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Celestial navigation, also known as astronavigation, is a method of fixing your position based on the stars. It has developed over over several thousand years to help sailors cross featureless oceans without having to rely on estimated calculations(dead reckoning). Celestial navigation uses angular measurements(sights) taken between a visible celestial body (Sun, Moon, or a star) and the visible horizon. The angle measured between the Sun and the visible horizon is most commonly used.
The measured angle between the celestial body and the visible horizon is directly, geometrically related to the distance between the celestial body’s geometrical position(GP), and the observers position. After some computations(sight reduction) this measurement is used to plot a line of position(LOP) on a navigational chart. The LOP is actually a short segment of a very large circle on the Earth which surrounds the GP of the observed celestial body. An observer located anywhere on the circumference of this circle on the Earth, measuring the angle of the same celestial body above the horizon at that instant of time, would observe that body to be at exactly the same angle above the horizon. This is called the altitude-intercept method.
Practical celestial navigation requires a marine chronometer to measure time, a sextant to measure the angles, an almanac giving schedules of the coordinates of celestial objects, a set of sight reduction tables to help perform the height and azimuth computations, and a chart of the region. With sight reduction tables, the only math required are addition and subtraction. Small computers, laptops, and scientific calculators enable modern navigators to “reduce” sextant sights in minute. Modern practical navigators usually use celestial navigation in combination with satellite navigation to correct dead reckoning.
The best use for celestial navigation is in the event of equipment or electrical failure. Using it allows a navigator to get to a port by simply taking sun lines a few times a day and advancing them by dead reckoning to get a crude running fix.
We have written many articles about celestial navigation for Universe Today. Here’s an article about azimuth, and here’s an article about octans.
We’ve also recorded an entire episode of Astronomy Cast all about the Coordinate Systems. Listen here, Episode 170: Coordinate Systems.