Kepler's second law of motion

  • Planets sweeps out equal areas in equal times.



    Is the area being calculated here the area of a triangle? You draw a straight line from the center of the sun to the earth at point A. The Earth moves to Point B and you have another straight line from point A to point B and the finally another straight line from point B to the Sun? Even though the Earth makes an arc, I guess we make it a straight line.



    I guess my question is whether I have understood the second law correctly? That the area of a triangle was being calculated in an elliptical orbit, no matter how elliptical the orbit is.


    No, it's not a triangle - it's the pie shape. Kepler thought of that :)

  • userLTK

    userLTK Correct answer

    6 years ago

    Hohmannfan's answer is correct, but as I understand your question, you do understand the general idea correctly and the answer to your question is yes, no matter how eccentric the orbit, the planet spans equal areas over equal time, at least nearly perfectly. (more on that later)


    You're mistake is in calling the sectors "triangles". They're sectors of an ellipse from a focal point, not from the center. I'm not sure what the mathematical term is for those shapes, but calling them triangles, while convenient, is incorrect, calling them sectors of an ellipse is problematic too because a sector without clarification implies it's initiated from the center point, not a focal point. So, near as I could find, "Sector from a focal point of an ellipse" is correct, but long winded. "Kepler area" would probably do the trick. Wikipedia just puts up a picture and calls them "shaded areas" or "shaded sectors".


    Kepler Ellipse


    As for deviations from Kepler's law. Mercury is close enough to the sun that relativity caused an observed deviation from it's predicted elliptical orbit. Late 19th and early 20th century astronomers tried to explain that by adding a planet (Vulcan), but they couldn't find that theoretical planet. Einstein's laws later explained Mercury's observed deviation.


    Also, planets pull on each other and that's been known at least since Newton's time, as he did a fair bit of work on it but never solved it to his satisfaction. Earth, for example, speeds up if Venus and Jupiter are ahead of it, and slows down when they are behind it. These are called orbital perturbations and are simple in principal, quite complicated mathematically. An unexplained deviation in Uranus' elliptical orbit lead to the discovery of the planet Neptune. That's not a flaw in Kepler's law. His law is so exact that when a deviation to an elliptical orbit is observed, we know there's another gravitational body out there and we even know about where to look.


    the technical mathematical term is surely "pie slice shapes" !

    @JoeBlow I know your joking, but pie, assuming you find an ellipsoid one, are usually sliced from the center, not the focal point.

    You're kidding man, I always slice from the focus! :) But yes that's an excellent point.

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Content dated before 7/24/2021 11:53 AM