What kind of noun is longitude?
The word longitude is a singular, common, abstract noun; a word for an imaginary line to measure the distance east or west of the Greenwich meridian; a word for a thing.
The compound noun 'prime meridian' is a common noun as a general word for an artificial line adopted as a reference line from which longitude east and west is measured. The compound noun 'Prime Meridian' (capitalized) is a proper noun as a word for the meridian passing through Greenwich, England, designated as the zero (0°) longitude.
The noun 'longitude' is a common noun, a general word for the imaginary lines that run between the North and South Poles to measure east-west position of the Earth. A proper noun is the name of a specific person, place, or thing. The noun Earth is a proper noun, the name of a planet; the nouns North Pole and South Pole are proper nouns, the names of specific places.