# Orbit of a comet

1. Nov 22, 2006

### Logarythmic

If I have the eccentricity and the perihelion of an orbit given, how can I compute the aphelion?

Last edited: Nov 22, 2006
2. Nov 22, 2006

### Tomsk

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.

3. Nov 22, 2006

### Logarythmic

Can I not just use the relation for an ellipse:

$$e = \frac{d}{a}$$

where d is the distance from the focal point to the center and a is the semi major axis?

4. Nov 22, 2006

### Tomsk

Well yes if you know a and d. You'd get the same result. You might need J as well though.

5. Nov 22, 2006

### Logarythmic

I just used the radial equation

$$r_a = \frac{a(1 - e^2)}{1 + e \cos \pi} = a(1+e)$$

$$r_a = \frac{r_p(1+e)}{1-e}$$
$$a = \frac{r_a + r_p}{2}$$
But for $$r_p = 0,2301 AU$$ I get $$r_a = 2988 AU$$ and this is wrong. I should get $$r_a =4699 AU$$. Am I too tired or what is this?
I have that $$e = 0,999846$$