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Homework Help: Halley's comet

  1. Sep 25, 2007 #1
    1. The problem statement, all variables and given/known data
    When I plug in all of the parameters for Halley's comet (from Wikipedia) into Kepler's third law a get a semimajor axis of 38.56 AU when it should be about 17? Can someone else try it and see if I am crazy?


    2. Relevant equations



    3. The attempt at a solution
     
    Last edited: Sep 25, 2007
  2. jcsd
  3. Sep 25, 2007 #2

    Dick

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    What parameters are you trying to plug into what equation?
     
  4. Sep 25, 2007 #3
    mass of halley's comet = negligable
    mass of the sun
    G
    T = 76 years
     
  5. Sep 25, 2007 #4

    Dick

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    I get that 76^2 is pretty close to 17.8^3. Perhaps you are crazy. :) Remember that the earth semimajor axis is 1 AU and it's period is 1 year.
     
  6. Sep 25, 2007 #5
    OK here are the details:

    38.5654 = (T^2/(4 pi^2) * G * (Ms))^(1/3)/(1.4*10^11)

    where T is the period in seconds, Ms = 1.991*10^31 and G = 6.674 * 10^(-11)
    what am I doing wrong?
     
  7. Sep 25, 2007 #6

    dynamicsolo

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    The mass of the Sun is 1.99*10^30 kg... (Your result for a is off by very nearly the cube root of 10.)
     
    Last edited: Sep 25, 2007
  8. Sep 25, 2007 #7

    Dick

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    You beat me! I just figured that out. But, ehrenfest, for solar orbits if you work in AU and years, the constant proportionality k in R^3=k*T^2, is one.
     
  9. Sep 25, 2007 #8

    dynamicsolo

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    My training's largely in astrophysics, so I have the solar mass by heart. I would usually take the proportionality approach myself as well, though...
     
  10. Sep 25, 2007 #9

    Dick

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    Funny, my training is in cosmology, so I know it's like to ten the fifty some proton masses. And fifty plus what I forget. Good job.
     
  11. Sep 25, 2007 #10
    Ahh! 30 minutes of frustration because my short-term memory is not good enough to look at a computer screen and then write down a two-digit number without botching a digit!

    Thanks guys.
     
  12. Sep 25, 2007 #11

    dynamicsolo

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    Close enough... ;-) When I was an undergraduate, cosmology was called "the science where you're happy when your order of magnitude is right to an order of magnitude". Nowadays we speak of "precision" cosmology -- what an age we live in...
     
  13. Sep 25, 2007 #12

    dynamicsolo

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    Everybody makes copying errors (when they're not making *sign* errors), so I know just how you feel...
     
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