APOD: Comet Meets Moon and Morning Star (2015 Dec 12)

[Ron-Astro Pharmacist]

I am not trying to dispute the well-proven shape of elliptical orbits but what makes the opposite end of the ellipse so narrow? I can see how the suns's gravity causes it on the solar end but (on the other end) you'd think it would be a more gradual effect - causing it to be closer to egg shaped as gravity diminishes a comet's speed much more slowly.

That's probably covered in orbital mechanics 101 but Kepler's family banned me from understanding ellipses.

[neufer]

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I am not trying to dispute the well-proven shape of elliptical orbits but what makes the opposite end of the ellipse so narrow? I can see how the suns's gravity causes it on the solar end but (on the other end) you'd think it would be a more gradual effect - causing it to be closer to egg shaped as gravity diminishes a comet's speed much more slowly.

The opposite end of the ellipse is narrow to make up for the fact that the comet is moving so slowly at that point.

The apparent symmetry goes away by
including the equal area in equal time segments.

[Nitpicker]

I am not trying to dispute the well-proven shape of elliptical orbits but what makes the opposite end of the ellipse so narrow? I can see how the suns's gravity causes it on the solar end but (on the other end) you'd think it would be a more gradual effect - causing it to be closer to egg shaped as gravity diminishes a comet's speed much more slowly.

That's probably covered in orbital mechanics 101 but Kepler's family banned me from understanding ellipses.

It is what it is because that's what Newton's kinetic model says it is, to a good approximation. And it holds up to experiment fairly well (which is how Kepler came up with his kinematic laws, before Newton's kinetic ones). But bear in mind that the comet travels much more slowly when distant from the Sun, and much faster when closer, so maybe that is the answer you are seeking.

[Ron-Astro Pharmacist]

Thanks for the replies. Equal and opposite forces didn't seem to apply but the first two of Kepler's Laws forced some sense into me.