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Author Topic: If all motion is relative, how can we tell whether the Earth is spinning around the sun or vice versa?  (Read 14297 times)

Johnny Midnight

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Johnny Midnight  asked the Naked Scientists:
   
If all motion is relative, how can we tell whether the Earth is spinning around the sun or vice versa?
What do you think?


 

Offline Soul Surfer

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Neither is true.  Both the earth and the sun are orbiting around their common centre of gravity.  The presence of all the other planets just makes it more complicated :-)
 

Offline syhprum

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If there was some mysterious Ęther relative to which the Earth was non rotating the orbital speed of bodies more remote than 55AU would exceed the speed of light.
« Last Edit: 20/01/2009 21:28:43 by syhprum »
 

Offline demadone

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Actually in those lines, the earth may even be considered the center of the universe. So Galileo was falsely accusing those Catholics of telling lies. The earth is the center of the universe, relative to an earthly observer. As simply as the sky tells it. Poor Catholics
 

Offline yor_on

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Offline lightarrow

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Actually in those lines, the earth may even be considered the center of the universe. So Galileo was falsely accusing those Catholics of telling lies. The earth is the center of the universe, relative to an earthly observer. As simply as the sky tells it. Poor Catholics
Ok. Can you please explain me then why on January (for example) the stars you see on the sky during the night are not the same that you see on July?
 

Offline swansont

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"All motion is relative" refers to inertial frames, as long as you want to apply Newton's laws.  While you can't tell who is moving if the velocity is constant, you can (in principle) tell if you are accelerating.  You have to make up pseudoforces to account for motion in an accelerating frame. 
« Last Edit: 31/01/2009 19:27:46 by swansont »
 

Offline Vern

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Quote from: Soul Surfer
Neither is true.  Both the earth and the sun are orbiting around their common centre of gravity.  The presence of all the other planets just makes it more complicated :-)
I'm sure someone has figured out how far from the sun that common centre is. I wonder if it is outside the sun's surface.

 

Offline Soul Surfer

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No it is not in the case of the earth sun system only but the centre of gravity of the earth moon system is above the surface of the earth.  The moon's orbit is also always concave with respect to the sun so the earth moon system could well be considered to be a double planet
« Last Edit: 01/02/2009 12:11:31 by Soul Surfer »
 

Offline lightarrow

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Quote from: Soul Surfer
Neither is true.  Both the earth and the sun are orbiting around their common centre of gravity.  The presence of all the other planets just makes it more complicated :-)
I'm sure someone has figured out how far from the sun that common centre is. I wonder if it is outside the sun's surface.
Taking, according to wiki, Sun mass ≈ 2*1030 kg, Earth mass ≈ 3*1024 kg and Sun-Earth average distance ≈ 150 millions km, with a mental compute the centre of mass is 450 km from the Sun's centre. Sun's radius is ≈ 700.000 km, so...
 

Offline Vern

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Quote from: lightarrow
Taking, according to wiki, Sun mass ≈ 2*1030 kg, Earth mass ≈ 3*1024 kg and Sun-Earth average distance ≈ 150 millions km, with a mental compute the centre of mass is 450 km from the Sun's centre. Sun's radius is ≈ 700.000 km, so...
That rules out an experiment where we float a craft at the centre of mass of the system:)
 

Offline syhprum

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Lightarrow

Somewhere the Earth has lost half its mass, one of those dreaded LHC blackholes ?
 

Offline Vern

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Lightarrow

Somewhere the Earth has lost half its mass, one of those dreaded LHC blackholes ?
Yep; looks like it :) but we still can't launch our common-centre-of-orbit craft.

Quote
What is the mass of the Earth?

Mass is the amount of matter in an object. The mass of the Earth is estimated to be 6 sextillion, 588 quintillion short tons (6.6 sextillion short tons) or 5.97 x 1024 kilograms. A short ton equals 2,000 pounds as opposed to a long ton, the unit used in Great Britain, which equals 2,240 pounds. The Earth has an average density of 5.52 grams per cubic centimeter, which is 5.52 times the average density of water (the standard).

The method for calculating the mass of the Earth was adopted by the International Astronomical Union in 1964 and recognized by the International Union of Geodesy and Geophysics in 1967.

Sources: Asimov, Isaac. Asimov's Guide to the Earth and Space, pp. 34-35; Emiliani, Cesare. The Scientific Companion, p. 167; Famighetti, Robert, ed. The World Almanac and Book of Facts 1995, p. 269.

« Last Edit: 01/02/2009 16:57:19 by Vern »
 

Offline syhprum

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If we are patient and wait for the sun to collapse into a neutron star it will be possible but I don't know if it is sufficiently massive to collapse that much.
 

Offline lightarrow

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Lightarrow

Somewhere the Earth has lost half its mass, one of those dreaded LHC blackholes ?
:)Yes, you are right! The result is correct because I computed with the right value, 6*1024 kg, but then, dividing for Sun's mass, a number 3 remained "attached" in my memory. Bad RAM  :)
 

Offline yor_on

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Don't spank your computer.
Spank your ram.

 

Offline lightarrow

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Don't spank your computer.
Spank your ram.


I thought the meaning was that one... :)
 

Offline syhprum

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Soulsurfer

"but the centre of gravity of the earth moon system is above the surface of the earth"
I believe this is not correct.
 

lyner

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Soulsurfer

"but the centre of gravity of the earth moon system is above the surface of the earth"
I believe this is not correct.
That is correct.
i.e. that that is not correct
i.e. you are correct

i.e the cm is below the surface
 

lyner

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No it is not in the case of the earth sun system only but the centre of gravity of the earth moon system is above the surface of the earth.  The moon's orbit is also always concave with respect to the sun so the earth moon system could well be considered to be a double planet

What do you mean "concave"? Do you mean there are cusps?
 

Offline Soul Surfer

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if you plot the moons orbit with respect to the sun (not the earth) it follows a wavy curved line close to the earth.  the curvature with respect to the sun varies but it is always concave and never convex
 

lyner

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Unless it has cusps, how can it be 'concave' but not circular? Is there a picture of what you say?
 

Offline Soul Surfer

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I have tried to do a drawing but my skills with the freehand mouse.  let me describe in more detail

The moon goes round the earth in an approximately circular orbit in 28 days

The earth goes round the sun in a 400 times larger almost circular orbit in one year during this period the moon goes round the earth approximately 13 times

Draw the track of the moon with respect to the sun it always moves in the same direction as the earth with respect to the sun  it moves ahead of the earth and behind the earth, closer to the sun than the earth and farther from the sun than the earth  This track is a wavy line. look at the radius of curvature of this line it varies but is always approximately towards the sun (as is the radius of curvature of the earth's orbit) and never points away from the sun
« Last Edit: 06/02/2009 09:28:52 by Soul Surfer »
 

lyner

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I think what you mean is that the Moon's orbit around Earth produces a variation   of the curvature of the Moon's Solar orbit. But that this variation is less than the curvature of the Earth's orbit (mean orbit of the Moon). So this means that the Earth's curvature +/- the variation (wobble) is still always positive (i.e. towards the Sun).
Strange, but I don't remember anyone pointing this out before. It seems fair enough, though, now I think about it. The magnitude of this wobble is only 0.2% of the radius of the Earth's orbit and rough sketches always exaggerate this and would suggest otherwise.

This won't always happen for all planets and moons. If the Earth were much more massive and the Moon had the same orbital period then its orbit could be much greater than 350,000km. That would produce a proper 'curly wurly' orbit.
 

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