Earth's orbit

1. May 12, 2014

wordsworm

How do scientists measure the distance between the earth and sun as it orbits? Do they use visual distance or do they account for distortions?

2. May 12, 2014

Staff Emeritus
Until recently, it was radar. It's now measured by the motion of space probes and geometry.

3. May 12, 2014

ChrisVer

http://curious.astro.cornell.edu/question.php?number=400 [Broken]

Last edited by a moderator: May 6, 2017
4. May 12, 2014

wordsworm

I have been a little obsessed with the question of whether the earth is attracted to the sun or the image of the sun. I had just 1 science class that I took and it was geology (of the solar system). I asked him if we orbited the image of the sun or the real sun. He told me that we orbited the image of the sun: that gravitons travel at the same speed as light.

It got me to thinking that if that is true, then when the earth is in Periapsis stage and Apoapsis stage, we would see the sun at different points compared to where the actual sun is. Since light travels at 300,000km/s, and there is a difference of 5 million kms in those two stages, that we'd be seeing a slightly different sun, since it itself is moving at around 225-250km/s. If we were looking at the sun and comparing the two images at those two stages, there would be a difference of almost 5,000km that the image would travel as it caught up to the actual sun. Since the actual sun would be closest to the earth when it appeared in Periapsis, it would also mean that it would be closer than it looks, while the contrary would be true in Apoapsis.

What's more, I thought, it would mean that the sun itself would red and blue shift based, at least in part, on this difference of real vs visual sun.

I guess all of this wondering that adds up to only 5,000km probably doesn't mean much... but I still wonder if they consider this aspect in measuring our orbit around the sun.

5. May 13, 2014

D H

Staff Emeritus
Your friend is wrong. It is true that gravitational waves travel at the speed of light, and that gravitons, if they exist, would also travel at the speed of light. However, the direction of the gravitational acceleration is still pretty much toward where the Sun is now rather than where the Sun appears to be. There's a whole lot more to general relativity than speed of light. Orbits would not be stable if speed of light was the only difference between Newtonian gravity (where gravitation is instantaneous) and general relativity.

For smallish masses separated by largish distances that are moving at slowish velocities, the dominant gravitational effect is that of a static field. That smallish masses, largish distances, and slowish velocities: That's our solar system. From a heliocentric perspective, the Earth moves through the Sun's gravitation field, a field that is already in place. From a geocentric perspective, this means the Earth more or less gravitates toward where the Sun is rather than where it appears to be.

6. May 13, 2014

wordsworm

DH: It was not my friend who said this, just a geology professor. If it is true that we are actually attracted to the sun itself rather than the image of the sun, then how can a 'particle' (graviton) that's never been seen or measured be assumed to be moving at light speed? And, more importantly, if we are attracted to the sun and not the image, as I have thought for some years now, then it would suggest that gravity moves at an infinite speed. And, if that is true also, I have an experiment I would really like to try to see if my own theory is correct or if Einstein's is.

For a long time I have wrestled with relativity, and found myself thinking that logically, it does not work for precisely the reason already brought up. While it describes light very well, and may describe how the appearance of time dilating, I don't think time itself dilates at all or that the universe is regulated by any speed. For instance, one of the experiments showing time dilation was with an airplane taking off with an accurate clock. However, I find that one suspect: you can fly an airplane up into the air with an atomic clock to measure time on the airplane, but we cannot fly the earth off of an airplane with an atomic clock to verify the reason for dilation was relativity or was it just that atomic clocks react to perhaps different kinds of stress, such as velocity? It seems impossible to use this as a falsifiable experiment. At least to my ears, if gravity moves faster than light, at an infinite speed occupying all space in diminishing quantity, then how does that not indicate that light is by far not the fastest form of energy in the universe?

And, what if there were things traveling faster than light in the universe? How would we be able to see them at all? If a ship were heading towards us at the speed of light, it would get to us just as the light was. If it were going faster, then it would be ahead of that light, and as it passed us, it would red shift so badly that you wouldn't be able to see it at all. And all that is assuming that the object itself either reflects or emits any light at all.

Anyways, thanks a lot for answering my question, DH.

I have another one if you're up for it. It's possibly a lot more complicated, but I cannot find anyone who can answer it for me. I have my guess, as I did above, but I would really like confirmation or criticism. If I had a magnetic bullet, and I fired it through a tube past a generator which in turn generated electricity from said bullet's velocity, the bullet would slow down, and it would cause the generator itself to follow it, would the kinetic energy of the two objects in motion be less than if there had been no generator at all? That is to say, does a generator really steal kinetic energy?

Last edited: May 13, 2014
7. May 13, 2014

Drakkith

Staff Emeritus
You have a few misconceptions.

1. Particles that mediate forces do so in the form of "virtual particles", not real particles. Virtual particles are a very difficult concept to understand without getting into the details of the math of various Quantum theories, but I can tell you that virtual particles are not bound by the normal laws of "real particles", such as traveling under c. Honestly it's better if you don't even think of them as traveling at all. Merely understand that in Quantum theories that use virtual particles the interaction between objects is described as if the two objects "exchanged" one of the mediating particles. No particle is actually emitted from one object and absorbed by the other. (This sounds really, really weird, and probably makes no sense, so I encourage you to look up more information on this)

2. Time dilation is a very real effect that has been measured countless times. The operation of the GPS system, any particle accelerator, and even measurements of astronomical objects like pulsars have verified this. This includes time dilation due to relative velocity and gravitational time dilation. Consider that if Relativity is wrong, why do ALL experiments ever performed match our predictions using that theory? This includes using atomic clocks, decaying particles, or any other method of timekeeping.

Also, Relativity accurately predicts other effects other than time dilation. The precession of Mercury's orbit and the orbital decay rate of orbiting pulsars are two I can think of right off the top of my head.

3. We know of no object capable of going FTL, so your assumptions about what would happen if an FTL object moved past us is simply pure speculation and doesn't represent reality in the slightest.

8. May 13, 2014

D H

Staff Emeritus
What is this "graviton" of which you speak? It's a hypothetical particle; it may or may not exist. Note very well: There is no such thing as a graviton in general relativity. General relativity is a not a quantum theory. It is a geometrical theory of gravitation.

This is *not*, repeat not, the site to espouse your theory. Reread the site rules.

In Newtonian mechanics, gravitation is an action-at-a-distance (infinite speed) type of force. Gravitation is not even a force in general relativity. It is instead a consequence of the curvature of space time.

Even before Einstein, there were hints that Newton's theory of gravitation was not quite right. The observed orbit of Mercury deviates by a precession of 43 arcsecond per century from the very best predictions of Newtonian mechanics. That is a very small deviation, but it is real. This deviation disappears with general relativity. That general relativity solved this outstanding problem was one of the key reasons why it was accepted so quickly. There are many other instances where general relativity does a better job than Newtonian gravity. General relativity has passed every test thrown at it. It is a profoundly accurate theory.

9. May 14, 2014

UltrafastPED

The good geology professor's lecture contained an error!

See "Aberration and the Speed of Gravity" by Steve Carlip: http://xxx.lanl.gov/abs/gr-qc/9909087

From the first page:

"It is well known that if a charged source moves at a constant velocity, the electric ﬁeld experienced by a test particle points toward the source’s “instantaneous” position rather than its retarded position. Lorentz
invariance demands that this be the case, since one may just as well think of the charge as being at rest while the test particle moves. This eﬀect does not mean that the electric ﬁeld propagates instantaneously; rather, the ﬁeld of a moving charge has a velocity-dependent component that cancels the eﬀect of propagation delay to ﬁrst order."

The derivation of this "well known result" can be found in the Feynman "Lectures on Physics"; Carlip also carries out a detailed derivation in section 1. In section 2 he carries out a similar analysis for gravitation, using the full machinery of General Relativity, showing why the "gravitational acceleration" felt by us points to the "instantaneous" location of the gravitational source.

That is: the actual sun and the image of the sun are in the same place; and this does not require instantaneous speeds for gravity any more than it does for light.

10. May 14, 2014

D H

Staff Emeritus
That should be "the actual Sun and the gravitational image of the Sun are in the same place." The visual image of the Sun as perceived from an orbiting is not in the same place as the actual Sun. The aberration of light results in the Poynting-Robertson effect (Wikipedia article), which causes small particles to spiral inward towards the Sun.

11. May 14, 2014

UltrafastPED

The image of the sun which we see is not that of the infalling dust particles; I think that is a separate effect from what we see from earth.

12. May 14, 2014

D H

Staff Emeritus
The Poynting-Robertson effect results from the aberration of light when viewed from the perspective of a frame instantaneously co-moving with the dust particle. It's something different (anisotropic thermal radiation by the moving dust particle) when viewed from the perspective of a heliocentric frame.