Latitude and Longitude. In Geography, denote the angular distances of a place on the earth from the equator and first meridian respectively. The latitude of a place is the angle subtended at the centre of the earth by the arc of the meridian from the equator to the place in question. The longitude of a place is the angle at the earth's axis between the plane of the first meridian and that of the meridian of the place. Latitude is reckoned from the equator to the poles, the equator having 0° lat., and the poles 90° N. and 90° S. respectively. Longitude is reckoned along the equator or along a parallel of latitude from the first meridian; but as nature has not in this case supplied us with a fixed starting-point, it is necessary to fix upon one in an arbitrary manner. Cardinal Richelieu in the 17th century proposed to use the meridian of Ferro, one of the Canary Isles, for this purpose, as this meridian lay to the west of all the Old World and to the east of America. The Arab geographers had also reckoned longitude from the 'Fortunate Isles.' For convenience the meridian of Ferro was subsequently reckoned as exactly 20° W. of Paris, and thus lost its independent character. The meridian of Greenwich came into widest use, being universal as the zero of longitude in sea-charts and in the land maps made in the United Kingdom and the United States. Large scale maps of the United States are usually marked with longitudes west from Greenwich and also the number of degrees from Washington. One set of engraved meridians serves for this purpose, as Washington lies 77° W. of Greenwich. By the decision of a conference of delegates from almost all the civilised countries in the world, held at Washington in 1884, the meridian of Greenwich was accepted as the universal prime meridian, from which longitudes were measured to + 180° (or 180° E.) and - 180° (180° W.); the French delegate dissented, and in France maps are still drawn to the prime meridian of Paris, although reference marks to Greenwich longitude are now usually added. On German maps the meridian of Berlin was sometimes employed, in Italian maps that of Rome, and in Russian maps that of Pulkova Observatory (St Petersburg) is still commonly used together with that of Ferro.
The determination of both latitude and longitude depends upon astronomical observation. The principle on which the more usual methods of finding the latitude depend will be understood from the following considerations: To an observer at the earth's equator the celestial poles are in the horizon, and the meridian point of the equator is in the zenith. If now he travel northwards over one degree of the meridian the north celestial pole will appear one degree above the horizon, while the meridian point of the equator will decline one degree southwards; and so on, until, when he reached the terrestrial pole, the pole of the heavens would be in the zenith, and the equator in the horizon. The same thing is true with regard to the southern hemisphere. It thus appears that to determine the latitude of a place we have only to find the altitude of the pole, or the zenith distance of the meridian point of the equator (the complement of its altitude). The method most usual with navigators and travellers is, by means of a sextant, to observe the meridian altitude of a star whose declination or distance from the equator is known; or of the sun, whose declination at the time may be found from the Nautical Almanac; the sum or difference (according to the direction of the declination) of the altitude and declination gives the meridian altitude of the equator, which is the co-latitude—i.e. when subtracted from 90° leaves the latitude.
The determination of the longitude is less easy, and long presented insuperable practical difficulties. All methods depend on measuring the difference between local time and the time of the first meridian, which, reduced to degrees (at the rate of 360° per day, or 15° for every hour, or 1° for 4 minutes), gives the longitude. Eclipses of the sun, moon, or Jupiter's satellites, occultations of fixed stars by the moon, the time occupied in the moon's transit over the meridian, &c. are occurrences the exact period of which are calculated in advance in Greenwich time. When one of these phenomena is observed the true Greenwich time can at once be obtained from the Nautical Almanac, and the local time from direct observation is the only other datum required. The longitude of stations on land connected by telegraph with an observatory is most readily and accurately determined by an exchange of time signals; the exact position of every observatory is always ascertained to a high degree of accuracy by repeated observations of celestial phenomena. The two methods in use among travellers and on board ship are remarkable for their combination of simplicity with accuracy. The first and most common consists merely in determining at what hour on the chronometer (which is set to Greenwich time) the sun crosses the meridian. If, when the sun is on the meridian, at the place of observation, the chronometer points to 3 hours 52 minutes, the difference of longitude is 58°, and the longitude will be W., as the sun has arrived over the place later than at Greenwich; similarly, if the sun be over the meridian of a place at 9 hours 40 minutes A.M., the longitude is 35° E. (by the chronometer). The accuracy of this method depends evidently upon the correctness of time-keepers (see HOROLOGY). The other method—that of 'lunar distances'—is much used at sea in order to check the results of chronometer measurements, and may be thus explained: The angular distance of the moon from certain fixed stars is calculated with great accuracy (about three years in advance) for every three hours of Greenwich time, and published in the Nautical Almanac. The moon's distance from some one star having been observed, and corrected for refraction and parallax, and the local time having also been noted, the difference between this local time and that time in the table which corresponds to the same distance gives the longitude. When applied to a heavenly body, the terms latitude and longitude have the same relations to the ecliptic and its poles, and to the point on the ecliptic called the Equinoctial (q.v.), that terrestrial latitude and longitude have to the equator and a first meridian. The positions of a heavenly body relatively to the equator are called its Declination (q.v.) and Right Ascension (q.v.). See also DEGREE.