LATITUDE (Lat. latitude, breadth, from lotus, °Lat. stlatus, broad) AND LONGITUDE (Lat. longitude, length, from lengus, long; con nected with Goth. laggs, 011G., Ger. lung, AS., Eng. long, Skt. OPers. dranga, °Church Slay. dhigii, Lith. ilgas, long). Geographical terms used in specifying the position of places on the earth's surface. Longitude is the angle at the pole between two great circles drawn on the earth's surface. passing through the poIes, and touching respectively the place whose longitude is in question and the place selected as the origin of longitudes. Latitude is the angular distance of a place north or south of the equator. The geographic latitude is determined as follows: In the figure, let S be any assumed point on the sur face of the earth; OPQP is the section of the earth through the meridian of the place S; 0 Q is the plane of the equator: I' P is the polar axis; and C is the centre of the earth. If T T' is the tangent to the merid ian at S and S C' is perpendicular to T T' at S. then the angle S C' Q is the latitude of the place S. This differs from the true or geocentric latitude, which is the angle S C Q, and the difference is 11' 30" at the latitude of 45°. Thegeoccntric latitude is used in navigation only in the correction of sights for lunar distances by the old methods. Latitude is reckoned from the equator to the poles, a place on the equator having latitude 0°, and the poles 90° N. and 90° S. respectively. Longitude is hest measured along the equator from the prime meridian; but as nature has not, as in the case of latitude, supplied us with a fixed starting-point, each nation has chosen its own prime meridian; thus, in the United States. in Great Britain and her colonies. in Germany, 1101 land. and other States, longitude is reckoned from the meridian which passes through Greenwich; in France, from that through Paris. etc.: and in many old charts, as 1 as in German atlases down to a recent date, from Ferro (one of the Canary Isles), the meridian of which (17° 40' W. from Greenwich) is the conventional dividing line between the Eastern and Western hemi spheres. or from the iMadeira Isles. It is reck oned east and west from 0° to 180°, though astronomers reckon from 0° west to 300° west, and never use east longitude. It will easily be seen that if the latitude and longitude of a place be given, its exact position is known, for the lati tude confines its position to a circle called a parallel of latitude passing round the earth at a uniform fixed distance front the equator, and the longitude shows what point of this circle is intersected by the meridian of the place, the place being at the intersection.
The measurement both of latitude and longi tude depends upon astronomical observation. The principle on which the more usual methods of finding the latitude depend will he understood from the following considerations: To an observer at the earth's equator, the celestial poles are in the horizon, and the highest point of the equator is in the zenith. If now lie travel northward over one degree of the meridian, the north celes tial pole will appear one degree above the hori zon, while the highest point of the equator will decline one degree southward; 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 highest point of the equator (which is the same thing as the complement of its altitude). The altitude of the pole is found most directly by observing the greatest and least altitudes of the polar star (see PotEs), or of any circumpolar star (q.v.), and (correction being made for refraction) tak ing half the sum. The method most usual with navigators and travelers is to observe the me ridian altitude of a star whose declination or dis tance from the equator i- 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 eodatitude. Other methods of finding the latitude at sea re quire more or less trigonometrical calculation. For very precise latitude determinations astron omers and geodesists employ an instrument called a 'zenith telescope,' with which the dif ference of meridional zenith distance can be measured micrometrically for certain pairs of stars. Front this difference the latitude can be computed. if the declinations of the stars are known. See NAVIGATION.