Why do lines of longitude run only 180 degree half circles around the earth while lines of latitude run in full 360 degree circles?

Answer 1

The numbering systems for longitude and latitude are simply different expressions of a circle. Without these intersecting lines, there would be no way to precisely position any location on the globe.

Longitude measures distance east and west of the Prime Meridian -- 0 degrees, the line that essentially 'separates the Earth into two halves', except that its continuation -- the International Date Line -- 180 degrees, isn't straight, for political reasons.

Latitude measures distance north and south of the Equator -- 0 degrees, which indeed 'separates the Earth into two halves'.

Combining longitude and latitude, you can, indeed, find any location on our planet.

Answer 2

Lines of longitude converge at the poles, which causes them to form half circles that meet at the North and South Poles. In contrast, lines of latitude are parallel to the Equator and circle the Earth fully because the circumference of the Earth remains the same as you move north or south from the Equator.