Meridian
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The highest point of the great circle which the sun describes from rising to setting, and which highest point it crosses at apparent noon; an imaginary great circle of the sphere which passes through the earth's axis and the observer's zenith; the highest point of anything.
First Meridian. The meridian from which longitude is reckoned, its choice being arbitrary.
Celestial Meridian. A great circle of the celestial sphere passing through the poles.
Prime Meridian. The same as first meridian.
Terrestrial Meridian. Same as meridian line.
