# What are some common ways to add variable descriptions with equations? [closed]

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}


Like this?

% arara: pdflatex

\documentclass{article}
\usepackage{array}
\newcommand{\where}[1]{where~\begin{tabular}[t]{@{}>{$}c<{$}@{ }l@{}}#1\end{tabular}}

\begin{document}
\begin{itemize}
\item Periapse
\begin{itemize}
\item The point of closest approach to the foci
$r_p = a(1-e)$
\where{a & is semi-major axis \\e & is eccentricity}
\end{itemize}
\item Apoapse
\end{itemize}
\end{document}