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Kepler's first law states that the orbit of every planet is an ellipse with the Sun at one of the two foci. Is there anything significant about the location, or any unusual properties about the other focus?


marked as duplicate by Pulsar, BMS, ACuriousMind, Brandon Enright, user10851 Nov 3 '14 at 21:10

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What I know is, putting the sun at one of the two foci is purely for mathematical convenience, because Kepler's laws derive naturally from vector formulations of Newton's laws.

so my answer to you is that no, the second focus has no physical significance, as far as classical mechanics is concerned. maybe general relativity has another answer. but this is as far as I can go.

  • 1
    $\begingroup$ +1. By the way, in general relativity the orbits of planets aren't even ellipses. $\endgroup$ – Prof. Legolasov Nov 3 '14 at 19:12

First of all, there are no two focus in reality, it is just assumed that Sun is at one of the focii. For your understanding, go to this link: http://www.youtube.com/watch?v=XBxDDXG3sTQ. The video just gives the idea. On the paper you have to consider one focus for one ellipse. Fous is nothing but position of the Sun. Based on the position of the Sun your ellipse is formed. It is you to decide which focus to chose.

  • $\begingroup$ The video does not explain anything. It just shows how ellipse is defined and how to construct it on paper. $\endgroup$ – Jan Hudec Nov 3 '14 at 19:25
  • $\begingroup$ Of course there are no two foci in reality. Reality does not care that we consider some points special and call them foci. But there always are two points we would call foci (unless the orbit is perfectly circular in which case there is just a centre). $\endgroup$ – Jan Hudec Nov 3 '14 at 19:26
  • $\begingroup$ But both the focus have different ellipses $\endgroup$ – Sushant23 Nov 3 '14 at 19:31

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