The relativity of celestial motion

As a non-astronomer, I have a simple question.

Since motion is relative and it is sometimes difficult to perceive
which object is really moving and which (if any) is stationary
(example being when you are the passenger in a train and and you see
the train next to you "move away", you don't know whether your train
is in motion or the train being observed until you can view a
stationary object for reference).

With the above in mind, how do we really know that the Earth revolves
around the sun, verses the sun rotating around the Earth?

Is it primarily because of Occam's Razor? In other words, do we accept
this primarily because the mathematics around the physicis are much
simpler if we accept that the earth is revolving the sun vs the sun
revolving around it?

"Ludwig77" wrote in message
om...
As a non-astronomer, I have a simple question.

Since motion is relative and it is sometimes difficult to perceive
which object is really moving and which (if any) is stationary
(example being when you are the passenger in a train and and you see
the train next to you "move away", you don't know whether your train
is in motion or the train being observed until you can view a
stationary object for reference).

With the above in mind, how do we really know that the Earth revolves
around the sun, verses the sun rotating around the Earth?

Is it primarily because of Occam's Razor? In other words, do we accept
this primarily because the mathematics around the physicis are much
simpler if we accept that the earth is revolving the sun vs the sun
revolving around it?

If it were just us and the sun, I think this would be argue-able (is that a
word?) But since we have other planets, and we can observe their behavior we
can determine who is revolving around whom.

BV.

(Ludwig77) wrote in
om:

As a non-astronomer, I have a simple question.

Since motion is relative and it is sometimes difficult to perceive
which object is really moving and which (if any) is stationary
(example being when you are the passenger in a train and and you see
the train next to you "move away", you don't know whether your train
is in motion or the train being observed until you can view a
stationary object for reference).

With the above in mind, how do we really know that the Earth revolves
around the sun, verses the sun rotating around the Earth?

Actually, the sun and the earth both rotate around their mutual center of
gravity. But to address your question more directly, you are correct that
for two objects in uniform relative motion we can't distinguish and say
that one is stationary and the other is moving. We can, however, detect
acceleration. If one object is rotating around the other, this can be
measured unambiguously. Likewise, in your example of two trains that are
initially stationary with respect to one another, it can be determined
which changes speed and starts to move with respect to the other.

--
Steve Gray

(Steven Gray) wrote in message ...
(Ludwig77) wrote in
om:

As a non-astronomer, I have a simple question.

Since motion is relative and it is sometimes difficult to perceive
which object is really moving and which (if any) is stationary
(example being when you are the passenger in a train and and you see
the train next to you "move away", you don't know whether your train
is in motion or the train being observed until you can view a
stationary object for reference).

With the above in mind, how do we really know that the Earth revolves
around the sun, verses the sun rotating around the Earth?

Actually, the sun and the earth both rotate around their mutual center of
gravity. But to address your question more directly, you are correct that
for two objects in uniform relative motion we can't distinguish and say
that one is stationary and the other is moving. We can, however, detect
acceleration. If one object is rotating around the other, this can be
measured unambiguously. Likewise, in your example of two trains that are
initially stationary with respect to one another, it can be determined
which changes speed and starts to move with respect to the other.

I'm not disputing your point, just trying to understand more clearly.

If I were in one of the trains, and one of them was just starting to
move (accelarating), how could I as an observer determine which train
was moving without looking at a stationary object or the train's
wheels?

Secondly, have we been able to observe the acceleration of the planets
in order to determine that they are orbiting around the sun as opposed
to the alternative?

"Ludwig77" wrote in message
om...

I'm not disputing your point, just trying to understand more clearly.

If I were in one of the trains, and one of them was just starting to
move (accelarating), how could I as an observer determine which train
was moving without looking at a stationary object or the train's
wheels?

An accelerometer would detect it. An oversimplified example would
be a massive plumb-bob suspended from the ceiling of the trains.
In the train that accelerates, the plumb-bob would show an angle
with respect to the vertical.


Secondly, have we been able to observe the acceleration of the planets
in order to determine that they are orbiting around the sun as opposed
to the alternative?

Stellar aberration measured from the surface of the Earth
shows that the Earth is in motion around the Sun (yearly
cycle). The rest can be easily inferred from the observed
geometry of the planets as they move.

An accelerometer would detect it. An oversimplified example would
be a massive plumb-bob suspended from the ceiling of the trains.
In the train that accelerates, the plumb-bob would show an angle
with respect to the vertical.

Makes sense to me but is the earth in acceleration or is it now in uniform motion?



Secondly, have we been able to observe the acceleration of the planets
in order to determine that they are orbiting around the sun as opposed
to the alternative?

Stellar aberration measured from the surface of the Earth
shows that the Earth is in motion around the Sun (yearly
cycle). The rest can be easily inferred from the observed
geometry of the planets as they move.

What is steller aberration?

"Ludwig77" wrote in message
om...

An accelerometer would detect it. An oversimplified example would
be a massive plumb-bob suspended from the ceiling of the trains.
In the train that accelerates, the plumb-bob would show an angle
with respect to the vertical.

Makes sense to me but is the earth in acceleration or is it now in uniform
motion?

For the thought experiment we're conducting, the Earth
is taken to be in uniform inertial motion.




Secondly, have we been able to observe the acceleration of the planets
in order to determine that they are orbiting around the sun as opposed
to the alternative?

Stellar aberration measured from the surface of the Earth
shows that the Earth is in motion around the Sun (yearly
cycle). The rest can be easily inferred from the observed
geometry of the planets as they move.

What is steller aberration?

The motion of the Earth around its orbit causes small
shifts in position of the "fixed" stars. This is
due to telescopes having to tilt slightly to allow
for the addition of the Earth's velocity vector to
that of the light's.

http://scienceworld.wolfram.com/phys...berration.html
http://www.mathpages.com/rr/s2-05/2-05.htm
http://www.globalserve.net/~bumblebe...berration.html

.... etc. A google search will bag loads of links.

"Greg Neill" wrote in
:

Makes sense to me but is the earth in acceleration or is it now in
uniform motion?

For the thought experiment we're conducting, the Earth
is taken to be in uniform inertial motion.

Lest there be some misunderstanding on the part of the OP, when we're
talking about the Earth rotating around the sun, the Earth is seen to be
(and is) constantly accelerating. When we're talking about the train
example, the acceleration of the Earth is small enough to be neglected.

--
Steve Gray

"Ludwig77" wrote in message...
om...
. . .
What is steller aberration?

Think of yourself holding an umbrella while it's raining. You
are standing still, and the rain is dropping vertically, no wind.
As you begin to walk and go faster and faster, you may note
that the rain hits you at more and more of an angle. You might
even have to tilt your umbrella a bit to keep from getting your
face wet. This effect might be called "raindrop aberration."

Now think of the light from the stars (stellar) as being like the
rain. The same angle effect (aberration) is caused by Earth's
motion through space.

Back in the 18th century scientists were trying to prove that
the Earth moved. One theory dealt with the "parallax" of
stars. If you view a star from one side of the Sun, then 6mos
later view it again, then there ought to be a displacement due
to parallax. But their telescopes were too small to measure
the parallax of even the nearest stars.

One of those who tried was British astronomer James Bradley
(1693-1762). Using a telescope 212 feet long, Bradley tried
to measure the small displacement of stars in the course of the
year and actually detected such a displacement. What he found,
however, could not be parallax, because the displacement did
not coincide with what would be expected if it were the result
of Earth's changing position in its orbit.

So Bradley looked for an alternate explanation and one
occurred to him in 1728: The displacement arose because the
telescope had to be tipped slightly to catch the light as the Earth
moved (this is called adjusting to the aberration of light), just as
an umbrella must be tipped when you're walking briskly through
a rain in which the drops are falling vertically. The amount by
which the telescope must be tipped depends on the ratio of the
speed of the Earth in its orbit to the speed of light.

This meant that although Bradley had not detected parallax, he
had discovered a new way of calculating the speed of light, since
the speed of the Earth in its orbit was known and the amount of
the tipping of the telescope was known, too. This was the first
determination of the speed of light since Roemer's a half-century
before (1675) and it was a more accurate measurement. Bradley's
figure was 176,000 miles per second, only 5 percent less than the
true value.

What's more, the existence of light aberration was just as strong a
proof that the Earth was moving as the existence of stellar parallax
would have been.

hth

happy days and...
starry starry nights!

--
a Secret of the Universe...
so please don't breathe a word of this--
the Moon above will smile perverse
whene'er it sees two lovers kiss;
(breathe not a single word of this!)

Paine Ellsworth