Overview
European geographers divided the globe into a grid, and numbered the lines. The horizontal lines were the lines of latitude, and the vertical one the lines of longitude. This made it easier for navigators to identify their position at sea.
To find their latitude and longitude, they measured angles between the horizon and stars and planets in the sky. The octant, an instrument invented by John Hadley in 1731, measured angles of up to 90 degrees. The sextant, a later development, measured angles of up to 120 degrees.