Meridian, simply means line of longtitude. Long definition: A (geographic) meridian (or line of longitude) is the half of an imaginary great circle on the Earth's surface, a coordinate line terminated by the North Pole and the South Pole, connecting points of equal longitude, as measured in angular degrees east or west of the Prime Meridian. The term meridian comes from the Latin meridies, meaning "midday"; the subsolar point passes through a given meridian at solar noon, midway between the times of sunrise and sunset on that meridian.[ Methods of determining longitude and Earth rotation Traditional optical instruments for measuring Earth rotation included visual and photographic zenith tubes, circumzenithals, Danjon astrolabes, and transit telescopes, such as the Airy Transit Circle. These instruments measured local sidereal time by the passage of specially designated “clock stars” across a plane of reference established in part by the local direction of gravity. Th...