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Old October 20th 18, 04:05 PM posted to sci.astro.amateur
Quadibloc
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Default OT? Amateur Astronomy

On Thursday, October 18, 2018 at 11:35:06 PM UTC-6, Gerald Kelleher wrote:

A person capable of using their intelligence will recognise that the RA/Dec
framework is in competition with the Lat/Long system for the basic answer to the
question 'How long does it take the Earth to turn once ?'. The reasonable answer
is once in 24 hours with quite a long explanation covering timekeeping from its
emergence in antiquity to demonstrate the rules which tie cyclical timekeeping
to cyclical dynamics to a close approximation.


The naive answer is indeed 24 hours.

If the Earth didn't turn, wouldn't one part of the Earth always face the Sun,
the way Mercury was once thought to do, and the way one side of the Moon always
faces the Earth?

A more complicated explanation, noting that the Earth and Moon have orbits that
aren't perfect circles, and that this has the consequence of *libration in
longitude* for the Moon, and the *Equation of Time* for the Earth, explains why
such a naive viewpoint has a problem.

If the Moon "didn't rotate" it would stay still - it would always face exactly
in one direction. But if not rotating means facing the Earth, well, it wiggles a
bit from side to side.

If the Earth took 24 hours to rotate, then it would take 24 hours between
succesive crossings of a meridian by the Sun. Not more nor less. It wouldn't
cross the meridian up to 15 minutes early for part of the year, and up to 15
minutes late in another part of the year.

At least, not unless something was pushing on the Moon to make it wiggle, or
something was pushing on the Earth to speed it up and slow it down.

Of course, the idea in that preceding paragraph was enunciated clearly by your
bete noire, Isaac Newton, and while you are ready to admit that Newtonian
mechanics might be useful on the Earth's sublunary sphere, you find it improper
or even impious to apply the same principles to the skies.

Neptune wouldn't have been discovered if its gravity, pulling on Uranus, didn't
cause changes in the orbit of Uranus. The Moon's orbit reflects both motions of
the Moon's nodes and of its perigee point that are due to the Sun's gravity.

That the planets and their natural satellites move in response to the same
physical laws that apply to boulders and bricks and, yes, even cannonballs isn't
an opinion or a "disastrous error", it's a fact confirmed repeatedly by long
experience.

John Savard
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