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Adding Leap Seconds
Name: Stephen J.
Status: other
Age: 40s
Location: N/A
Country: N/A
Date: Thursday, August 22, 2002
Question:
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?
Replies:
Stephen,
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:
http://tycho.usno.navy.mil/leapsec.html
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
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Update: June 2012
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