4 ms·

> it's never an ellipse because the Earth is not a point mass. At least classically, a sphere is indistinguishable (gravitationally) from a point mass while y

> it's never an ellipse because the Earth is not a point mass. At least classically, a sphere is indistinguishable (gravitationally) from a point mass while you're outside it. The earth is pretty sphere-ish, locally speaking.

libraryofbabel 6y ago
> The earth is pretty sphere-ish, locally speaking. Kinda. There are mountains, they have their own gravitational attraction, and it can be measured... even in the 18th century! https://en.m.wikipedia.org/wiki/Schiehallion_experiment https://en.m.wikipedia.org/wiki/Schiehallion_experiment It all depends on how pedantic one wants to be :)
- perl4ever 6y ago
  The first time I heard of the concept, it wasn't actually that, it was how in the 60s, they had to correct for mascons in the moon while orbiting.
  - garmaine 6y ago
    There are very few stable orbits close to the lunar surface. Basically a couple of polar orbits with very specific parameters, and that's it. The rest get so perturbed by gravitational anomalies that they fall out of orbit after a few months or years--faster than low Earth orbit where there is atmospheric drag! https://www.lpi.usra.edu/lunar/documents/NTRS/collection2/NASA_TP_3394.pdf https://www.lpi.usra.edu/lunar/documents/NTRS/collection2/NA...
  - Sharlin 6y ago
    Yeah, the moon's gravitational field is quite lumpy compared to Earth's, due to its smaller size and the way it is believed to have formed. Plus you can orbit much closer to the surface due to the lack of atmosphere, so the lumps are steeper.
Tyr42 6y ago
And a ball thrown in the air follows a parabola, locally speaking. At least, you're better off correcting for air resistance before you sub in the ellipse equation.
Koshkin 6y ago
Locally speaking, the earth is flat.
- dllthomas 6y ago
  But if you're speaking globally, you've already defined yourself out of calling the earth flat.
CydeWeys 6y ago
Only if the sphere has uniform density. The Earth very much does not have uniform density.

libraryofbabel 6y ago

> The earth is pretty sphere-ish, locally speaking. Kinda. There are mountains, they have their own gravitational attraction, and it can be measured... even in the 18th century! https://en.m.wikipedia.org/wiki/Schiehallion_experiment https://en.m.wikipedia.org/wiki/Schiehallion_experiment It all depends on how pedantic one wants to be :)

perl4ever 6y ago
The first time I heard of the concept, it wasn't actually that, it was how in the 60s, they had to correct for mascons in the moon while orbiting.
- garmaine 6y ago
  There are very few stable orbits close to the lunar surface. Basically a couple of polar orbits with very specific parameters, and that's it. The rest get so perturbed by gravitational anomalies that they fall out of orbit after a few months or years--faster than low Earth orbit where there is atmospheric drag! https://www.lpi.usra.edu/lunar/documents/NTRS/collection2/NASA_TP_3394.pdf https://www.lpi.usra.edu/lunar/documents/NTRS/collection2/NA...
- Sharlin 6y ago
  Yeah, the moon's gravitational field is quite lumpy compared to Earth's, due to its smaller size and the way it is believed to have formed. Plus you can orbit much closer to the surface due to the lack of atmosphere, so the lumps are steeper.

Tyr42 6y ago

And a ball thrown in the air follows a parabola, locally speaking. At least, you're better off correcting for air resistance before you sub in the ellipse equation.

Koshkin 6y ago

Locally speaking, the earth is flat.

dllthomas 6y ago
But if you're speaking globally, you've already defined yourself out of calling the earth flat.

CydeWeys 6y ago

Only if the sphere has uniform density. The Earth very much does not have uniform density.