Celestial navigation is a form of navigation which involves using the known locations of celestial bodies to determine one's own location on Earth. This navigational technique has been used for thousands of years, and only really fell out of vogue in the late 20th century, when more accurate navigational methods began to be developed. Instruction in celestial navigation continues to be offered in some regions of the world, primarily as a point of historical interest.
When people navigate in an area without known landmarks, such as the middle of the ocean, they have no way of knowing where they are simply by looking around them. Celestial navigation resolves this problem, by allowing navigators to take several sightings of celestial bodies, and use information from these sightings to determine where they are relative to these objects, and therefore where they are on Earth.
The Sun and Moon can be used in celestial navigation, along with the planets and some stars. In order for celestial navigation to work, the navigator needs several tools. The first is an extensive chart which provides information about key celestial bodies and their positions in relation to Earth. The next is some sort of instrument to take sightings, such as a sextant. Navigators also need a clock, because sightings are useless without some sort of timeframe, since the angle of a sighting changes as the Earth and that object move through space.
In order to figure out where one is, several sightings of known bodies are taken. Then, the navigator uses basic mathematics to determine his or her position. If, for example, two sightings or “fixes” are taken, the navigator can determine the “line of position” for each object. This line is roughly elliptical in shape, and it shows where on Earth someone would have to stand to see that object at the observed angle. The navigator looks for the places where the lines of position intersect, using the process of elimination to find the intersection point where he or she is standing; if one intersection is in the Pacific Ocean and the other is in the Atlantic, for example, the mariner can usually figure out where the ship is located.
Typically, at least three fixes are taken in celestial navigation, to ensure that the navigator narrows the position down as much as possible. The process is tedious, and it requires patience and good math skills, because a single mistake can be quite catastrophic, especially in dangerous waters. A good navigator can narrow possible positions on Earth down to a pretty narrow space, but thanks to the use of Global Positioning Satellites, it is possible to get even better data in much less time with a computer.