Page:The New International Encyclopædia 1st ed. v. 11.djvu/901

* LATITUDE AND LONGITUDE. SI- LATITUDE AND LONGITUDE. LATITUDE (Lat. latitudo, Ijivailtli, from latus, OLat. stiulus. l)roa(l) aM) LONGITUDE (Lat. longitudo, length, from loiiyun, long; ton- nected with Goth, laggs, OHG., Ger. lang, AS., Eng. long, Skt. dirgha, OPers. draiiga, OChurch Slav, dlityii, Lith. ilgas, long). Geographical terms used in specifying the position of places on the earth's surface. Longitude is the angle at the pole iM'tween two great circles drawn on the earth's surface, passing through the [)olcs. 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; O P Q P is the section of the earth through the meridian of the place S; i) is the plane of the equator ; P 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°. The geocentric 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 best 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. Hol- 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 well as in German atlases down to a recent date, from Ferro (one of the Canary Isles), the meridian of which (17° 40' W. from (ircenwich) is the conventional dividing line between the Eastern and Western hemi- spheres, or from the JIadeira Isles. It is reck- oned east and west from 0° to 180°. though astronomers reckon from 0° west to 3t')0° west, and never use cast longitude. It will easily be seen that if the latitude and longituile 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 from the equator, and the longitude shows what point of this circle is intersected by the meridian of the place, the place lieing 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 be 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 he 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 altitxide). The altitude of the pole is found most directly by observing the greatest and least altitudes of the polar star (see Poles), 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 is known : or of the sun, whose declination at the time may be found from the Xauticiil Almannr; 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. 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. From this difference the latitude can be computed, if the declinations of the stars are known. See Xavigatiox. To understand the determination of longitude by observation, it is necessary to remember that differences of longitude correspond to differences of time. Thus, if a place be in longitude 15° west of Greenwich, its local time will be one hour slow of Greenwich time. Similarly .30° cor- respond to two hours, etc. (See I.nter.natioxal Date-Lixe.) To find the longitude in any place, it is thus only necessary to ascertain how much its local time is fast or slow of Green- wich. On shipboard the navigator uses a chro- nometer, the error of which in Greenwich mean time and its daily rate of gain or loss arc ascer- tained before leaving port. Anywhere at sea he can find out his local time at any moment by oliserving the sun with a sextant, and thence de- termining the local time. This local time he com- pares with the Greenwich time shown at the same moment by the chronometer; and the difference in hours, multiplied by l.i. is then the longitude in degrees. Longitudes on land are determined liy astronomers and geodesists on the same prin- ciple, only here the comparison of local with Greenwich time can be made more accurately by direct telegraphic comparison of the standard Greenwich clock with the clock or chronometer at the obser-ing station. If the latter station is very far from Greenwich, its time is usually compared telegraphically not with Greenwich itself, but with some nearer place whose longi- tude has already been determined. The above methods of determininc; longitude are so superior in precision to all others that they are prac- tically the only ones now in use. See Navioa- TTOX. When applied to a heavenly body, the terms latitude and longitude have the same relations