Home >> Chamber's Encyclopedia, Volume 8 >> Krupps Steel to Lawrence_4 >> Latitude and Longitude

Latitude and Longitude

meridian, equator, time, sun, distance, greenwich, pole, difference, altitude and poles

LATITUDE AND LONGITUDE, in geography, denote the angular distances of a place on the earth from the equator and first meridian respectively; the angular distance in lonoitude being found by supposing a plane to pass through the place, the earth's center, and tire poles, and measuring the angle made by this plane with the plane of the first meridian; the angular distance in latitude being found in the same manner, but substi tuting the two extremities of an equatorial diameter for the poles; or, more simply, lati tude is the angle made by two lines drawn from the earth's center—the one to the place, the other to the equator at the point where it is crossed by the meridian of the place. latitude is reckoned from the equator to the poles, a place on the equator having let. 0°, and the poles 90° n. and 90° s. respectively. Longitude is reckoned along the equator from the first 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 first meridian: thus, in Great Britain and her colonies, in Holland, and other maritime states, longitude is reckoned from the meridian which passes through Greenwich; in France, from that through Paris, etc.; and in many old charts, from Ferro (one of the Canary isles), or from the Madeira isles. It is reckoned e. and w. from 0° to 180°, though astronomers reckon from 0' w. to 360° w., and never use east longitude. It will easily be seen that if the latitude and longitude of a place be given, its exact position can be determined, for the latitude fixes its position to a circle passing round the earth at a uniform fixed distance from the equator (called a parallel of latitude), and the longitude shows what point of this circle is to be intersected by the meridian of the place, the place being at the inter Section.

The determination both of latitude and longitude depends upon astronomical observation. The principle on which the more usual methods of finding the lati tude 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 merid ian, the north celestial pole will appear one degree above the horizon, while the merid ian 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 alti fude of the pole, or the zenith distance of the meridian 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 POLE), or of any circumpolar star, and (correction being made for refraction), taking half the sum. Similarly, half the sum of the greatest and least meridian altitudes of the sun, at the two solstices, corrected for refraction and parallax, gives the altitude of the meridian point of the equator. The method most usual with navigators and travelers is 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. Other methods of finding the latitude require more or less trigonometrical

calculation.

The determination of the longitude is by no means so readily accomplished. Various methods have at different times been proposed, most of which are only fitted for observ. atories. Among these may be classed those which depend upon the determination of the local time of the occurrence of certain celestial phenomena, such as the eclipses of the sun, moon, or Jupiter's satellites, occultations of f xed stars by the moon, the time occupied in the moon's transit over the meridian, etc.; and comparing the observed local time with the calculated time of the occurrence, at some station whose longitude is known (e.g., Greenwich), the difference of time when reduced to degrees, minutes, and seconds, at the rate of 360° to 24 hours, gives the difference of longitude. The two methods in use among travelers and on board ship are remarkable for their combination •of simplicity with accuracy. The first consists merely in determining at what hour on the chronometer (which is set to the time at Greenwich, or some place of known longi tude) the sun crosses the meridian. It is evident that as the sun completes a revolution, or 360°, in 24 hours, he will move over 15° degrees in 1 hour, or 1° in 4 minutes. Now, if the watch be set to Greenwich time—viz., point to 12 o'clock when the sun is on the .meridian of Greenwich, and if at some other place, when the sun is on the meridian .there, the watch 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 and 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 WATcnEs). The other method— that of "lunar distances"—may be briefly explained as follows: The 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 Alma nac. The moon's distance from some one star having been observed, and corrected for Tefraction 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, which may be converted into degrees as before. It may also be mentioned that the lonoitude of all places connected by telegraph with the reckoning-point can be •easily found by from the latter a signal to an observer in the place, at a certain fixed time (reckoned in solar time at the reckoning-point), and by the observer instantly and accurately the local time at which the signal arrived; the difference of the two times, reduced in the way shown above, will give the longitude, the time occupied in the transmission of the signal being so small as to be negleted. 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 equinox (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 arc called its declination (q.v.) and right ascension (q.v.).