how do we know that earth’s orbit is stable?

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As far as i know earth’s orbis is stable.

If i understand it correctly it means that small perturbations in earth’s position or mass won’t change its orbit (like a spring effect that goes back to its rest state unless you break it), but if i simply balance out sun’s newtonian gravity vs the centrifugal force any little change would modify earth’s orbital speed and distance from the sun, so there’s something i’m missing, otherwise anytime a meteor hits or we send stuff to space we would be changing earth’s orbit.

So, IF my initial statement is correct, earth’s orbit doesn’t change for small perturbation, how do we know so? Secondly how big a perturbation would you need to change earth’s orbit?

On the other hand, if earth’s orbit changes with any minumum change, how big of a change in orbit would be needed for us to sebsibly perceive it?
Thanks

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Well the thing is that our orbit isn’t fully stable. Our planet actually has measurable jitter in our orbit and our planet will undergo massive changes in its orbital path. The issue is timescale. On the human timescale these imperceptible changes (while measurable) are insignificant. The orbital changes takes millions of rotations (years) to affect. At that timescale (thousands of human lifetimes back to back) the orbital path appears stable.

Well it depends what you mean by stable. If you mean stable as in it’s largely the same over very long periods of time, then yes. The Earth will not be crashing into the sun or flung out of the solar system or anything like that, at least for a few billion years. If you mean stable as in it’s perfectly 100% unchanged ever, then it’s *not* stable. All of the bodies in our solar system interact with each other gravitationally, and that changes the orbits of those bodies by small but measurable amounts. Earth particularly is gravitationally perturbed by Venus due to its proximity and Jupiter due to its mass. The Venus-Jupiter gravitational pull slightly alters Earth’s orbit ever 405,000 years.

Earth’s orbit isn’t self stabilizing in the way that you propose. Any little change will affect the orbital parameters, but as the Earth is extremely heavy and going extremely fast it’s really hard for us to make any measurable difference.

The only kind of self stabilizing orbits you’ll find are ones around Lagrange points, specifically L4 and L5. The ballet of gravitational forces from the Earth and the Sun work together to pull nearby small objects into that specific orbit, and a small perturbation will be corrected with time.

If I understand you question correctly. The earth as it revolves around the sun isnt a perfect circle, so say the scientists at nasa. It’s more of an “elipse” it does get closer to the sun by a measurable distance a few million miles closer and the reverse it gets father away. The earth and its neighbor MARS often their orbits bring them very close together. I think if I remember correctly it’s why the long wait to try and land people on mars it’s better if it’s the closest to earth as possible.

It’s not “stable” in the sense of springing back to the way it was after any little bump.

It *is* “stable” in the sense that any small change to an elliptical (oval-shaped) orbit is still an elliptical orbit. Inward-spiraling orbits are not a thing in the vacuum of space, and it would take the mother of all bumps to shift us onto a hyperbolic (escaping from the solar system) orbit.

Changes in Earth’s orbit over time *are* perceptible. For example, if we define “winter” astronomically as “the duration between the December solstice and the March equinox”, winter lasted 128,157 minutes in 1972 but only 128,134 minutes in 2022. That’s a 23-minute difference! (Caused mainly by gravitational interactions with the Moon and other planets.)

Source: playing around with cheap online gravity simulators,

I think, what happens is, the perturbations induced by the “players”, in the solar system. [i.e. Jupiter, Mars, numerous small “near-Earth” asteroids] in sum, approach some average value. This value is “probably” in the literal sense, close to 0.

So basically, unless there is an overwhelming influence, from a massive and near object gravitationally, we should be fine.

How that could happen, and us being in a sticky situation: Possibly, in the far future, is if there is an unfortunate orbital coincidence that “constructively interferes”, then, yes, we could have a problem.