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How can I calculate velocity of planet using orbit time?

  1. Mar 28, 2015 #1
    Hi. I'm just a hobbier of astronomy and have a question about elliptical orbit.
    In Before, I had calculated the elipse of a planet successfully.
    In this case, I wonder that can calculate velocity(red vector) using orbit time(or position on elipse).

    Let me assume that we know orbit's semi-major, minor axis(properties of black elipse), orbit period
    and green dot's location or elapsed time from passing perihelion or aphelion.

    I had thought that just differentiate the orbit elipse by time t but differentiate elipse is impossible operation as I know.
    so can't catch clue of this problem.

    Would anyone give me some hints?
  2. jcsd
  3. Mar 28, 2015 #2

    Simon Bridge

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    HInt: Kepler.
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