meridian

Meridian is a term with several closely related meanings in geography, astronomy, and related fields. In geography, a meridian is a half-circle line running from the North Pole to the South Pole that connects places sharing the same longitude. Meridians form the vertical lines of longitude used to specify position east or west on the globe. The global network of meridians underpins map projections and navigation, with longitudes measured in degrees from a reference meridian.

The best-known meridian is the Prime Meridian, defined as 0 degrees longitude. It was established by convention at the Royal Observatory in Greenwich, England, and was adopted as the international standard in 1884. Longitudes are measured east or west from this line, while the opposite side of the globe centers on the 180th meridian and serves as the basis for the International Date Line, though the line is adjusted to accommodate political boundaries.

In astronomy, the celestial meridian is the great circle on the celestial sphere that passes through the north and south celestial poles and the observer’s zenith. A celestial object crosses the meridian at its transit, reaching its highest point in the sky. The Sun’s crossing of the local meridian marks solar noon, which can vary from clock noon due to the equation of time.

Other uses include historical instruments such as the meridian circle, used in measuring celestial coordinates, and the term’s presence in traditional Chinese medicine, where meridians are described as energy channels—a concept not supported by mainstream science.