# Why earth rotates around ?

#### sneez

It is said that sun pulls on earth by gravity. Here 'gravity' i mean some attractive field mediated by something. (newton theory i guess)

But then im told that earth rotates round sun because of space distortion by sun. (the famous rubber sheet example)

Which one is it?

and How does the search for gravitons go with the curved space hypothesis [why do we look for them if gravitational attraction is only illusion caused by curved space] ? How does the curved space produce tidal effects on moons rotating planets?

and if its not too much, How/what/why produces this curvature in space? [Does the graviton act on space or would the graviton act directly on the object?]

Even little bit of clarification is appreciated

#### Ki Man

in 8th grade science i was told that the planets rotate because of the way the dust-disk that all planets were formed from rotated in the same direction around the sun. (or something like that, my memory is bad and so was my teacher)

Except Venus, which was supposedly hit by one of the original prototype planets that caused it to tip over and switch rotation

#### Werg22

Stick with Newtonian gravitation right now, space distortion is a relativistic concept.

#### LURCH

I agree with Werg22, If you're just learning, stick with Newton's gravity; it explains enough so that you can get a grasp fo what's happening, make accurate predictions, etc.

But just keep in mind that the "rubber sheet example" makes all the same predictions, and is a slightly better understanding (for those to whom Newton's "force" of gravity isn't good enough).

#### Integral

Staff Emeritus
Gold Member
The earth revolves around the sun, because it does.

Newton found that he could predict the positions of heavenly bodies by using a central force he called gravity.

Einstein found that he could model the same system by assuming that space is curved by massive bodies. The fact is that Einstein's model is more general and works over a wider range of conditions then Newtons model. What we have found, is that Newtons model is an approximation to Einstein's and can be arrived at by dropping "extra" terms that show up when using Einstein's model.

So as long as you are aware of what conditions are suited to Newtons laws they are useful and accurate. Einstein's theory yield more accurate results in all cases but they are much harder to apply. In the world we live in very precise measurements are required to separate the two models.

#### sneez

Well, i appreciate responses but i would like to know what is theory about the apparent contradiction?

If space is curved and [as im told einstein incorporates newton theory in it], what would be the role of gravitons? And how curvature alone can produce tidal effects on planets? Streching to me is force, i dont understand how curvature can produce force? [i understand i can produce apparent force, ie. two man walking on parallel longitudes on a sphere].

Is there anyway to explain these to me without relativistic math?

#### LURCH

That image of two men walking is a good place to start. Suppose those two men were trying to carry one object by holing on to both ends. As each man walks along his own line of longitude, the load they share gets stretched.

In a similar way, the side of the Earth that is closest to the Sun travels through a space that is slightly more curved than the side that is farthest away from the Sun. It is the difference between these two curves that causes tidal forces (the stretching of the Earth's surface).

#### nannoh

sneez said:
How does the search for gravitons go with the curved space hypothesis [why do we look for them if gravitational attraction is only illusion caused by curved space] ?
This definition of a graviton may help with part of your question:

In physics, the graviton is a hypothetical elementary particle that transmits the force of gravity in most quantum gravity systems. In order to do this, one theory posits that gravitons have to be always-attractive (gravity never pushes), work over any distance (gravity is universal) and come in unlimited numbers (to provide high strengths near stars). In quantum theory, these requirements define an even-spin (spin 2 in this case) boson with a rest mass of zero.
en.wikipedia.org/wiki/Graviton

And I thought a graviton was an increment of measurement measuring the amount of gravity caused by n amount of mass. Wrong?

#### russ_watters

Mentor
sneez said:
Well, i appreciate responses but i would like to know what is theory about the apparent contradiction?
What apparent contradiction? It just seems to me that you don't understand the issue...
If space is curved and [as im told einstein incorporates newton theory in it], what would be the role of gravitons?
Those are questions that physics is still trying to answer, and at your level of knowledge it may be best to leave them alone for now, as others suggested.
And how curvature alone can produce tidal effects on planets? Streching to me is force, i dont understand how curvature can produce force? [i understand i can produce apparent force, ie. two man walking on parallel longitudes on a sphere].
The curvature is not easy to visualize, so it is best to use analogies such as the bowling ball on a trampoline. A marble placed near a bowling ball will get "pulled" toward the bowling ball due to the curvature of the trampoline. Two marbles placed different distances from the bowling ball will be pulled with different forces. The difference between those forces (which can be felt if you connect the marbles with a string) is the tidal force.

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#### DaveC426913

Gold Member
Integral said:
The earth revolves around the sun, because it does.
This is an answer not to be passed by too quickly.

It is important that you recognize that both Newton's and Einstein's theories are simply models that we use to make predictions. Newton's is an excellent predictor in 99% of circumstances, but Einstein's is more like 99.9%. Neither actually claims to be the actual cause of the Earth going round the Sun.

As to what actually causes the Earth to revolve around the Sun, well, that's a question for future physicists.

#### Dr.Brain

and what explains the earth's motion around its own axis??? ..I read that earth rotates aroun the c.m of the earth-moon system which lies within the earth itself , so earth rotates arnd its own axis...is it true??..

#### russ_watters

Mentor
Well, the earth's rotation and the moon's existence/orbit are likely unrelated as the earth and moon probably did not form from the same disk of material (iirc, the leading current theory is that the moon was formed from the debris of an asteroid colliding with the earth).

But, the earth's rotation and its revolution around the sun do have the same cause: the gravitational collapse of an asymetric cloud of debris induces a torque and thus the rotation and revolution of the earth. That's why most planets rotate in the same direction as they revolve.

#### rathlawath

hey dudes ............ i am not any genius or any thing ............. but would like to think a lot and i got this strange feeling that earth revolves around sun because it obeys gyroscopic effect ...................... i mean to say that as earth rotates around its own axis..........this will cause the earth to rotate around sun...................

this can be seen in many experiments conducted by Eric Laithwaite .......... any object revolving around its own axis.........would be traveling in a path ..... like earth revolving around sun.........

#### DaveC426913

Gold Member
hey dudes ............ i am not any genius or any thing ............. but would like to think a lot and i got this strange feeling that earth revolves around sun because it obeys gyroscopic effect ...................... i mean to say that as earth rotates around its own axis..........this will cause the earth to rotate around sun...................

this can be seen in many experiments conducted by Eric Laithwaite .......... any object revolving around its own axis.........would be traveling in a path ..... like earth revolving around sun.........
So how does one explain Mercury, which rotates in the opposite direction to its revolution. Hamster wheel?

#### Renge Ishyo

Okay so here is a question for the physics types on here: The spinning earth generates a magnetic field. I am assuming that other spinning planets/the sun generate a magnetic field. The question is, wouldn't this contribute to an attraction or a repulsion of the planets towards or away from one another (in the same way that wires with currents through them attract or repel each other)? I realize the distances between planets are quite large and the magnetic force between planets might be very weak, but even a small change in the orbit of these planets could cause a collapse towards one another...so why doesn't this happen?

#### DaveC426913

Gold Member
Okay so here is a question for the physics types on here: The spinning earth generates a magnetic field. I am assuming that other spinning planets/the sun generate a magnetic field. The question is, wouldn't this contribute to an attraction or a repulsion of the planets towards or away from one another (in the same way that wires with currents through them attract or repel each other)? I realize the distances between planets are quite large and the magnetic force between planets might be very weak, but even a small change in the orbit of these planets could cause a collapse towards one another...so why doesn't this happen?
1] The Earth's magnetic field is not caused by its spin.
2] The magnetic field is bipolar, meaning is has both north and south. The net effect is zero.
3] Gravity drops off as the square of distance. Magnetic fields drop off as the cube of the distance. i.e. at an arbitrary distance x, if the gravitational force has dropped off to 1/100th, the magnetic field will have dropped of to 1/1000th. If gravity is 1/1000th, then magnetic field is 1/31,617th.

And after all that,
4] Who says it isn't having an effect? Planets are perturbed all the time by each other and by jets of gas and energy from the sun. These are taken intro account. They do not send planets spinning out of control on collision courses.

#### Renge Ishyo

A couple of Q's on these:

1] The Earth's magnetic field is not caused by its spin.
2] The magnetic field is bipolar, meaning is has both north and south. The net effect is zero.
1. At least when I last learned it, the magnetic field is proposed to be generated by liquid metal moving in a circular pattern near the earth's core. Of course, for all I know this view might be outdated.
2. I thought all magnets have both a north and south pole? Or is this idea outdated too (<---- feels old).

#### muppet

Yes, all magnets have a north and a south pole. I can't comment on the origin of the Earth's magnetic field except to say that just metal moving in a circular pattern wouldn't create one- there would need to be a motion of net charge.

#### H0T_S0UP

Yes, all magnets have a north and a south pole. I can't comment on the origin of the Earth's magnetic field except to say that just metal moving in a circular pattern wouldn't create one- there would need to be a motion of net charge.
For earths magnetic field I would imagine it more like a sea of liquid metal (iron perhaps?) rather than a synchronized flow influenced by convection currents and such. As far as science knows all magnets must have two poles since a monopole has not been discovered nor created yet.

#### arildno

Homework Helper
Gold Member
Dearly Missed
hey dudes ............ i am not any genius or any thing ............. but would like to think a lot and i got this strange feeling that earth revolves around sun because it obeys gyroscopic effect ...................... i mean to say that as earth rotates around its own axis..........this will cause the earth to rotate around sun...................

this can be seen in many experiments conducted by Eric Laithwaite .......... any object revolving around its own axis.........would be traveling in a path ..... like earth revolving around sun.........
Wholly incorrect.
I assume that you are speaking of the Magnus effect, which makes a rotating ball curve its path.

The Magnus effect requires that the ball to rotate in a fluid (like the air), and is generated by the pressure differential set up in that medium.

Now, to be sure, the space-time of Einstein is not some sort of Newtonian rigid box, but you'd certainly not get at earth rotation velocities effects like the Magnus effect out of general relativity.

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