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What are north and south lines that form half a circle between north pole and south pole? ?

What are North-South lines that form half circles between the north and south pole.

Public Comments

1. longitudes
2. do you mean longitude and latitude? Or you referring to the equator? Im a bit confused
3. Longitudes. As opposed to latitudes, longitudes divide the earth in two half parts. That's what they are also called "great circle" and the shortest line between two points on earth. To only way to measure a longitude is to measure the time angle with an arbitrary line of reference. That has been many places in the past but today, by international agreement, it is Greenwich, near London, England. That longitude is called the prime meridian. Note that it was not until John Harrison's invention of the first marine chronometer at the end of the 18th century that longitude could be assessed accurately.
4. lines of longitude