Every so often February has 29 days instead of 28, including this year. We all know it’s called a leap year, but why does it actually happen?
Here’s an easy to digest breakdown of what leads to February periodically inheriting an extra day.
As we all know, or so we think, the calendar year is 365 days, which loosely translates to the amount of time it takes for the Earth to complete a single rotation around the sun.
But, according to Geologist Dr. Bob Craddock 365 is actually a round number.
In reality, “it takes Earth 365.242190 days to orbit the Sun, or 365 days 5 hours 48 minutes and 56 seconds,” Dr. Craddock wrote in an article on the National Air and Space Museum’s website.
This means the extra five hours 48 minutes and 56 seconds need to be accounted for, and if they weren’t the seasons would begin to drift, which would have severe consequences.
“Over a period of about 700 years our summers, which we’ve come to expect in June in the northern hemisphere, would begin to occur in December,” according to Craddock.
But, by adding an extra day roughly every four years our regular calendar year aligns with the leap year, and the seasons remain consistent.
However, there’s a little more to it.
According to Craddock, relatively straightforward math will show that over four years the difference between the calendar years and the sidereal (leap) year is not exactly 24 hours, it’s actually 23.262222 hours.
This means a leap day is rounded up, making the calendar longer by 44 minutes. Over time, this would also cause the seasons to drift in our calendar.
This is why leap years don’t actually occur every four years.
“The rule is that if the year is divisible by 100 and not divisible by 400, the leap year is skipped. The year 2000 was a leap year, for example, but the years 1700, 1800, and 1900 were not,” Dr. Craddock said.
This means the next leap year is 2028 and the next time a leap year will be skipped is 2100.
