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I know that Earths gravity pulls down the Moon towards the Earths center and Moon is falling on Earth, but because of its change in velocity it never fall on Earth but orbits around Earth.

Now I don't understand why the Moon can not change its orbit around the Earth to some other planets. As a planet comes near to Moon where the new planets gravity is more than Earths gravity. And the Same situation's why not applying on other planets like Mars and Earth when they are Perihelion point, change it's orbits and orbiting each other or one orbit to another.

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  • $\begingroup$ The short answer is "they do". All the bodies in the solar system (and the wider universe for that matter) are affecting each other gravitationally. In fact, the gravity of other planets does have a subtle effect on the Earth's orbit, but these effects are relatively weak. The planets never come close enough to each other for it to have any serious effect. The Earth's gravitational effect on the moon is always far stronger than any other planet simply because the moon is much closer to the Earth. $\endgroup$
    – JeneralJames
    Commented Nov 30, 2016 at 11:05
  • $\begingroup$ If they do affecting each other (with weak gravitational force), so then why not it change the speed, velocity or force of other planet. $\endgroup$
    – Imran Khan
    Commented Nov 30, 2016 at 13:58
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    $\begingroup$ @Imran: what makes you think it hasn't changed speed, velocity or force of another planet? $\endgroup$
    – Kyle Kanos
    Commented Nov 30, 2016 at 14:21
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    $\begingroup$ @Imran: that's the thing, there are affects, but because our lifetimes are incredibly short compared to planetary orbits, you'd never notice it. The forces are just too small to stop a planet, as you could do some math using Newton's gravitational law between earth and, say, Jupiter. $\endgroup$
    – Kyle Kanos
    Commented Nov 30, 2016 at 14:59
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    $\begingroup$ Similar to this physics.stackexchange.com/q/295569 but with its own particularities. Since gravity decreases with the inverse square law, distance between objects is of paramount importance. $\endgroup$
    – J. Manuel
    Commented Nov 30, 2016 at 16:09

3 Answers 3

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The higher the mass/distance-between-objects-squared the higher their mutual influence under gravity.

In solar system (excluding the earth) the planet having bigger mass/distance-to-moon-squared is Jupiter. So, let’s consider Jupiter-to-Moon Vs Earth-to-Moon influence only.

The closest distance between earth (moon) and Júpiter is $5.9×10^8 km$. Considering also

$m_{earth}=6×10^{24} kg$;

$m_{jupiter}=1.9×10^{27} kg$;

$d_{earth-moon}=3.8×10^{5} km$;

$d_{jupiter-moon}=6.3×10^{8} km$;

Now, if α is the ratio between Earth and Jupiter’s gravitational influence, then

$$\rm α= \frac{M_{earth}}{M_{jupiter}}(\frac{d_{jupiter-moon}}{d_{earth-moon}})^2=7.4×10^{3}$$

Therefore, Earth exerts a force ten thousands times stronger than any other planet into Moon, so we are not going to lose her soon (though we may lose her as Lawrence B. Crowell said).

PS: Every object in solar system (I’m avoiding using the universe), do act on others making slight changes in orbit, speed, velocity depending on how strong the interaction is, as a matter of fact, Neptune (and then Pluto) was predicted before observation just because of the anomalies it caused in the obit of Uranus.

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In effect this will happen in a way at least in theory. The orbital angular momentum of the Earth's rotation is being transferred to the moon. This is due to tidal interaction between the Earth and the moon. The orbit of the moon is being nudged outwards. Think of a line from the center of the sun through the Earth. The moon crosses this line twice in its orbit around the Earth. However, once the moon is sufficiently far out that its velocity is equal or lower than this line then it is effectively free of the Earth.

This will happen in 50 billion years, which means it will be interrupted by the sun in its red giant phase where it will likely swallow up Earth and moon.

There are these libration points where centripetal forces and gravitation of many body systems balance. These Lagrange points can be stable or saddle points that are unstable in one direction and stable in the other. In the solar system the planets have their Lagrange points which perburb each other as they come close. It is then possible for a mass to hop from one to the other and move around the solar system this way. This is the interplanetary transport network which has been proposed as a way to get spaceprobes to perform long term explorations. It is possible that over a long period of time that trojan asteroids as Lagrange points might migrate around the solar system, and in the early solar system this may have involved larger bodies comparable to moons around some of the planets.

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  • $\begingroup$ I don't think the rotational angular momentum of the earth is sufficient to unbind the moon. $\endgroup$
    – DilithiumMatrix
    Commented Nov 30, 2016 at 18:21
  • $\begingroup$ You might be right. I wrote the $50$ billion years based on memory of this. It might be instead that in $50$ billion years the Earth-moon system might be completely tidally locked and the moon migrates out to its maximal distance. $\endgroup$
    – Lawrence B. Crowell
    Commented Dec 1, 2016 at 11:02
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I think it is because we are literally closer

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    $\begingroup$ This seems like an answer to me, not a comment. Although brief, it does answer the actual question (as does J Manuel's comment, which is an fact an answer) without beating about the bush. $\endgroup$
    – sammy gerbil
    Commented Dec 1, 2016 at 3:10

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