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Name: Stephen J.
Status: other
Age: 40s
Location: N/A
Country: N/A
Date: Thursday, August 22, 2002

Can you explain why we have added 21 Leap Seconds in the last 27 years yet the rate deceleration is about 1.4 milliseconds per every 100 years?


The stated rate of negative acceleration refers to actual negative acceleration of the Earth's orbit due to running into space debris (dust, meteorites, etc.). The 21 Leap Seconds are a correction for the fact that the Earth's orbit is not exactly 365.2425 days long. The 0.24 day shift is taken care of with the extra day every fourth year (leap year), except for multiples of 100. The years 1700, 1800, and 1900 were not leap years. To compensate for the extra 0.0025 days, years that are multiples of 400 are leap years. The year 2000 was a leap year. Although this is VERY close to the Earth's orbit, it is not exact. It is a tiny fraction of a second different. Also, tides and such slow the rotation of the Earth over time. In a century's time, a day becomes 1.4 milliseconds longer. That is a shift of about 26 seconds per century.

The world has a system of atomic clocks ticking off exact seconds. They do not keep track of days or years, only seconds. After several years pass, these exact atomic clocks begin to disagree with time based on the sun and stars. When the atomic clock system and the actual orbit system disagree by more than 0.5 seconds, a leap second is added or removed to keep the atomic time aligned with solar time. A good on-line discussion of this can be found at:

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

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