Why an extra 24 hours is added to the calendar every 4 years


TAMPA, Fla. (WFLA) – Every four years, an extra day is added to the month of February known as a leap day. This is to account for the trip around the sun being slightly longer than our typical 365 day calendar.

The trip actually takes around 365.25 days but the 0.25 days goes unaccounted for during the three years leading up to a leap year. If a leap day wasn’t recognized, eventually the seasons wouldn’t line up with their current months. Holidays like Christmas and July 4th would eventually be celebrated in warmer temps and colder temps respectively.

The leap day was implemented around 46 B.C. by Julius Caesar but in 1582 A.D. a new rule was announced by Pope Gregory XIII.

A closer look of the earth’s orbit reveals it takes 365.2422 days, which is .0078 days less than 365.25. These small numbers might not sound like much year to year but over thousands of years, it does add up if they aren’t accounted for.

By not accounting for the extra 0.0078 days between 46 B.C. and 1582 A.D. per year, it added an extra 12.7 days.

The new rule in 1582 A.D. stated that years ending in “00” should not be a leap year unless they are divisible by 400. This is why the years 2000 and 1600, for example, were leap years but the years in between (1900, 1800 etc.) were not.

Adding the leap year every 4 years accounts for the ~0.25 days we lose every year. But by not having a leap year on the century years that are not divisible by 400, it eliminates the extra 0.0078 days of error added during a leap year.

Even with the new rule, the average error is still about 25.9 seconds too much each year. At this rate, it takes 3,333 years to bump the calendar one day off.

The math is broken down from NASA here.

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