Determining Latitude by the North Star

In this illustration from a 16th century portolan atlas, a seaman is determining latitude using a cross-staff. This device allowed him to measure the "altitude" of Polaris or the North Star. Polaris is directly overhead at the North Pole (90° of latitude); in other words, the angle between Polaris and the horizon at the North Pole is 90°. This angle is called "the altitude" of Polaris. At the Equator (0° of latitude), the North Star is on the horizon, making an angle of 0°.

For any point between the Equator and the North Pole, latitude is obtained simply by measuring the altitude of Polaris: at 30°N the star is 30° above the horizon, at 63°N, it is 63° above the horizon, and so on. Before the use of the cross-staff pictured, mariners and others who needed to determine latitude used flat pieces of wood with holes to sight through to locate Polaris and with pieces of cord attached to measure angles.