Hour angle

Definition

Fix an observer on the Earth and a direction $u$ on the celestial sphere, not at a celestial pole. The local meridian is the half great circle from the north celestial pole through the zenith to the south celestial pole. The hour circle of $u$ is the half great circle from the north celestial pole through $u$ to the south celestial pole. The hour angle $H$ of $u$ is the angle along the celestial equator from the local meridian to the hour circle of $u$, measured westward. It is given in hours, with $1^{\mathrm h} = 15^\circ$, either in $[0^{\mathrm h}, 24^{\mathrm h})$ or in $(-12^{\mathrm h}, 12^{\mathrm h}]$.