Celestial navigation: Difference between revisions

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[[Image:Marine sextant.svg|thumb|right|300px|A [[sextant]]]]

'''Celestial navigation''', also known as '''astronavigation''', is the ancient art and science of position fixing that enables a navigator to transition through a space without having to rely on estimated calculations, or [[dead reckoning]], to know their position. Celestial [[navigation]] uses "sights," or angular measurements taken between a celestial body (the sun, the moon, a planet or a star) and the visible horizon. The sun is most commonly used, but navigators can also use the moon, a planet or one of 57 [[navigational stars]] whose coordinates are tabulated in the [[Nautical almanac|Nauticalnautical Almanacalmanac]] and Airair Almanacsalmanacs.

Celestial navigation is the use of angular measurements (sights) between celestial bodies and the visible horizon to locate one's position on the globe, on land as well as at sea. At a given time, any celestial body is located directly over one point on the Earth's surface. The latitude and longitude of that point is known as the celestial body’s geographic position (GP), the ___location of which can be determined from tables in the Nautical or Air Almanac for that year.

The measured angle between the celestial body and the visible horizon is directly related to the distance between the celestial body's GP and the observer's position. After some computations, referred to as [[sight reduction]], this measurement is used to plot a [[Position line|line of position (LOP)]] on a [[Nautical chart|navigational chart]] or plotting work sheet, the observer's position being somewhere on that line. (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 the same angle above the horizon.) Sights on two celestial bodies give two such lines on the chart, intersecting at the observer's position (actually, the two circles would result in two points of intersection arising from sights on two stars described above, but one can be discarded since it will be far from the estimated position—see the figure at "[[Celestial_navigation#Example|example"]] below). Most navigators will use sights of three to five stars, if they're available, since that will result in only one common intersection and minimize the chance for error. That premise is the basis for the most commonly used method of celestial navigation, and is referred to as the "Altitude'altitude-Interceptintercept Methodmethod'."

There are several other methods of celestial navigation which will also provide position finding using [[sextant]] observations, such as the "Noonnoon Sight"sight, and the more archaic [[Lunar distance (navigation)|Lunarlunar Distancedistance method]]. [[Joshua Slocum]] used the Lunarlunar Distancedistance method during the first ever recorded single-handed circumnavigation of the world. Unlike the Altitudealtitude-Interceptintercept Methodmethod, the noon sight and lunar distance methods do not require accurate knowledge of time. The altitude-intercept method of celestial navigation requires that the observer know exact Greenwich Mean Time (GMT) at the moment of his observation of the celestial body, to the second—since every four seconds that the time source (commonly a chronometer or in aircraft, an accurate "hack watch") is in error, the position will be off by approximately one nautical mile.

==Example==