On Dec 2, 11:42*am, Glenn Knickerbocker wrote:
On 12/02/2010 01:09 PM, oriel36 wrote:

There are only two motions involved,intrinsic rotation which causes
the day/night cycle with no more than a full 365 rotations in a
year,given that daily rotation is independent of the orbital motion of
the Earth.

Is it? *Given 365 of those daily rotations in a 365-day calendar year,
are the ones in January when the orbital motion is fastest really no
longer than the ones in July when it's slowest?

Oh, my. Much later in this thread, rec.arts.sf.written got added to
the crossposting. I had no idea that the distinguished Oriel36 was a
participant in the thread earlier.

In fact, as the Earth orbits the Sun in approximately 365 1/4 days,
the rotations of the Earth proceed at a completely uniform pace. The
Earth's orbital revolution around the Sun speeds up and slows down,
because when the Earth is closer to the Sun, its loss of potential
energy is balanced by an increase of kinetic energy of orbital motion.

But the Earth's rotation is unaffected. The stars will move through
the night sky so as to return to their previous positions after 23
hours, 56 minutes, and 4 seconds at any time of the year. The Earth is
a big heavy ball of rock, and so making it speed up or slow down would
require prodigious amounts of energy, and somewhere to which to
transfer the angular momentum involved.

Now, though, the Earth's orbital motion _will_ lead (along with the
inclination of the Earth's axis to the Ecliptic, in a way too
complicated to explain here, but there is an explanation at

http://www.quadibloc.com/science/eot.htm

on my very own web page if you're interested) to something known as
the Equation of Time, which means that you have to correct your
sundial by up to 15 minutes if you want to set your watch by it.

John Savard