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Homework Help: Orbit of a comet

  1. Nov 22, 2006 #1
    If I have the eccentricity and the perihelion of an orbit given, how can I compute the aphelion?
    Last edited: Nov 22, 2006
  2. jcsd
  3. Nov 22, 2006 #2
    You need to think of what angles correspond the the perihelion and aphelion. Then use the relation between r, e, theta and l to get the aphelion.
  4. Nov 22, 2006 #3
    Can I not just use the relation for an ellipse:

    [tex]e = \frac{d}{a}[/tex]

    where d is the distance from the focal point to the center and a is the semi major axis?
  5. Nov 22, 2006 #4
    Well yes if you know a and d. You'd get the same result. You might need J as well though.
  6. Nov 22, 2006 #5
    I just used the radial equation

    [tex]r_a = \frac{a(1 - e^2)}{1 + e \cos \pi} = a(1+e)[/tex]

    This leads to

    [tex] r_a = \frac{r_p(1+e)}{1-e}[/tex]

    where I have used that

    [tex]a = \frac{r_a + r_p}{2}[/tex]

    But for [tex]r_p = 0,2301 AU[/tex] I get [tex]r_a = 2988 AU[/tex] and this is wrong. I should get [tex]r_a =4699 AU[/tex]. Am I too tired or what is this?
    I have that [tex]e = 0,999846[/tex]
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