Leap Day Explained- a moment of science

Storm Team 2

The calendar. Nice and neat, it details the typically 365 days a year, until it adds one every 4 years and an estimated 4 million people finally get to celebrate their birthday after a few years off! We know why it’s special for them but for everyone else- why do we go through all this trouble of an extra day?

To explain, we must take a trip back millennia and to the stars above in this week’s moment of science. 

A year ≠ a year

A day normal or special, is marked by one full rotation of the Earth. A year- marked by one full rotation around the sun. During that round trip around our star, the Earth rotates roughly 365 times- leading to 365 days on the calendar. Roughly…

The more accurate number is 365 and a quarter rotations in one go around the sun. This fundamental difference between what’s on the calendar and what’s actually happening causes some problems as there’s no such thing as a quarter day! Each year, if left unchecked, that quarter rotation of the Earth would add up- causing the calendar to fall out of sync! The seasons would drift away from their normal months- winter and summer starting later and later!  

Correcting the calendar

Thankfully this was corrected long ago- 45 B.C. to be exact as Julius Caesar added the “leap day” we know and love- accounting for that quarter day gained each year by ignoring it for 3 years, and adding a full day every 4. 

This correction wasn’t completely accurate either as it overcorrected- a year isn’t 365.25 days but 365.2422- a full 11 minutes and 14 seconds shorter! 

That seemingly minute error added up! By the 16th century, the spring equinox was occurring 10 days earlier than it should have. 

Finally tired of this calendar confusion, Pope Gregory XIII in 1582 made some revisions to the leap year established by Caesar in his “Julian calendar .”  This “Gregorian Calendar” is the one we still use today with a leap year occurring every four years, unless that year falls on a new century.

 As such, 1700, 1800, 1900 had 28 days in February and not 29. This adjustment undercorrects, and therefore one extra rule is included- if a century year is divisible by 400, the previous rule does not apply and it IS a leap year. This skipping around is why we saw a leap year in 2000 but will not see one in 2100.  

Which puts us here today at 29 days in February 2020- calendars finally fully synchronized to the heavens above…right? Let’s just say it’s close enough. 

An additional day will need to be added in 4909 to adjust for the 0.0001% error with the Gregorian calendar.  Timekeepers have more adjustments to work on before then- adding leap seconds every so often as the Earth’s rotation rate varies.

Storm Team 2 Meteorologist David Dickson

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