Zenith distance ($z$)

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 $0\le z\le 180^{\circ }$. 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.