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Longitude by chronometer is a method, in navigation, of determining longitude using a marine chronometer, which was developed by John Harrison during the first half of the eighteenth century. It is an astronomical method of calculating the longitude at which a position line, drawn from a sight by sextant of any celestial body, crosses the observer's assumed latitude. In order to calculate the position line, the time of the sight must be known so that the celestial position i.e. the Greenwich Hour Angle (Celestial Longitude - measured in a westerly direction from Greenwich) and Declination (Celestial Latitude - measured north or south of the equational or celestial equator), of the observed celestial body is known. All that can be derived from a single sight is a single position line, which can be achieved at any time during daylight, when both the sea horizon and the sun are visible. To achieve a fix, more than one celestial body and the sea horizon must be visible. This is usually only possible at dawn and dusk. The angle between the sea horizon and the celestial body is measured with a sextant and the time noted. The Sextant reading is known as the 'Sextant Altitude'. This is corrected by use of tables to a 'True Altitude' . The actual declination and hour angle of the celestial body are found from astronomical tables for the time of the measurement and together with the 'True Altitude' are put into a formula with the assumed latitude. This formula calculates the 'True Hour Angle' which is compared to the assumed longitude providing a correction to the assumed longitude. This correction is applied to the assumed position so that a position line can be drawn through the assumed latitude at the corrected longitude at 90° to the azimuth (bearing) on the celestial body. The observer's position is somewhere along the position line, not necessarily at the found longitude at the assumed latitude. If two or more sights or measurements are taken within a few minutes of each other a 'fix' can be obtained and the observer's position determined as the point where the position lines cross. The azimuth (bearing) of the celestial body is also determined by use of astronomical tables and for which the time must also be known. From this it can be seen that a navigator will need to know the time very accurately so that the position of the observed celestial body is known just as accurately. The position of the sun is given in degrees and minutes north or south of the equational or celestial equator and east or west of Greenwich, established by the English as the Prime Meridian. The desperate need for an accurate chronometer was finally met in the mid 18th century when an Englishman, John Harrison, produced a series of chronometers that culminated in his celebrated model H-4 that satisfied the requirements for a ship-board standard time-keeper. Other nations, notably France, proposed its own reference longitudes as a standard, although the world’s navigators have generally come to accept the reference longitudes tabulated by the British. The reference longitude adopted by the British became known as the Prime Meridian and is now accepted by most nations as the starting point for all longitude measurements. The Prime Meridian of zero degrees longitude runs along the meridian passing through the Royal Observatory at Greenwich, England. Longitude is measured east and west from the Prime Meridian. To determine "longitude by chronometer", a navigator requires a chronometer set to the local time at the Prime Meridian. Local time at the Prime Meridian has historically been called Greenwich Mean Time (GMT), but now, due to international sensitivities, has been renamed as Coordinated Universal Time (UTC), and is known colloquially as "zulu time". ==Noon sight for Longitude== Noon on the Prime Meridian occurs at 1200 hours UTC. The Sun moves west from that point at a rate of 15 degrees each hour. Therefore, solar noon at 15 degrees west longitude would take place at exactly 1300 hours UTC. Solar noon at 30 degrees west longitude would take place at 1400 hours UTC. A navigator uses his sextant to track the rise of the Sun in the sky to determine the exact moment that it reaches its highest point in the sky—local apparent noon. The navigator then notes the UTC (on his chronometer) at this local apparent noon. By subtracting ''from the UTC of local apparent noon'', 1200 UTC and multiplying the result by the Sun’s movement of 15 degrees for each hour's difference, a navigator can calculate the number of degrees of longitude the Sun has crossed from the Prime Meridian to his current meridian of longitude. For example, if the navigator reads 1704 hours UTC on his chronometer at his local apparent noon, he can subtract 1200 hours UTC to arrive at 5 hours and 4 minutes of travel time for the Sun at a rate of 15 degrees per hour or one degree in 4 minutes. Multiplication results in a calculation of 75 degrees west longitude plus one additional degree of west longitude to account for the :04 minutes of time past 1700 hours for a total of 76 degrees west longitude. In the time lapse from local apparent noon at the Prime Meridian to the local apparent noon at the navigator's position, the Sun has travelled 76 degrees west. Incidentally, with the same sextant sight values, the UTC of local apparent noon and the Nautical Almanac, the navigator can also determine his latitude thereby achieving a positional fix with a single noon shot of the Sun. The significance of the noon sight of the Sun has made it an integral component of nautical lore. Longitude cannot accurately be measured at noon, when it is very easy to determine the observer's latitude without knowing the exact time. At noon the sun's change of altitude is very slow so determining the exact time that the sun is at its zenith is impossible to measure to the degree of accuracy necessary to give an accurate longitude. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Longitude by chronometer」の詳細全文を読む スポンサード リンク
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