Date: Tue, 23 Dec 1997 10:21:29
From: Simon Cassidy <scassidy@EARTHLINK.NET>
Subject: Re: Query (Peter's reply)

Peter Meyer wrote:
> Since the average length of a year in the Gregorian Calendar is 365.2425
> mean solar days, the calendar slips by .0003 m.s.days per year, on average.
> Thus 3000 to 4000 years is needed for the calendar to slip by a day.

Simon notes: I assume that by "mean solar day" Peter means a real Universal calendar day as regulated by the earth's rotation wrt the sun (i.e. that which is referred to by 'a day' in normal usage).

While the Gregorian calendar may be slipping by 0.0003 mean solar days per astronomer's mean solar year, this does not mean that 3000 to 4000 years will be needed for the calendar to slip by a day! Since the astronomer's mean solar year is decreasing in length and the mean solar day is increasing in length (both with respect to atomic time) it is manifest that the slippage (between the Gregorian average calendar year (fixed at 365.2425 days) and the astronomer's mean solar year (measured in mean solar days) is increasing with time and thus the Gregorian calendar will slip by a day in about 2000 years with respect to the astronomer's mean solar year.

However the true slippage of the Gregorian calendar must be measured against the Vernal Equinox year (measured in mean solar days) and not against the astronomer's mean solar year (how many times do I have to say this!) since the intent of the Gregorian calendar was, and is, to keep the Vernal Equinox steady in the calendar. The mean Vernal Equinox year length is currently 365.2424 days and will remain so for at least the next 2000 years. Thus the true slippage of the Gregorian calendar over the next 2000 years will almost certainly be less than six hours (a fifth to one quarter of a day).

The accuracy of a calendar must be measured in its own terms! One would not measure the accuracy of a synodic lunar calendar (like the GLC or the Jewish calendar) aginst the sidereal month, so please dont measure the accuracy of the Gregorian calendar against the wrong kind of year! Or if you must, please state explicitly what you are measuring the Gregorian calendar against!

--Dee's Y'rs, Simon Cassidy, 1053 47th. St. Emeryville Ca. 94608.
ph.510-547-0684. email:

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