Past exam of the mathematics course of the University of Cambridge / 2024 / iii / Paper 316 / 1 / ii / Solution

Resolving $\mathbf{r}=a(\cos f{\mathbf{e}}_1+\sin f{\mathbf{e}}_2)$ into the sky coordinates givesIn particular, $z=0$ at the orbital nodes, and $dz/df=a\sin I>0$ at $f=0$, so that node is indeed the ascending node.