From: spam@merlyn.demon.co.uk
JRS: In article <420f9de1.19872454@supernews.seanet.com>, dated Sat, 12
Feb 2005 06:12:43, seen in news:sci.space.science, Derek Lyons
posted :
>"David Grossmann" wrote:
>
>>I have heard every 19 years (235 lunar months) the moon and the solar
>>year are synchronised. However, after looking at the times for new
>>moons in the almanacs, and comparing new moon times with a year and a
>>year 19 years later, the new moon can be off as much as one day. March
>>16 in the first year and March 15 in the 20th, which is 19 years later.
>>Why is this? Is there any truth to the 19-year cyle?
>
>IIRC it's caused by the fact that the Gregorian calendar isn't
>precisely synched to sideral time. It does however stay within
>plus-or-minus a day or two.
The Gregorian Calendar year cycle has nothing to do with the Moon,
although that rather loosely caused the division into months; nor with
Sidereal Time.
It is fairly precisely, though irregularly, synched to solar time.
That is a further cause of variation of date omitted in my previous
article; the error I calculated as being the difference between 235
lunar months and 19 solar years does not include the fluctuation in the
length of a Gregorian year, which may be either (about) six hours
shorter or eighteen hours longer than the average. As 19 is not evenly
divisible by 4, that fluctuation must be added in considering the civil
date/time of New Moons.
The Metonic period of 19 years must not be confused with the period of
the Regression of the Nodes (18.61 years) nor with the Saros (18.03
years, 223 lunations) which is the usual repeat interval for Eclipses.
--
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