Solution
= Solution
Resolving $\mathbf r=a(\cos f\,\mathbf e_1+\sin f\,\mathbf e_2)$ into the sky coordinates gives
$$
\boxed{x=a(\cos\Omega\cos f-\sin\Omega\cos I\sin f)},
$$
$$
\boxed{y=a(\sin\Omega\cos f+\cos\Omega\cos I\sin f)},
$$
$$
\boxed{z=a\sin I\sin f}.
$$
In particular, $z=0$ at the <orbital nodes>, and $dz/df=a\sin I\gt0$ at $f=0$, so that node is indeed the <ascending node>.