From: HAIRSTON%UTDSSA.DECNET@relay.the.net
Subject: Calendar and Zodiac
Date: Wed, 29 Jul 92 18:18:45 GMT
Organization: [via International Space University]
X-Added: Forwarded by Space Digest
Original-Sender: isu@VACATION.VENARI.CS.CMU.EDU
Distribution: sci

   A lot of the confusion here about the calendar comes from the fact 
that there are THREE separate "years" being discussed here.  There is
the "sidereal year" which is the time it take the earth to go around
the sun relative to the distant stars (i.e.--how long does it take 
the earth to go from being on the line from the sun to distant star X 
to coming back to that line).  Then there is the "tropical year" which
is the time it takes the earth to go from a given solstice or equinox
back to that again (i.e.-- how long does it take from the instant that
the north pole is most tipped towards the sun (northern summer solstice)
until that happens again).  And last, the "calendar year" which is
how long is it between New Years parties.  The calendar year can be
defined however you want, but the Julian/Gregorian follows the tropical
year so that northern summer solstice will stay in the month of June.

   For the record here are the times:
   Sidereal year  365d 6h  9m 10s
   Tropical year  365d 5h 48m 46s
   Calendar year  365d  (or 366d on leap years)


   The trouble starts if you take one year as the "sidereal year" or
the time it takes the Earth to return to the same orientation relative
to the stars.  Say at exactly the spring equinox this year that we 
were lined up between the sun and star X.  365 days 5 hours 48 minutes
and 46 seconds later it is exactly the spring equinox again, BUT we still
have to move for another 20 minutes and 24 seconds to get back to the line
between the sun and star X.  The spring equinox in 1994 will occur
40 minutes 48 seconds before we get back to the line between the sun
and star X, and so on.  Thus the locations of the equinoxes and solstices
slowly move around the Earth's orbit relative to the fixed stars.  This is
due to the precession of the axis and it takes about 24000 years to
go around once. (This means that in 14000 AD Orion will be a summer
constellation in the northern hemisphere.)

   Since neither of these years is divisible by an integer number of days
you have to have an occasional "leap day" to put the calendar back in
synch with the seasons.  The early romans had a rather laid-back approach, 
they just ran the calendar during the spring, summer and fall.  Once winter 
arrived they stopped keeping track until the warm weather returned and then 
started it up again.  It was a nice, self adjusting, but not too accurate
system.  As they became an empire, they need to regulate time better and
the Julian calendar was formalized about the first century BC.  It worked 
by having a leap day once every four years.  Trouble with that is that
once you "leaped", your calendar was about 45 minutes AHEAD of the tropical
year.  This added up to a being a full day ahead after about 130 years.  
Rome fell and the Church kept the calendar in spite of the error and just
lived with it until the 1500's when Pope Gregory (being advised by the
astronomer Clavius) reformed the calendar.  First he lopped ten days off
to push the equinoxes back to about March 21 and September 21, and then he
decreed that only century years that were divisible by 400 would be leap 
years.  This keeps us in synch with the tropical year to one day every 3300 
years (so in 4800 AD we'll have to adjust one special leap year).  All the 
Catholic countries went along with this, but the Protestant and Eastern
Orthodox ones didn't.  England didn't join the pack until the 1700's, and
by then were eleven day out of step with the new Gregorian system.
(George Washington was born on February 11th under the old calendar.)
Russia didn't switch until after the Russian revolution in 1917 (which is why 
the October Revolution was always celebrated in November).  For more info on
all of this, go check out Daniel J. Boorstin's book "The Discoverers" and
read the section on "Time".
________________________________________________________________________
Marc Hairston--Center for Space Science--University of Texas at Dallas
"We cannot continue to have graduate student turning elderly waiting for
mission launches." --NASA Head, Daniel Goldin
Newsgroups: sci.space