= Kepler's first law
{c}
{title2=$r=a(1-e^2)/(1+e\cos f)$}
The bound relative <Kepler orbit> is an <ellipse> with the attracting centre at one <focus of a conic>. In <polar coordinates>, its separation is the displayed function of <true anomaly> $f$, the angle measured from <periapsis>. For $a=1$ and $e=0.6$, it becomes $r=0.64/(1+0.6\cos f)$, giving $0.4$ at $f=0$ and $1.6$ at $f=\pi$. Setting <orbital eccentricity> to zero gives a <circular orbit>. This specifies the path; <Kepler's second law> specifies how it is traversed.
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