How did sailors find their latitude by looking at the sky?

Sailors found their latitude by measuring how high a star or the Sun stood above the horizon. In the northern hemisphere the star of choice was Polaris, the North Star. It lies nearly in line with Earth's rotation axis above the North Pole, so it stands almost motionless while the other northern stars appear to circle around it. Its height above the horizon therefore gives the approximate latitude of the observer. Measure that angle, and you know how far north you are.

The tools were simple. One of the earliest was the kamal, used by Arab navigators from the late 9th century and in the Indian Ocean from the 10th. It was a small wooden card on a string with knots tied along it. The sailor held one end of the string in his teeth and moved the card along it until its lower edge sat on the horizon and its upper edge hid the star. The knot he reached told him the angle, and the angle told him his latitude. The kamal is described as the earliest known tool for sailing along a line of latitude.

There was a catch: Polaris is not visible south of the equator. Sailors therefore began taking the height of the Sun instead, as Portuguese and Spanish navigators did on their long voyages with the mariner's astrolabe, a graduated circle used to measure the sun's noon altitude.

Latitude, though, is only half of a position. East and west are a different matter, because celestial navigation can only give longitude when the time at the prime meridian is accurately known. Without a clock that could be trusted, sailors had to estimate how far they had come east or west from their speed, heading and time elapsed, a method whose errors grew with every mile.