Where does the extra 4 minutes go in a 23h 56minute day?

Earth orbits the sun once every 365.256 days. This is how long it takes the Sun to travel once around the sky through each of the zodiacal constellations, and end up in exactly the same star field it started from in the sky. The calendar, however, only has 365 honest-to-God slots. In terms of hours, one rotation of Earth takes 23 hours, 56 minutes and four seconds. This means that there is 0.256 days left over each year, which after 4 years add up to 4 x 0.256 = 1.024 days. Every four years, we decide to add a 'Leap Day' at the end of February, which normally got short-changed and only had 28 days in a standard year.

This still leaves us at the end of 4 years with 0.024 days or 34.4 minutes extra. This extra bit of unaccounted time in our calendar, which is trying to keep up with the exact motion of the Sun in the sky, is made up in a rather non-intuitive way. Every two centuries we add a second day. 1600 was a leap year, 1700, 1800 and 1900 were not, but in 2000 we had 366 days with February 29th added. This follows the rule that century years divisible by 400 ARE leap years to make up for that extra 0.024 days we still have after 4 years.

This answer was updated in 2011. See my books: The Astronomy Cafe (1998) and Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011

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