What is the other focus in an elliptical orbit?

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Discussion Overview

The discussion revolves around the nature of elliptical orbits in the context of planetary motion, specifically addressing the concept of the second focus in an ellipse and whether it has any physical significance. Participants explore the implications of Kepler's laws and the dimensionality of orbits.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that while the Sun occupies one focus of the elliptical orbits of planets, the nature of the second focus is unclear and questions what, if anything, exists there.
  • Another participant references an article on Kepler's laws, suggesting there may be a generalization to three-dimensional motion, but questions its necessity.
  • A different participant expresses understanding that there is no physical object at the second focus, indicating a conceptual clarification on the matter.
  • One participant argues that while orbits are described as elliptical, they are actually approximately elliptical and subject to perturbations from other planets, particularly Jupiter, which affects the stability of these orbits over time.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the significance of the second focus in elliptical orbits. There are competing views regarding the nature of planetary orbits, with some suggesting they are strictly elliptical and others proposing that they are only approximately so due to perturbations.

Contextual Notes

There are unresolved questions regarding the necessity of considering three-dimensional generalizations of orbital motion and the implications of gravitational forces on the stability of orbits.

ShadowDatsas
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Hello.
I've read in many books that the orbits of the planets of our Solar system are ellipses.
With the one Focus as the Sun.
Well, what is the other one? I mean every ellipse has two focuses...
 
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Yay! It's good enough to understand the half!
Thanks, now I see that there isn't something at the second focus. (like a mass or a something)
Thanks.
 
Hippasos said:
I guess there is generalization to 3D as well somewhere...

Or is that really necessary?

It isn't. Gravity is a central force. The velocity vector can always be represented as a sum of two vectors, one parallel to and the other normal to this displacement between the two bodies. The force between the two bodies is directed against the displacement vector between the two bodies. The bodies will always on the plane defined by the initial displacement and velocity vectors.

Planets do not really follow elliptical orbits. The orbits are instead approximately elliptical and the planes slowly (very slowly) wobble. The planets (particularly Jupiter) perturb the orbits of the other planets.
 

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