A Matter of Degrees, Pt. 2

[William Karlin]

On Thu, 25 Sep 1997, Vaska Tumir wrote:

> Help!  Does the earth actually describe a circle in its orbital path round
> the sun?  Am I confusing this with some other stuff --  or is it the case
> that the earth's solar merry-go-round is not a true circle at all?  


  Nope.  It sure doesn't, mildly elliptical with the sun at one foci.  The
effect of the non-circular orbit is small, but observable -- especially
now with our sophisicated observational and time-keeping abilties.  

  to further complicate matters the ellipse the earth traces out around
the sun changes orientation slowly (it precesses) as does the relative
position of the north pole to the "fixed" background stars (which are
really moving around themselves). The precession motion of the north pole
has a period of about 26,000 years! Our north pole star was no where near
the north pole around 6,000 years ago.  Add to this all the effects of the
other planets' gravitational pulls on us, and you see that the actual
motion of the earth is extremely complicated.  So *any* attempt to base a
system of measurement or time-keeping on this motion (which the ancients
saw as being absolute) would run into problems.  Better to pick some
standard here on earth and make minor adjustments as needed (such as leap
seconds).

  All this has made me apprieciate what astronomers (including ol' M&D)
were able to figure out back in the day...quite the accomplishment.

  One last note, Eratosthenes proved the earth was round (and calculated
it's size to within 2%) using the 360 degrees in a circle rule -- this
back around 200 B.C.E.
  

  a big fan of the "sciento-fascist" (ouch!) metric system,

will