The imaginary line is called the Tropic of Cancer because when the Sun reaches the zenith at this latitude, it is entering the astrological sign of Cancer (summer solstice in the northern hemisphere). When it was named, the Sun was also in the direction of the constellation Cancer (Latin for crab).
The prime meridian is the line of 0° longitude, the starting point for measuring distance both east and west around Earth. The prime meridian is arbitrary, meaning it could be chosen to be anywhere.
Null Island is an imaginary island located at 0°N 0°E (hence “Null”) in the South Atlantic Ocean. This point is where the Equator meets the Prime Meridian.
The transit circle was built by the 7th Astronomer Royal, Sir George Biddell Airy, in 1850. The cross-hairs in the eyepiece of the Transit Circle precisely defined Longitude 0° for the world.
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