Multiple lines of position can be intersected to obtain a position known as a celestial fix. The first is to obtain a line of position by use of a sextant observation and the techniques of celestial navigation. These stars are typically used in two ways by the navigator. This list provides information on the name, approximate position in the celestial sphere, and apparent magnitude of the 58 selected stars in tabular form and by star charts. Information for another 115 stars, known as "tabulated stars", is also available to the navigator. Criteria in the choice of stars includes their distribution across the celestial sphere, brightness, and ease of identification.
The selection of the stars is made by Her Majesty's Nautical Almanac Office and the US Naval Observatory, in the production of the yearly Nautical Almanac which the two organizations have published jointly since 1958. Of these, 58 stars are known in the field of navigational astronomy as "selected stars", including 19 stars of the first magnitude, 38 stars of the second magnitude, and Polaris. Under optimal conditions, approximately 6,000 stars are visible to the naked eye of an observer on Earth. Selected navigation stars (except Polaris) listed on a U.S. In practical use for sight reductions whilst at sea, tables can further assist a navigator by giving approximate altitudes (angles above the horizon) and azimuths (degrees as read from the compass) from an assumed or estimated position, usually helping to quickly determine the location and then quickly identify a particular navigational star that may be useful for a sight reduction. In addition to tables, star charts provide an aid to the navigator in identifying the navigational stars, showing constellations, relative positions, and brightness. A second group of 115 "tabulated stars" can also be used for celestial navigation, but are often less familiar to the navigator and require extra calculations.Īlthough Polaris can quickly and simply give a solution for latitude in the northern hemisphere, it can not participate in giving a position fix including longitude - it is for this reason it is excluded from the list of 57 primary navigational stars, each of which can be used to produce (in conjunction with each other, known time in relation to the prime meridian and a set of sight reduction tables) an actual latitudinal and longitudinal positional fix.įor purposes of identification, the positions of navigational stars - expressed as declination and sidereal hour angle - are often rounded to the nearest degree. The other 57 selected stars have daily positions given in nautical almanacs, aiding the navigator in efficiently performing observations on them.
When navigating in the Northern Hemisphere, a simple and quick technique can be used with Polaris to determine the observers latitude or, for larger maritime vessels can be used to calculate any gyrocompass error that may exist. The star Polaris, often called either the "Pole Star" or the "North Star", is treated specially due to its proximity to the north celestial pole. Many of the selected stars were named in antiquity by the Babylonians, Greeks, Romans, and Arabs. Of the approximately 6,000 stars visible to the naked eye under optimal conditions, these selected stars are among the brightest and span 38 constellations of the celestial sphere from the declination of −70° to +89°. The selected stars for navigation are often used for sextant observations.įifty-seven navigational stars and additionally the star Polaris are given a special status in the field of celestial navigation.
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