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Name: Carlo
Status: educator
Age: 30s
Location: N/A
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Date: 1999 - 2000


Question:
Hi, according to the previous question, the exact time for a day is 23 h 56 min... so in 365 days (a year) we have about 24 h and I'd like to know the reason why every 4 years we have a leap year with only one day, thanks


Replies:
A year is approximately 365.24 solar days. So, approximately every four 365-day years, the earth is ane day short of a full revolution around the sun. Therefore, we extend the numbering of the year by one day to start the new year in the same spot.

The 23 h 56 min day is the "siderial" day, the time from fixed-star to fixed-star. This is because the time it takes the Earth to rotate once about its axis is a little shorter than the 24-hour "solar" day we're accustomed to. A "solar" day is defined as the average time from high noon to high noon, that is, the interval between the times when the Sun is highest in the sky. Because the Earth orbits the Sun, by the time it completes one revolution, the location on the earth closest to the sun has shifted a bit. The Earth then needs to rotate a little more to make it line up again. The time for one true rotation is the interval between the times that a distant star is highest in the sky, "fixed star to fixed star." This interval is known as a "siderial" day, which is 23 hours, 56 minutes, 4.06 seconds.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois



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