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Homework Help: Keplers Laws

  1. Feb 21, 2008 #1
    [SOLVED] Keplers Laws

    Please identify the length of the semi-major axis, the semi-minor axis, and the focal length of the ellipse. Specify each of these quantities in millions of km (Mkm).

    The semi-major axis (a) is the distance to the center of the ellipse along the major axis. The center of the ellipse is halfway between the two foci (one of which is the Sun).

    1. The distance from the Earth to the Sun is 150 million km (Mkm)
    2. The distance from Mars to the Sun is 230 million km (Mkm)


    To determine the length of the semi-major axis first determine the length of the major axis (using the information above) and divide it in half.

    To determine the focal length, subtract the Earth's distance to the sun from the semi-major axis.

    To determine the length of the semi-minor axis, use the equation below.

    f2 = a2 - b2 or b2 = (a2-f2) 1/2 or b = SQRT (a2 - f2)

    Totally Lost can you please help me.
  2. jcsd
  3. Feb 21, 2008 #2


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    We're not going to do your homework for you. You need to show your work, show us your thoughts, and explain to us exactly why you're stuck.

    - Warren
  4. Feb 21, 2008 #3

    I am confused because I am unsure sure if I am supposed to find the earths major axis. This is what I did:
    By using Kepler's Third law and the mass of the Sun the universal constant G.

    2a = cube root of ((GMT^2)/(4pi^2))
    which gives me 2.99 x 10^-11 m.

    But I'm not sure is this is the correct approach. As I did not take into consideration the distances?

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