25 difference, our calendar gradually gets out of sync with the seasons. One orbit of Earth around the Sun takes approximately 365.25 days-a little more than our Gregorian calendar’s nice, round number of 365. Because the calendar does not account for the extra quarter of a day that the Earth requires to complete its orbit around the Sun, it doesn’t completely align with the solar year.īecause of this. The short explanation for why we need leap years is that our calendar needs to stay aligned with the astronomical seasons. If a year satisfies both the rules above, then it is a leap year. (For this reason, the years 1700, 1800, and 1900 were not leap years, but the years 16 were.) Years that are divisible by 100 (century years such as 1900 or 2000) cannot be leap years unless they are also divisible by 400.A year may be a leap year if it is evenly divisible by 4.However, there is a little more to it than that. Generally, a leap year happens every four years, which, thankfully, is a fairly simple pattern to remember. Additionally, a leap year does not end and begin on the same day of the week, as a non–leap year does. (Keep reading for a longer explanation.)īecause of this extra day, a leap year has 366 days instead of 365. Without this extra day, our calendar and the seasons would gradually get out of sync. Simply put, a leap year is a year with an extra day-February 29-which is added nearly every four years to the calendar year.Īdding an extra day every four years keeps our calendar aligned correctly with the astronomical seasons, since a year according to the Gregorian calendar (365 days) and a year according to Earth’s orbit around the Sun (approximately 365.25 days) are not the exact same length of time.
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