celestial navigation

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The sextant, the chronometer (nowadays possibly a quartz watch), the nautical almanac, and navigation tables (now perhaps some form of calculator or computer) in one form or another remain the essential requirements for celestial navigation, by means of which, provided the heavenly bodies themselves are visible, the navigator can find his position anywhere in the world. For centuries, navigation by sextant, or one of its predecessors, was the only position-fixing system in the deep ocean and all the great voyages of exploration by sea have found their way by it. The practice has now largely been replaced by satellite navigation, in its current (2004) form GPS (Global Positioning System) or, less frequently, the Russian Glonass. The reason for the present supremacy of GPS is clear. For the first time in the long history of navigation a system exists that will give the navigator a position within a few metres, by day or by night, in any weather and in any part of the world. That astronomical navigation has not yet become obsolete is largely due to the fact that it remains self-contained, is wholly independent of any national or political authority, and is not subject to system failures.

For all the brilliance of its conception and performance satellite navigation as we know it remains for one reason or another vulnerable. As things stand, for example, GPS, which is provided free by the US Department of Defense, is still classed as a weapon-targeting system and could be withdrawn at any time. So a back-up still remains a requirement and celestial navigation remains at any rate one wholly viable alternative.

The practice of celestial navigation at sea remains in principle very much as it was in the days of Captain Cook. But things have become easier. Time is universally available and the problem of longitude has long been solved. The ephemeris is now presented in the nautical almanac in such a manner that only the most elementary mathematics are required to solve the nautical triangle. Perhaps, above all, the concepts of the position line and the intercept have illuminated and simplified the whole business of astronomical position finding.

The predicted apparent positions of the heavenly bodies used in navigation, namely the sun, moon, planets, Aries, and the selected navigational stars, are tabulated for hourly intervals in the nautical almanac. The coordinates used are Greenwich Hour Angle (GHA) and declination which are adjusted by interpolation tables to the time of observation. The values extracted are used to enter the reduction tables (or calculator as the case may be). Each sextant observation or sight will produce a position line, somewhere along which the observer's position must lie. Two position lines taken at roughly the same time will indicate just where along the first position line the observer lies, and three sights in a series will normally produce a cocked hat which gives a more reliable fix. A single position line can be advanced along the ship's track and crossed with a later observation to give a position. Where the sun is used this is known as a sun-run-sun. The same principle of the transferred position line is of course frequently used in coastal navigation.


Subjects: Maritime History.

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