![]() ![]() At the North Pole, which is 90 degrees latitude, Polaris is directly overhead at an altitude of 90 degrees. Polaris always stays within 1 degree of celestial North Pole. Was measured either at noon (the "noon sight") or from the angle between Polaris, the North Star or the Pole Star, and the horizon. The object's position is then looked up in the Nautical Almanac for that particular time and after allowance for refraction, instrument error and other errors, a position circle on the Earth's surface is calculated. To determine the position of a ship or aircraft by celestial navigation or astronavigation, the navigator uses a sextant to take a "sight" to measure the apparent height of the celestial object above the horizon, and notes the time from a marine chronometer. The Sokkia Corporation's annual "Celestial Observation Handbook and Ephemeris" tabulates daily celestial coordinates (to a tenth of an arc-second) for the Sun and nine stars, Polaris and eight others. The "Air Almanac" of the United States and Great Britain tabulates celestial coordinates at 10 minute intervals. More recent editions have kept up with the changes in technology - the 1924 edition for example had extensive advertisements for coaling stations. of the Hydrographic Department of the Admiralty" provided a useful compendium of information. This combination of trade advertising, and information "by permission. A good example would be Brown's - which commenced in 1877 - and is still produced annually, its early twentieth century subtitle being "Harbour and Dock Guide and Advertiser and Daily Tide Tables". Also commercial almanacs were produced that combined other information. In Great Britain a nautical almanac has been published annually by the HM Nautical Almanac Office, ever since the first edition was published in 1767. The navigator can extrapolate by means of navigational tables to acquire the position of each object for each minute of time. The Sun, Moon and Planets are perceived independently and are therefore specified separately however, only the star Aries is specified, while the other stars are assigned a set angular distance. The Almanac specifies for each whole hour of the year the position on the Earth's surface at which each body is directly overhead. These almanacs describe the positions and movements of celestial bodies, including the sun, moon, planets, and 57 stars chosen for their ease of identification and wide spacing. ![]() This geographic position is known as the celestial object's sub-point and its location, defined by latitude and longitude, can be determined from the tables of nautical or air almanacs. At any given instant of time, any celestial object such as the Earth's Moon, the planet Jupiter, or the navigational star Spica, one of the brightest stars in the nighttime sky, will be located directly over a particular geographic position on the Earth. Astronavigation calculates angles between objects in the sky (celestial objects) and the horizon to locate one's position on the globe. With the rise of the British Navy and merchant marine, the Greenwich Meridian or Prime Meridian at Greenwich, England eventually became the starting location for most celestial almanacs. In ancient times, the vessel's home port or home capital was used as the known location. Although the Sun is the most often measured celestial object, more sophisticated navigators are prepared to use our Moon, other planets, or one of 57 "navigational stars" described in nautical almanacs to observe the positions of these celestial objects relative to the observer and a known location. Astronavigation uses angular measurements or sightings between the horizon and a common celestial object to perform navigational problem-solving. , is a position-fixing process that has enabled sailors to cross featureless oceans with certainty and target unsighted land with precision. ![]()
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