Meridian
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Geography
Any semicircle on Earth that connects the North and South Poles.
The so-called prime meridian runs through the English town of Greenwich (London) and is the basis for measuring longitude.
Astronomy
The circle on the Celestial Sphere that runs through both celestial poles and also, from any particular location, through the zenith and nadir. The celestial poles are the points of intersection between the Earth's extended axis and the celestial sphere.
A planet reaches its highest point when crossing the celestial meridian - once again from the location of an observer. This is called culmination.