In this appendix to my previous post “From Dancing Planet to Kepler’s Laws“, we derive the polar form for an ellipse that has a rectangular coordinate system’s origin as one of its foci.
Fig. 1
We start with the ellipse shown in Fig. 1. Namely,
Clearly,
After shifting the origin to the right by , the ellipse has the new origin as one of its foci (Fig. 2).
Fig. 2
Since , the ellipse in is
Substituting in (2) by
yields equation
Replacing by respectively, the equation becomes
Fig. 3
Solving (3) for (see Fig. 3) gives
or .
The first solution
.
Let
,
we have
.
The second solution is not valid since it suggests that :
.