# The sun's gravitational pull on the earth

1. Oct 19, 2006

### Mono182

if the sun was to vanish, what would happen to the earth. I've been told that the earth would still rotate around as if there was a sun, was atleast 8 minutes because thats the time it take for the sun's light to reach the earth. Since nothing is faster than the speed of light, the earth will stay in orbit. is that statement true or is there a better way of explaining it?

2. Oct 19, 2006

### billiards

I think that might be true, of course there would be no way to tell that the sun had vanished before those 8 minutes were up, and besides, stars don't just vanish like that.

3. Oct 19, 2006

### Danger

True, it's an impossible situation. Gravity does travel at the speed of light, though, so any disturbance in the sun's field would take 8 minutes to be noticed here.

4. Oct 19, 2006

### Mono182

so gravity does have a speed?

5. Oct 19, 2006

### scott1

In Genreal Realtivity the speed of light is also the speed of gravity

6. Oct 20, 2006

### Labguy

Which is exactly what became several L-O-N-G threads on the General Astronomy forum. Check over there for about all the opinions and questions you would ever want to read....

7. Oct 20, 2006

### Danger

Thanks. I knew there was at least one around somewhere, but I couldn't remember where.

8. Oct 20, 2006

### George Jones

Staff Emeritus
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9. Oct 20, 2006

### Labguy

Try putting an F-18 down on a rolling Carrier...

10. Oct 20, 2006

### Danger

No thanks; I get seasick.

11. Oct 21, 2006

### mijoon

May I add that the Earth would be squished by the tidal forces at around the 8 minute mark.

12. Oct 21, 2006

### Danger

You lost me on that one. If there is no gravitational source, there are no tidal forces.
By the bye, it's nice to see someone finally spell our country properly.

13. Oct 22, 2006

### Labguy

Not squished necessarily, but total disaster. As of now, the Earth is "compressed" by tidal forces between Sun and Moon in a vectored direction.

If the Sun's gravity were to "let go" 499 seconds (mean) after the Sun disappeared, the tidal forces would be instantly gone and the Earth's crust, et. al. would "spring out" and tear the hell out of about everything..:yuck:

14. Oct 22, 2006

### Danger

Hmmm... I don't even know whether that would fall under astronomy or planetology or what, but it completely escaped my home-made education. I would have expected the opposite effect, as in the compressive force of gravity no longer being counteracted by tidal force, but with the rotation still keeping the planet 'expanded'. At most, I figured that there would be a slight inward settling of the crust (still devestating to civilization, of course).
Thanks for the tune-in.

15. Oct 22, 2006

### Labguy

That's about all I really meant.

16. Oct 22, 2006

### LURCH

But would this cause the Earth's crust to move any more than it does on a daily basis? Tidal forces compress and extend the crust once for each planetary revoltion, don't they?

17. Oct 22, 2006

### Labguy

Yes, but slowly, not instantly..

18. Oct 23, 2006

### rbj

so think of $c$ as not simply the "speed of light" or the "speed of electromagnetic propagation" (which is where it came from originally), but as the speed of propagation of all things "instantaneous".

whether you and your signalling partner are each holding an electric charge (and you're wiggling them back and forth to signal the other) or you and your signalling partner are holding planets and wiggling them around to send "gravity signals", any instantaneous action from whatever source moves at a speed of $c$.

19. Oct 23, 2006

### sdemjanenko

would we even feel the "release" i mean as it is we are falling anyway. If there is no gravity we just feel like we are falling. So if we get released i don't think the earth would react in any significant and disasterous way.

20. Oct 23, 2006

### Staff: Mentor

Correct. It is a little bit of a tricky thing: when you are in orbit, you are falling towards the object you are orbiting and feel nothing. If the object blinks out of existence, you no longer orbit and you feel no forces pulling you toward something that doesn't exist. So you'd notice no change in the forces on you.

21. Oct 23, 2006

### LURCH

Oh yeah, hadn't thought of that.

But then again, from what I've heard the Earth is still bouncing back from deformations caused by the last Ice Age, so the subsidence of the tidal bulge might be a very gradual thing, taking thousands of years.

22. Oct 23, 2006

### Quaoar

Pretty sure the sudden absence of the Sun would NOT cause catastrophic rebound from the disappearance of the tidal forces. Quoth http://en.wikipedia.org/wiki/Tidal_acceleration" [Broken]:

...for the moon. If you do the math, the Sun's tidal forces are about half as strong. So basically, the sudden disappearance of the Sun would result in an elevation drop of no more than 1.6cm, which would also not be instantaneous due to the Earth's elasticity. In addition, the difference between local elevation changes would be tiny (Probably on the order of nanometers for two sides of an Earthquake fault).

Last edited by a moderator: May 2, 2017
23. Oct 24, 2006

### Mono182

hmm, since gravity travels at the speed of light, if the sun disappeared the earth would still orbit as if the sun was still there for atleast 8 mins, then we would feel the effects?

24. Oct 24, 2006

### Danger

Correct, but relax: it can't happen.

25. Oct 24, 2006

### rbj

but, even so, since orbiting or free-falling toward a graviational source is just an inertial movement from the perspective of GR, we wouldn't even feel the effects after those 8 minutes. the folks on the near side of the earth would miss the sunlight, but the folks on the far side wouldn't know any difference (except from communications) until the sun fails to rise when it's expected to.

what would be perceptually immediately remarkable is an observer near the axis of the Earth's revolution around the sun, equidistant from the earth and sun, and far enough away to see the circular or elliptical path of the Earth around the sun would see the earth continue in its elliptical orbit for 8 minutes after the sun was observed to disappear. that is a different observation than if the speed of gravity was infinite and the earth was observed to be moving in a straight line immediately after the sun disappears. that's the salient kernel of this thought experiment.

Last edited: Oct 24, 2006