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°
the meridian of a place on the Earth surface is the half circle passing through that place and ending at the poles
A line of longitude going north and south from pole to pole measured in degrees from the Prime Meridian or 0° See Longitude
An imaginary line drawn in the sky which joins the zenith[?] with the north and south celestial poles[?] See also: celestial equator, ecliptic
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
Being at, or pertaining to, midday; belonging to, or passing through, the highest point attained by the sun in his diurnal course
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
An imaginary line that begins on the northern point of the horizon, travels up to the observer's zenith, down to the southern point on the horizon