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Path: utzoo!linus!decvax!tektronix!tekchips!teklabs!abdali
From: abdali@teklabs.UUCP
Newsgroups: net.followup
Subject: Re: Info concerning our calendar year
Message-ID: <2450@teklabs.UUCP>
Date: Fri, 16-Sep-83 13:06:36 EDT
Article-I.D.: teklabs.2450
Posted: Fri Sep 16 13:06:36 1983
Date-Received: Sat, 17-Sep-83 09:22:46 EDT
Organization: Tektronix, Beaverton OR
Lines: 14


   The division of a year into 12 months is not by convention, but is
forced on us by the definition of year and month. Basically, the
month is derived from the motion of the moon and the year from the apparent
motion of the sun on the ecliptic. Next to the daily apparent
motion of the sun, the two most noticeable recurrent phenomena to provide
units of time have historically been: 1) The phases of the moon
and 2) the seasons. The cycle time for these are respectively used to 
define the month and the  year. It so turns out that a year contains
(very nearly) 12 months.
    This is a very simplified explanation. To be more rigorous, one
needs to distinguish between various kinds of year (e.g., solar,
sidereal) and month (synodal, etc.). Most textbooks of astronomy
give the detailed definitions. There is an excellent book giving the
historical development of calendars, but its title escapes me now.