The latitude--that is, the distance of any spot from the equator and the pole--might be found by a simple observation with the sextant.

The altitude of the sun at noon is found, and by a short calculation the position of the ship can be ascertained.
The sextant, which is the instrument universally used at sea, was gradually evolved from similar instruments used from the earliest times.

The object of this instrument has always been to find the angular distance between two bodies--that is to say, the angle contained by two straight lines, drawn from those bodies to meet in the observer's eye.

The simplest instrument of this kind may be well represented by a pair of compasses.

If the hinge is held to the eye, one leg pointed to the distant horizon, and the other leg pointed to the sun, the position of the two legs will show the angular distance of the sun from the horizon at the moment of observation.
Until the end of the seventeenth century, the instrument used was of this simple kind.