The upper meridian passing through the north and south celestial poles, and the observer’s zenith. This view is looking from the east towards the west for an observer in the Northern Hemisphere. The lower meridian is that part of the great circle below the horizon.

The upper meridian is the semi-circle above the observer’s horizon, and the lower meridian is the semi-circle below the horizon.

A meridian transit occurs when an object moves across the meridian. Meridian transits are used for timekeeping, with local noon is defined as the time at which the Sun crosses the upper meridian from east to west. An apparent solar day is the length of time between two successive upper meridian transits by the Sun.