Path: utzoo!attcan!uunet!wuarchive!emory!hubcap!ncrcae!ncrlnk!ncrwat!images1!jrl
From: jrl@images1.Waterloo.NCR.COM (john Latala)
Newsgroups: comp.sys.handhelds
Subject: Re: Leap Years
Message-ID: <1010@ncrwat.Waterloo.NCR.COM>
Date: 15 Aug 90 04:25:54 GMT
References: <31210047@hpcvia.CV.HP.COM> <6758.26c2a07b@jetson.uh.edu>
Sender: news@ncrwat.Waterloo.NCR.COM
Reply-To: john.Latala@Waterloo.NCR.COM
Organization: NCR Canada Ltd, Waterloo, Ontario, Canada.
Lines: 54
Keywords:

In article <6758.26c2a07b@jetson.uh.edu> math1i7@jetson.uh.edu writes:
>In article <31210047@hpcvia.CV.HP.COM>, scottb@hpcvia.CV.HP.COM (Scott_Burke) writes:
>> 
>> Would someone please verify that I have the leap year algorithm correct:
>> 
>>   February has 28 days UNLESS the year is divisible by   4, in which case
>>   February has 29 days UNLESS the year is divisible by 100, in which case
>>   February has 28 days UNLESS the year is divisible by 400, in which case
>>   February has 29 days.
>
>Scott, I have never heard about the 100 and 400 year rules... I had always
>assumed that:
>	IF 0 year 4 MOD == THEN 28 ELSE 29 END
>Are the 100 and 400 year rules there to correct for the fraction of
>a second off each year is from a "true" 365 or 366 days?

There's a neat pair of books about doing astronomical calculations:

	Practical Astronomy with your Calculator
and
	Practical Astronomy with your PC

both books are by Peter Duffet-Smith.

They have a nice section talking about date and time routines for
converting from 'normal' date to the astronomical Julian Date. In the
one section there's a nice disscussion about all the date stuff:

" ... Before 5th October, 1582 the Julian calendar was in general use in
Europe. By this calendar there were 365 days in each year except if the
year number was divisible by 4, then there were 366. This variation was
incorporated to make some allowance for the fact that the length of the
year, i.e. the time taken by the Earth to complete one orbit around the
Sun, is not a whole number of days. The length of the tropical year is
in fact 365.2422 days, and by adopting the convention of a leap year
every fourth year, the average Julian year is 365.25 days. This worked
quite well for hundreds of years, until 1582 astronomers were worried by
the accumulate error which had built up through the difference between
the tropical year and the Julian year. Accordingly, Pope Gregory decreed
that the dates 5-14th October 1582 inclusive were to be abolished (to
readjust the seasons to their correct positions) and that years ending
in two zeros (1700, 1800, etc.) were only leap years if divisible by
400. By this device, 400 civil years contained (400 * 365) + 100 - 3 =
146097 days. The average length of the civil year was then 146097 / 400
= 365.2425 days, a much better approximation to the length of the
tropical year."

Quoted without permission from "Practical Astronomy with your PC" by
Peter Duffet-Smith, Cambridge University Press, ISBN 0-521-38995-X
(paperback), Second Edition, Pages 15-16.

Any spelling errors and/or typos are probably mine......
--
john.Latala@Waterloo.NCR.COM