The only visual reference for determining one's position anywhere in the world is the sky. The Sun during the day and the stars at night appear to move on an immense sphere resting on the circle of the horizon. The heavenly bodies appear to rotate from east to west. Some bodies always remain visible, others rise and set, and all—in the course of time—change position relative to the horizon. These apparent movements are due to the Earth's rotation in the opposite direction, from west to east, while the varying positions of the bodies are due to the Earth's revolution around the Sun. The position of heavenly bodies is measured by altazimuth coordinates, which define the altitude above the horizon and the angular distance, called azimuth, from the direction of geographic north. There are two categories of stars. The first are stars that rise and set. The second are those that remain visible all night long: they are called circumpolar. The category to which a star belongs depends on the latitude of the observation point. At the Equator, all stars rise and set, whereas, at the poles, all stars are circumpolar. In the northern hemisphere, there is only a single star that remains practically motionless: it is the North Star, which lies at less than one degree from the Celestial North Pole and thus serves as the main reference for finding one's position both at sea and on land.