Why, when the planet is accelerated during the ‘close phase’ of its orbit, isn’t it then flung away? I get that gravity pulls/holds it in orbit but why hasn’t it flattened out into a spherical orbit?
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> Why, when the planet is accelerated during the ‘close phase’ of its orbit, isn’t it then flung away?
It is. That’s how it gets to the “far phase” of its orbit. At which point it’s decelerated, and “falls back down” into the “close phase” of its orbit. (In technical terms, the faster speed near periapsis helps the orbiting object get back to apoapsis and the slower speeds near apoapsis cause it to fall back to periapsis.)
> I get that gravity pulls/holds it in orbit but why hasn’t it flattened out into a spherical orbit?
There’s no force acting to do so, at least not in the idealized case we’re talking about. And a force that isn’t just the central pull of gravity is required to change an orbit.
To make an orbit more circular, you need to thrust (EDITED, had these backwards) ~~*forward*~~ *backward* (~~prograde~~ retrograde) at your closest point in the orbit (periapsis) or thrust ~~*backward*~~ *forward* (~~retrograde~~ prograde) at your most distant point in the orbit (apoapsis). This thrust is perpendicular to the pull of gravity, so gravity can never supply it, at least not in the idealized case we’re talking about. Some other force, like the pull of another object or the thrust of a rocket, is needed to do so.
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Because that is its stable orbit. I’m not an expert but look at it like this:
Something gets pulled toward something else as it flies by. It turns back around because of the pull. It wants to get closer but is flung away again by its trajectory and speed not being broken by the gravitational pull. That keeps happening over and over. If the pull and trajectory/speed are in sync the orbit becomes stable and keeps the object locked in that orbit.
But it being perfect circle would be almost impossible because the objects meet instead of starting in a perfect setup to have perfect circle trajectory.
All these closed orbits are equally valid. There’s no physical reason for eccentric orbits becoming naturally more circular. Eccentric orbits are fine.
Probably similar to why rectangles are more common than squares. Or is that overly simplified?
>Why are planets orbits elliptical and not circular?
Planetary orbits are elliptical because of the nature of gravity. Sir Isaac Newton discovered that the force of gravity between two objects depends on the *square* of the distance separating them.
What does that mean? To put it simply, let’s say that a planet and its sun are at a certain distance X from each other, and the force of gravity has been calculated to be Y. The precise numbers don’t matter, only the relationship between them.
If the planet is suddenly moved to be twice as far (the distance is now 2X), then we will find that the force of gravity drops to one-quarter (the force is now Y/4). We increased the distance by a factor of two, and so the force falls by a factor of 4, because 2^2 = 4.
Because of this changing force of gravity, you can use a branch of mathematics called *calculus* (something else Newton developed) to chart out the orbital path of anything you like, and you’ll find that most orbits are indeed elliptical.
Obligatory pedantry note: Circles are ellipses, too, just with an eccentricity of 0.
I feel like the simplest explanation is that a circle is just a special form of ellipse, with a very specific requirement that the two foci of the ellipse be at the same point. For that to occur takes some extremely specific circumstances, even excluding the fact that space includes significantly more than just the planet and orbited body in question. External forces, even if small, force some eccentricity into the orbit. Orbiting bodies pull on each other and cause them to ‘wobble’ slightly. The sun pulls on the earth, as the earth pulls on the sun. But our moon is also pulling on the earth, while being pulled on itself. The effect is substantially smaller, but does exist, and causes that orbital foci to very slightly wobble as well.
All elliptical orbits are circular, but not all circular. There is only a single single circular orbit compared to an infinite number of elliptical orbits. Even if orbits tended to circular over time due to loss of energy or angular momentum, we’d expect that the gravitational pull of the other planets to tweak the orbit out of perfect circularity–once again only one orbit is perfectly circular. That said, most planets have a circular orbit (even if strictly speaking they’re elliptic)–the Earth’s orbital eccentricity is only about 0.017. Check out this [link](https://qph.cf2.quoracdn.net/main-qimg-3b7c569b3087517a63bdb797d0cee441-c) to see a view of the Earth’s and Pluto’s orbits. Pluto has the highest eccentricity of any planet 0.248 and it still looks pretty circular)–NOTE: I am not an IAU member and still consider Pluto to be a planet.
Where the planet is moving the fastest (perihelion) the gravity is at its strongest, and where it is moving slowest the gravity is weakest.
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