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> So if you want to be really pedantic, it's never an ellipse because the Earth is not a point mass. This turns out to be not pedantic but very important if yo
by kens 6y ago
> So if you want to be really pedantic, it's never an ellipse because the Earth is not a point mass.
This turns out to be not pedantic but very important if you're guiding an ICBM. And when landing on the Moon, Apollo had to deal with irregularities in the Moon's gravity due to mass concentrations, called mascons.
(If you're interested in missile guidance, take a look at the book Inventing Accuracy. Among other things, it discusses some of the efforts to map the Earth's gravity field to increase missile accuracy for Trident and Minuteman missiles. I knew a physicist who worked on this.)
- SiVal 6y ago(Ah, yes, pedantry. It has its own gravity. You can't tell me it's not a force. I can't resist its pull.) It wouldn't be an ellipse even if Earth were a point mass. The gravity of the Moon and Sun, the gravitational lumpiness in the sky, has the same effect as the gravitational lumpiness underground. The combined result may be no closer to an ellipse than it is to a parabola.
- exDM69 6y agoBut on low earth orbit, the L2 term of earth's oblateness dominates by an order of magnitude compared to the moon and the sun, and the rest of the planets are negligible. Source is a table in the first chapters of Fundamentals of Astrodynamics.
- mcv 6y ago> "Ah, yes, pedantry. It has its own gravity. You can't tell me it's not a force. I can't resist its pull." It's not a force. From the right frame of mind[0], it's just a straight line. [0] Substituting for frame of reference.
- jhoechtl 6y agoTo extend the pendantry: Don't we have to account that the onserved object isn't a point mass either?
- jacobwilliamroy 6y agoDo we have to use this knowledge to toss nukes at each other? Surely there's something else we could be doing instead?
- Const-me 6y ago> Do we have to use this knowledge to toss nukes at each other? ICBMs were never used to toss nukes at people. Pretty sure their existence saved the world from WW3. > Surely there's something else we could be doing instead? The application most useful in everyway life is GPS.
- quirkot 6y agoICBMs... the black knight and the white knight of the cold war