I'm looking for a good way to add variable descriptions immediately following equations. In plain text, it might look something like this:
r_p = a(1-e)
where a is semi-major axis
(space) e is eccentricity (perhaps e is neatly aligned with a)
Further, I'm interested in placing this equation and the descriptions inside of a list:
\begin{itemize}
\item Periapse
\begin{itemize}
\item The point of closest approach to the foci
\[r_p = a(1-e)\]
(variable descriptions)
\end{itemize}
\item Apoapse
\begin{...
...
\end{itemize}