Why do we need leap years?

Why do we need leap years?

They keep the calendar year synchronized with the astronomical year, preventing us, for example, from celebrating Christmas in November.

There is a difference between a common calendar year and an astronomical year. The astronomical year, also called a tropical year, a solar year, or an equinoctial year, is the actual duration of the Earth's full rotation around the Sun, on its orbit (Earth's revolution around the Sun). It takes the Earth around ≈ 365.242216 calendar days, or around 365 days, 5 hours and 48 minutes to complete a rotation around the Sun (* see note below). Yet, the common year has only 365 days.

The additional day contained in a leap year keeps the calendar year synchronized with the astronomical year, compensating for the fact that a period of 365 days of a common calendar year is shorter than the astronomical year (with something slightly less than a quarter of a calendar day); otherwise, seasons would occur earlier than intended in the calendar year.

Leap years keep the calendar year in sync with the astronomical year, preventing us, for example, from celebrating Christmas in the month of November.


* Note: The actual duration of the Earth's revolution around the Sun.

The actual duration of the Earth's full rotation around the Sun, on its orbit, also slightly varies from year to year, sometimes up to 30 minutes, due to the Earth's rotation speed around the Sun, ie: measured from equinox (when the duration of a day is equal to the duration of a night), March 2015 - March 2016 took 365 days, 5 hours and around 45 minutes, yet March 2016 - March 2017 took 365 days, 5 hours and around 59 minutes (a difference of about 14 minutes between the two consecutive years).