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I need to find the width/x-radius of an orbit, which is ellipse shaped, knowing variables like the orbit's length(perihelion + aphelion in this case) and the eccentricity. I am not sure if this is even possible, I am pretty new to this. If there is already available data on this that you're aware of, it would solve my problem too.

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    $\begingroup$ By width, do you mean semi-major axis? $\endgroup$ Mar 25 at 1:44
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To find the width (semi-major axis, see Wikipedia) of an orbit, you only need two of the following: perihelion, aphelion, and eccentricity, represented by the variables $P, A, $ and $e$, respectively, and $a$ is the semi-major axis.

If you are given perihelion and aphelion, you can just take the average of the two: $a=\dfrac{A+P}{2}$

If you are given perihelion and eccentricity, you can calculate $a=\dfrac{P}{1-e}$. Similarly, given aphelion, the width of your orbit is $a=\dfrac{A}{1+e}$.

If you are talking about the other kind of width (Semi-minor axis), represented by $b$, then the formulas are:

  • Given $P \text{ and } A: b = \sqrt{A \cdot P}$
  • Given $P \text{ and } e: b = \sqrt{-\dfrac{P(1+e)}{e-1} \cdot P}$
  • Given $A \text{ and } e: b = \sqrt{-\dfrac{A(1-e)}{e+1} \cdot A}$

I hope this helps.

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