The meridian or noon circle is formed by the two poles of the earth and the zenith
. The meridian is thus at right angles to the equator and perpendicular to the horizon. The meridian forms noon with the highest point of the sun for all places that lie on the same meridian. Each meridian is divided into 360 degrees, which are used for geographical determination.
The reference meridian is the prime meridian, which passes through the London Borough of Greenwich and is used as the reference time. The corresponding time is Greenwich Mean Time (GMT). The meridian is used for GPS coordinates and indicates the longitude.