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Rules for Leap year and 30 February
This is from the ucisa-y2k@mailbase.ac.uk list
From K.F.Hartley@rl.ac.uk Mon Mar 23 10:22:20 1998
Date: Mon, 23 Mar 1998 09:08:00 -0000
From: K.F.Hartley@rl.ac.uk
To: ucisa-y2k@mailbase.ac.uk
Subject: RE: February 30th ???
As a former employee of the Royal Greenwich Observatory let me assure
you that there is no additional leap day in millenium years. It is
simply journalistic incompetence.
Here, as they say, is some text I prepared earlier:
The present "Gregorian" calendar was introduced by Pope Gregory in 1582,
but only adopted in Great Britain in 1752 and Russia in 1917. The problem
is that there is not a whole number of days (mean solar days) in a year
(tropical year), because there is no physical link between the Earth's spin
on its axis (day) and orbit around the Sun (year). In fact one year is
approximately 365.2422 days.
Inserting an extra day every four years makes the average calendar year
365.25 days long. This was introduced by Julius Caesar round about 50 BC.
By 1752 the calendar was 12 days out compared with the sun. In that year
September 2nd was followed by September 14th and there were riots in the
streets with people demanding "Give us back our 12 days".
Gregory agreed that 3 leap years in every 400 years would no longer be leap
years. That makes the average calendar year 365.2425 days, compared with
the correct value 365.2422. The rule chosen was that if the last two digits
of the year are both zero (1900, 2000, 2100 ...) then it is only a leap
year if the first two digits are divisible by 4. Thus 1600, 2000 are leap
years, but 1700, 1800, 1900 and 2100 are not. That is considered good
enough, for the time being.
Of course, if you are really serious about time and want to keep atomic
clocks in step with astronomical ones you have to insert a leap second once
or even twice a year because the earth is slowing down.
(I got this information from my favourite basic astronomy book by Roy and
Clarke, ISBN 0-85274-292-4)
Ken Hartley,
Oxfordshire
$Revision: 1.1 $ $Date: 1998/03/23 10:30:03 $
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