Re: Year 2000 Issue

["Otto G. Raabe" <ograabe@ucdavis.edu>]

June 9, 1997

As previously noted, the leap year rules are as follows: 

Years that are evenly divisible by 4 are normally leap years unless they are
also evenly divisible by 100, in which case they are not leap years unless
they are also evenly divisible by 400. Hence, per 1,000 years there will be
365,242.5 days with this system. 

Although 1900 was not a leap year, the last year of the 20th century, the
year 2000, will be a leap year. There will be a February 29,2000.

Leap years are necessary because of the time required for the earth to move
around the sun with respect to their relative positions. The tropical year,
which keeps the seasons in their right places, is defined by the time for
two consecutive returns of the sun to the vernal equinox (at the earth's
equator). That is 365 days, 5 hours, 48 minutes, and 46 seconds in 1990
(1997 World Almanac). It is decreasing by about 0.530 seconds per century.
Hence, the tropical year is now about 365.2422 days. With the leap year
system, there is an extra 3 days every 10,000 years. That's about an extra
26 seconds per year. We could approximately correct by not allowing years
divisible by 4,000 to be leap years. Luckily, we have astrophysicists and
atomic clocks to help keep us on time. 

Although 1900 was not a leap year, the last year of the 20th century, the
year 2000, will be a leap year. There will be a February 29,2000.

I hope this is all there is on this issue. 

Otto Raabe

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		Prof. Otto G. Raabe, Ph.D., CHP
		Institute of Toxicology & Environmental Health (ITEH)
		     (Street address: Old Davis Road)
		University of California, Davis, CA 95616
		Phone: 916-752-7754     FAX: 916-758-6140
		E-Mail: ograabe@ucdavis.edu
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