Re: Geosynchronous orbits

On May 17, 10:16 am, "Curtis Croulet" <calypte@xxxxxxxxxxxxxxxxxxxx>
wrote:
You totally miss the contradiction of, on the one hand, claiming there is no
external frame of reference for planetary motions, but then claiming a
planet moves counterclockwise.

That depends on what you mean by "moves".

He doesn't contradict himself when he says a planet *rotates*
counterclockwise. His claim is that there is no reference _external to
the Solar System_ that is legitimate to apply to the axial rotation of
a planet. But the Earth rotates on its axis, when compared to the
legitimate *internal* Solar System reference of the Sun,
counterclockwise with a period of 24 hours.

Of course, he does contradict himself when he (correctly, along with
Copernicus) claims that the planet Earth *revolves* counterclockwise,
with a period of one year, in an orbit around the Sun.

As best I can understand it, he believes that the only "legitimate"
way to understand the Solar System is by using a hierarchical model.
The planets orbit the Sun, and these orbits have the same periods as
everyone else accepts. He may still believe that even for that
purpose, using the fixed stars is illegitimate, and so instead a
dynamical criterion should be used - Kepler's square/cube law would
work for that purpose.

But the axial rotation of each planet needs to be understood, in his
opinion, in a coordinate system based on the planet as the center, and
the *direction from the planet to the Sun* as the reference.

This same coordinate system would, I presume, apply to the Moon's
orbit around the Earth. And the Moon's rotation would be understood
with the Moon as the center, and the direction from the Moon to the
Earth as the reference direction - leading to the conclusion that the
Moon *does not rotate on its axis*, given that it is tidally-locked
and always keeps one face to the Earth.

This emphasis on synodic periods rather than sidereal ones isn't
completely bizarre. It is, in fact, the natural first approach of a
layman to astronomy, which is why concepts such as the "sidereal day"
have to be introduced at an early point in astronomical textbooks.
And, in fact, those concepts sometimes _are_ introduced with...
unnecessary brutality, so that a younger reader of an astronomy
textbook is left with no real reason why the Earth's _real_ rotational
period is 23 hours, 56 minutes and 4 seconds... instead of the 24
hours he is used to from everyday life.

While I'm on the topic, what _is_ the real reason that astronomers say
the Earth's "real" rotational period is 23 hours, 56 minutes, and 4
seconds?

The Earth isn't attached to a rigid arm extending out from the Sun. It
rotates on its axis freely in space. The Sun's gravity will have tidal
effects on the Earth, but these effects are very slow ones, adding up
little by little over the course of many years.

And the Earth's orbit around the Sun isn't a perfect circle (nor is
its plane that of the Earth's equator).

So there is no natural motion which corresponds to axial rotation with
a fixed period of 24 hours - only the natural noon cycle, which
_averages_ to 24 hours over the course of a year, but which has the
irregularities known as the Equation of Time.

An axial rotation for the Earth that has a uniform 24 hour period
would have to be measured against a "mean Sun", which is not a real
physical object. The stars, though, are real physical objects, and
their directions from us are approximately constant due to their great
distance.

The Earth's uniform rotation can be appreciated, therefore, when the
stars are used as a reference; using the Sun, due to the Earth's
complicated orbital motion, leads to the complicated compound natural
noon cycle which includes the Equation of Time.

I have tried to explain this simply - using his own terms, such as
"natural noon", so he can follow what I am referring to - because I
still hope that someday he will understand what I am saying, and see
the truth.

John Savard
.

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