Binet equation
= Binet equation
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{wiki}
Binet’s equation rewrites a central-force orbit using $u(\theta)=1/r$. If $\ell=mr^2\dot\theta$ is angular momentum and the potential is $V(r)$, then
$$
u''+u=\frac{m}{\ell^2u^2}\frac{dV}{dr}\bigg|_{r=1/u}.
$$