A great circle is an imaginary circle on the surface of a sphere whose center is at the center of the sphere. The equator is a great circle. Great circles that pass through both the north and south poles are called meridians, or lines of longitude. For any point on the surface of Earth a meridian can be defined. The prime meridian, the starting point measuring the east-west locations of other meridians, marks the site of the old Royal Observatory in Greenwich, England. Longitude is expressed in degrees, minutes, and seconds of arc from 0 to 180 degrees eastward or westward from the prime meridian. For example, the GBT is located at 79 degrees, 50 minutes, 24 seconds of arc westward of the prime meridian: 79deg. 50' 24" W.
The starting point for measuring north-south locations on Earth is the equator (the equator is the imaginary circle around the Earth which is everywhere equidistant from the poles). Circles in parallel planes to that of the equator define north-south measurements called parallels, or lines of latitude. Latitude is also expressed in degrees, minutes, and seconds of the arc subtended from the center of the Earth. The GBT is located at 38 degrees, 25 minutes, 59 seconds of arc north of the equator: 38deg. 25' 59" N.
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