Dynamic elliptical orbits for the two-body Kepler problem:

In terms of the eccentric anomaly, the parametrization of the orbit in Cartesian coordinates is

\[ x(u) = a (\cos u - e) \hspace{4em} y(u) = a \sqrt{1-e^2} \sin u \]with semimajor axis \( a \) and semiminor axis \( b = a \sqrt{1-e^2} \). The ellipse is offset along the negative *x*-axis by \( c = a e \), which is just the distance to a focus of the ellipse. The statements are confirmed by evaluating

which is the expression for a shifted ellipse. The amount of the shift means the larger body is always at a focus (using reduced mass). The major axis of the ellipse remains a constant \( 2a \) as the eccentricity changes.

The orbiting body always stays in the invariant *xy*-plane, with the point of closest approach on the positive *x*-axis. Coordinate axes are included on the larger body simply to help orient the orbit in three-dimensional space.

Cartesian coordinates are not only simpler here but less tricky to employ. The radial variable can be written simply as a function of eccentric anomaly, but cannot be visualized as a polar plot without also including the dependence of the physical polar angle \( \phi \) on eccentric anomaly as well.

The eccentric anomaly has the same domain as the physical polar angle, but neither tranverse their domains evenly as a function of time. To get the actual physical behavior of the orbit, one must determine the eccentric anomaly in terms of constantly flowing time from the Kepler equation

\[ \omega t = u - e \sin u \]This equation cannot be solved explicitly in terms of known functions, and is generally solved numerically. That is why this example requires the higher-level mathematics of Math. The frequency is set equal to one to reduce an already involved calculation.

As a final note on the code, the path of the orbiting body is constructed using a JavaScript template literal in order to included variables easily. It could of course be constructed by adding together regular strings.

Complete code for this example: