Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current position on Earth, in the air, or at sea. Celestial navigation uses "sights," or timed angular measurements, taken typically between a celestial body (e.g., the sun, the moon, a planet, or a star) and the visible horizon. Celestial navigation can also take advantage of measurements between celestial bodies without reference to the Earths horizon, such as when the Moon and other selected bodies are used in the practice called "lunars" or the lunar distance method, used for determining precise time when time is unknown.
Mariners relied on the Sun and stars to tell time and determine their place on the featureless ocean. To find latitude, they came to use angle-measuring instruments, which became increasingly accurate toward the end of the 1400s. On a starry night in the northern hemisphere, a mariner could figure out latitude fairly easily by measuring the angle between the North Star and the horizon. On fair days, the Sun provided the most reliable readings.
Celestial navigation continues to be used by private yachtsmen, and particularly by long-distance cruising yachts around the world. For small cruising boat crews, celestial navigation is generally considered an essential skill when venturing beyond visual range of land. Although satellite navigation technology is reliable, offshore yachtsmen use celestial navigation as either a primary or backup means of navigation because it is not vulnerable to satellite destruction, cyber attack, electromagnetic pulse, or system failure.
