Right up until the 20th century, ships still depended on the sun, moon and stars to navigate the seas. Sailors used the earliest navigational instruments – the astrolabe, the cross-staff and the back-staff – to determine how far north or south they were from their point of departure. All instruments worked with the same principle: they measured the angle of the sun (or star) against the horizon.
As seafaring became increasingly important, sailors needed more precise methods to determine their position. They developed new instruments, such as the octant and sextant. The new instruments worked according to the same principle, but gave much more precise readings than their predecessors.