A north-south reference line It may be taken through the position of the instrument, or, in special cases, through a reference point (such as the Royal Observatory in Greenwich, England, which designates the Prime Meridian - 0o longitude)
great circle on the sky that goes through the celestial poles and the zenith point It separates the daytime motions of the Sun into ``a m '' and ``p m '' The azimuth of an object on the meridian in the northern sky = 0° and the azimuth of an object on the meridian in the southern sky = 180°
A line on the celestial sphere passing from the North Pole through the Zenith to the South Pole Can also be the center line of a planetary body drawn through it's poles
a imaginary line on the earth's surface passing through both geographic poles and through any given point on the planet, also called a line of longitude
A line of longitude on a spherical grid reference system Meridians also form great circles passing through the North and South Poles Meridian is often used as a synonym for "line of longitude" thus, 100th meridian, or Greenwich Meridian (See central meridian and Greenwich Meridian)
An imaginary line that passes directly north and south through an observer or specified location on Earth A plane extended from this line into space passes through the zenith (point above the observer)
A surveying term that establishes a line of reference The bearing is used to designate direction The bearing of a line is the acute horizontal angle between the meridian and the line Azimuths are angles measured clockwise from any meridian
The meridian is the imaginary line in the sky dividing east from west The meridian runs from north to south through a point directly overhead An object is at its highest point in the sky when is crosses the meridian (with the exception of objects near the celestial pole which can also cross the meridian at their lowest point when passing under Polaris, the north star)
{i} circle circumscribing the earth that passes through the poles; longitudinal half of the earth (from one pole to the other); meridian line through any celestial being; noon, midday; climax, climactic period
A great circle passing through the poles along which all points have the same longitude The prime meridian also passes through the Royal Observatory located in Greenwich, England
Great circle on the surface of the Earth passing through the geographical poles and any given point on the Earth's surface All points on a given meridian have the same longitude
an imaginary great circle on the surface of the earth passing through the north and south poles at right angles to the equator; "all points on the same meridian have the same longitude"