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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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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