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 he one hour slow of Greenwich time. Similarly 30° cor respond to two hours. etc. (See INTERNATIONAL DATE-LINE.) 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 are ascer tained before leaving port. Anywhere at sea he can find out his local time at any moment by observing 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 15. is then the longitude in degrees. Longitudes on land are determined by 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 eomparison of the standard Greenwich clock with the clock or chronometer at the observing 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 longitude are so superior in precision to all others that they are prac tically the only one now in use. See NAVIGA TION.
When applied to a heavenly body. the tennis latitude and longitude have the same relations to the celestial equator and its poles, and to the point on the ecliptic called the equinox (q.v.), that terrestrial latitude and longitude have to the equator and a first meridian. The corre sponding coordinates of a heavenly body rela tively to the celestial equator are called its declination (q.v.) and right ascension (q.v.).