Zenith distance
= Zenith distance
{title2=$z$}
<Zenith distance> is the <angle> from the <zenith> to a viewing direction: $z=\arccos(\mathbf n\cdot\mathbf e_U)=90^\circ-\gamma$, where $\gamma$ is <altitude> and $0\leq z\leq180^\circ$. The <astronomical horizon> has $z=90^\circ$, while the <zenith> has $z=0$. This unsigned separation supplies no <azimuth> and is distinct from the signed north/south angular readout used under <declination>.