From: rcb@druwa.ATT.COM (BakerRC)
Newsgroups: sci.space
Subject: Re: Calendar and Zodiak
Summary: let me try
Date: 29 Jul 92 17:32:28 GMT

In article <1992Jul28.043346.21710@unocal.com>, stgprao@xing.unocal.com (Richard Ottolini) writes:
> In article <1992Jul28.011553.19947@vdoe386.vak12ed.edu> ghasting@vdoe386.vak12ed.edu (George Hastings) writes:
> >To make up the VERY small difference due to precession, as well
> >as to adjust for the slowing of the Earth's rotation due to
> >tidal drag of the oceans (caused by the moon's gravitation),
> >from time to time they declare "leap-seconds" there was one
> >this year.
> >-- 
> 
> 
> Incorrect.  Precession advances the year appoximately 20 minutes per year.
> (24*60*365.2422)/26000.  There are other influences- quakes, solar activity,
> weather- that may cause the length of the year to vary one or two seconds
> per year.  Since we can measure the length of year to an accuracy of about
> a microsecond (one part in 10E14), a second is a noticable effect.
> Universal time based is based on atomic vibrations and independent of
> astronomical irregularity.


Also not quite right. Leap seconds are added to keep the DAY correct.
Let's start from the beginning.

We measure time in seconds. The second is now defined relative to cesium
clocks, and was made to approximately equal the second as it was then (1950s?).
However, since this [previous] second was defined in 1900, it approximately equals
1/86,400 of a mean solar day in 1900. Since then the day has been increasimg
at a rate of (1 second/ year) per century. Since it has been about a century
since the second was "defined", approximately 1 leap second a year
is added to keep the day correct, ie the mean sun crosses the
meridian at noon. If this was not done daytime would drift to the night hours.

So now we have an accurate second, and a little more than 86,400 of them
to make a mean solar day. Back to the original posters question, the
calendar is based upon the tropical year, the time between successive
vernal equinoxes (or other seasons). This is slightly
different then the sidereal (star) year due to precession as mentioned by others.
The calendar was developed to keep track of the seasons, not to
keep track of how many times the earth revolves around the sun.
Julius had a good approximation of 365.25 days, but by Pope Gregory's
time the error had built up to 11 days, and the seasons were
out of synch with tradition. Thus he took 11 days out of the calendar,
and changed the rules to make the year more closely correct at
365.2425 days, subracting 3 leap days every 400 years
(multiples of 100 are not leap years unless divisible by 400.)
That was a decent approximation then, but now the tropical year
is 365.2422 days and getting smaller (both due to the earth slowing down
it's rotation and the earth getting closer to the sun).

Thus the seasons stay in the same months because we make it that way,
that's what the calendar is supposed to represent.
Where the sun appears during
those seasons, and thus the months representing them, drift to different
constellations due to precession.

Hope this helps

Robin Radar Baker

Let's get Henry fired for talking about American politics :.) :.)