What is the Deal with Leap Year?

Approximately every four years, February has 29 days instead of 28 and the year has 366 days. The extra day is called “leap day” and the year it happens in is called “leap year.” But why do we have this?

It has to do with time, space, the seasons, and the mathematical rounding of numbers.

A calendar year is 365 days long (also called a “common year”), and represents the number of days it takes for Earth to orbit the Sun. A day, measured as 24 hours, is the amount of time it takes Earth to rotate on its axis. If you counted the days from January 1 through December 31 in an ordinary, common year calendar, there would be 365 days, each 24 hours long.

But the time needed for Earth to complete one orbit of the Sun doesn’t exactly match 365 days.

The calendar count of 365 days is a rounded number. The actual amount of time it takes Earth to make one orbit around the Sun is slightly longer at 365.242190 days (365 days, 5 hours 48 minutes and 56 seconds) which is called the “sidereal” year. The extra 5 hours 48 minutes and 56 seconds required for the orbit around the Sun needs to be accounted for to keep the calendar aligned with the astronomical seasons, and that is where leap year comes in.

Why Does Leap Year Matter?

Leap years are important to keep the calendar year and the sidereal year synchronized. The easiest way to do so is to round the calendar down to 365 days for three years, then round up to 366 days in the fourth year, which is the leap year. The extra day, called leap day, happens on February 29 in a leap year and makes up for the rounding of the three previous years. Adding the extra day realigns the calendar with the seasons.

Without leap years, the nearly six-hour rounding difference of time for each year would add up and the seasons of nature would drift on the calendar. After 100 years, the calendar and the seasons would be off by 25 days. Eventually, the northern hemisphere summer beginning in June would have gradually shifted and would begin in December instead. By adding an extra day every four years, the calendar remains adjusted to the sidereal year.

In general, if a year is evenly divisible by four it may be a leap year, but there is an exception.

Rounding comes into play again, because the time difference of 5 hours 48 minutes and 56 seconds over four years doesn’t add up to exactly 24 hours, but is 23.262222 hours instead. When a leap year is added every four years, the calendar is made longer by 44 minutes, and over time, that can also cause the seasons to drift on the calendar.

To avoid the drift caused by the 44-minute difference, there is another leap year rule to allow for skipping leap year. Century years such as 1800, 1900, 2000, etc., which are evenly divisible by 100 can’t be a leap year unless they are also evenly divisible by 400. With this calculation, the years 1600 and 2000 were leap years, but the years 1700, 1800, and 1900 were not. The next time leap year is skipped will be the year 2100.

Fun fact: the term “leapling” refers to a person born on leap day, and many people feel that being born on February 29 is a sign of good luck. Happy birthday leaplings!  

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