On Oct 5, 2:25*pm, oriel36 wrote:

Maybe some person here who is not enamored by an alternative value for
rotation through 360 degrees and subsequently a nonsensical 366 1/4
rotations per orbital circuit will explain to this guy what he needs
to know.

Many people could explain to _you_ what you need to know.

Yes, a naive person might well find it more natural to say that the
Moon doesn't rotate, because it keeps one face always towards the
Earth. And that the Earth rotates in 24 hours, because that's the
length of a day.

But the solar day isn't uniform in length by the time marked out by a
mechanical clock. It turns out that these non-uniformities can all be
accounted for by two factors - the foreshortening of the Earth's orbit
in the plane of the Earth's rotational equator, and the fact that the
Earth's orbit is elliptical, sweeping out equal areas in equal times,
following Kepler's Laws.

So the Earth's rotation is itself smooth and uniform within the time
scale given by any form of mechanical clock, whether a pendulum clock,
a quartz crystal clock, or an ammonia maser clock. (There are tiny
irregularities due to momentum transfer between the rocky Earth and
its atmosphere due to changes in seasonal prevailing winds, but that
is a detail we can ignore.)

As you yourself have noted, though, "there is no external reference"
for a _uniform_ rotation of the Earth with a 24 hour period. Such a
reference would buzz around the Sun in a shape that is a twisted
version of the analemma. But the fixed stars - or, to take the
technicality of stellar parallax into account, the directions from the
Sun to the fixed stars - _are_ an external reference for uniform
rotation of the Earth with a period of 23 hours, 56 minutes, and 4
seconds.

It is for this reason that astronomers count this as the period of the
Earth's rotation.

Similar reasoning involving libration explains why astronomers count
27 1/3 days, approximately, as the period of the Moon's rotation.

John Savard