Random musings: 515 Days Not Lost: How Leap Year Keeps Us On Track

I had planned on doing a blog post on Leap Day, but since Days of the Year already had one, I decided to use it.

26th Feb, 2016 | Posted by Brandon Withey in Blog

515 Days Not Lost: How Leap Year Keeps Us On Track

Once every four years there is the strangest occurrence, suddenly February has an additional day, and children all over the world are resigned to only having one birthday every four years as the 29th rolls around again. While we have all grown accustomed to living with the leap year as part of our normal calendars, most of us write it off to an oddity that comes to pass once every four years. But how many are aware of exactly why it happens? Today, we’re going to share with you exactly why that is.

The Changing Of The Calendar
While it’s strange to conceive of in modern days, the world didn’t always have one consistent calendar used all the year round. There are still remnants of that when you notice that not everyone observes the New Year on the same date.

Back in the beginning, people told the passing of the year by the position of the sun, how many moon cycles had gone by (Ever wondered where we got the word Month?) and the changing of the seasons. Ironically before we kept such specific track of things, the concept of a ‘Leap Year’ wasn’t necessary.

Civilization, however, was advancing, and ways to tell the passing of time accurately was needed. While many had been used, the first widely accepted and established one was the Julian Calendar, first put into place by its namesake Julius Caesar in 46BC. This calendar was established to correct the reckoning of the years, as the year used previously to this was only 355 days in length. As a result, the year 46BC had 445 days in it, to bring things back into alignment.

The Leap year
As many people know, the Greeks were absolutely superb astronomers, and had noticed that the year was not, in fact, 365 days, but 365.25. So every fourth year of 365 days was off by one full day, and thus a day was inserted in February to correct this every fourth year.

So We Use The Julian Calendar?
No, due to reasons far too long to go into here, the Julian Calendar was also inaccurate (mind you, only by .002%, but that adds up over the years), and in 1582 we changed over to the Gregorian Calendar, which we still use today. However, the establishing leap day remained in place, and we continue celebrating it to this very important day. How important you ask? Over the space of time in between, our calendar would be off by 515 days. In short, we’d have lapped the year and be halfway through doing it twice!

This is slightly inaccurate, the year is only 365.2422 days, so the rules for figuring out Leap Year are slightly more complicated. If a year is evenly divided by 4 (if the last 2 digits are evenly divisible by 4, the whole number is), then it's a leap year...2016 is divisible by 4, so this is a leap year. The next part of the rule is...if the year is also divisible by 100, it's not a leap year: 1700, 1800 and 1900 weren't leap years. The last part is...if the year is also divisible by 400, then it is a leap year: 1200, 1600, 2000 were all leap years. This means there are only 97 leap years in each 400 year cycle instead of 100...97/400 = .2425, so it's still off slightly...it will be off by a day in approximately 3,300 years!