Why Have a Leap Year?
Tom Dormer, Online Science Editor, explains the necessity of leap years and why February gets the extra day.
February 2020 will have 29 days due to 2020 being a leap year. But why do we need a leap year, and why did we put the extra day in February?
A leap year is a calendar year containing 1 more day than a ‘normal’ year (366 days instead of 365 days). The reason comes from the Earth’s Solar orbit not taking exactly 365 days, instead taking 365.25 days. By inserting an extra day once every four years (4 x 0.25 = 1 day), we take into account the extra 0.25 of a day.
The reason comes from the Earth’s Solar orbit not taking exactly 365 days, instead taking 365.25 days.
However, this leap year rule gets more confusing when we want to be more precise. To 4 significant figures, the Earth takes 365.2425 days to orbit the Sun. This means that by adding a leap year every four years, our calendar now overcompensates (is too long).
Therefore, if a leap year is divisible by 100, it reverts back to a normal year. However, if a leap year is also divisible by 400, it becomes a leap year again, as having non-leap years every 100 means our calendar is too short again.
In Summary:
- Leap Year every 4 years (*)
- * Unless divisible by 100 (**)
- ** Unless also divisible by 400
- * Unless divisible by 100 (**)
At this stage you could say we’re fastidious, but our calendars are accurate to 1 day in 3236 years (not too bad!)
So why add the day in February?
The answer goes back to the Romans and the reason why February is the shortest month.
Back in the 8th Century BC, the calendar only had 10 months with calendar makers not wanting to give the horrible winter days a month. Not surprisingly this didn’t work well with the seasons, and soon winter was in the middle of June.
The reason why we add the leap day to February is simply because it has the shortest number of days in the year.
Eventually the Romans added 2 months to the end of the winter period to account for the extra days. As February was the final month added, it got fewer days.
The reason why we add the leap day to February is simply because it has the shortest number of days in the year.
However, it’s easy to forget that all these problems have arisen because the Earth doesn’t orbit the Sun in an exact number of its own rotations (days).
