meridians

In geography, a meridian is any line on the Earth's surface that runs from the North Pole to the South Pole. Meridians are lines of longitude; they are all great circles that pass through both poles and converge at the poles.

The prime meridian, designated 0 degrees longitude, runs through the Royal Observatory, Greenwich, in London. Longitudes are measured east or west from this line up to 180 degrees. The 180th meridian roughly corresponds to the International Date Line. Greenwich was adopted as the international standard at the 1884 International Meridian Conference; other historical prime meridians exist in various places.

In astronomy, the local meridian is the great circle passing through the observer's zenith and the celestial poles. It is used to define a star's transit and to time astronomical observations.

In traditional Chinese medicine, meridians refer to energy pathways along which qi, or vital energy, is believed to flow. There are primary and various secondary and extraordinary meridians used in acupuncture and related practices. The scientific status of meridians as physical structures is debated, but they remain a central concept in traditional practice.