn't have leap years.

In just over 400 years, the calendar months drift backward so far that we'd be off by a factor of a full season. Thus, in the year 2425, we'd be observing Winter from March through June, Spring from June through September, Summer from September through December and Autumn from December through March.

It takes a long time to see significant impacts, but just adding that one day, every four years or so, avoids the whole problem.


We count on our calendars with precisely 365 days every year.

There's a problem with that, though. It doesn't take exactly 365 days for Earth to travel around the Sun.

Instead, it takes roughly 365 days, 5 hours, 48 minutes, and 45 seconds to complete that journey. That is what's known as a Tropical Year. That extra quarter of a day (or so) would cause real problems with our calendar if we didn't do something to compensate for it.

To keep things lined up correctly, we first add one extra day to the calendar every fourth year.

If Earth's year was just 365 days and 6 hours, that would be it. We need to take back a few leap days, though, due to the extra 11 minutes and 15 seconds we just introduced.

Specifically, each century year, for three centuries out of every four, we skip the leap year. To put it another way, if an impending leap year is evenly divisible by 100, we skip it, unless that year is also evenly divisible by 400.


Thus, the year 2000 was a leap year (evenly divisible by both 100 and 400), but we will skip having a leap year in the years 2100, 2200 and 2300 (evenly divisible by 100, but not by 400). Then we will once again have a leap year in 2400.

It seems a little complicated, but it is very effective. Just by adding this relatively simple equation to how we keep track of our years, the calendar can stay perfectly in sync with our astronomical seasons!

Then, just wait until we have to add another 'leap second'!

Sources: Dr. James O'Donoghue/Twitter

Editor's note: A previous version of this article used Earth's 'sidereal year' (365 days, 6 hours, 9 minutes and 10 seconds) in discussing the reason for leap years. It is the 'tropical year' (365 days, 5 hours, 48 minutes, and 45 seconds) that is actually used to determine how many leap years we have. This has been corrected. We apologize for any confusion.


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