LONGITUDE and LATITUDE. Thom terms moan different thine as applied to a point of the earth or a star in the heavens; and we must accordingly distinguish between geographical latitude and longitude and relestial latitude and longitude.
The latitude of a star in the heavens is its angular distance from the ecliptic, measured on a great circle drawn through the star and pole of the ecliptic. It differs from the DECLINATION only in this, that the ecliptic is used instead of the equator. Thu longitude of a star is the angle made by the circle on which latitude is measured with the circle which passes through the pole of the ecliptic and the vernal intersection of the equator and ecliptic. Thus a star on the ecliptic has no latitude, and one which lies directly between a pole of the ecliptic and the venial equinox has no longitude. The use of celestial longitudes and latitudes has in great measure been superseded by those of right asceusious and declinations.
The meaning of the term geographical longitude is the same whether we consider the earth as a sphere or a spheroid. It is the angle con tained between the plane of the meridian of the place, and that of some one meridian which is fixed on as the starting place. Thus we choose the observatory of Greenwich, and the French that of Paris, as being in the first meridian ; and while we express the relative position of the two observatories (in longitude) by saying that Paris is 2'20'24" east of Greenwich, the French describe Greenwich as 2' 20'24' west of Paris.
It is usual to measure terrestrial longitudes in time (ANGLE; Time], the whole circuit of the globe being suppoised described (as in the diurnal motion) in 24 hours. It is also usual to reckon longitudes to 180' east or west, without proceeding farther. Thus a motion in longi tude of 185° east will bring the traveller into 175' of west longitude. In astronomical writings, however, longitudes (both geographical and celestial) are measured all round the globe.
Supposing the earth to be a sphere,' the latitude of a place is the angle subtended at the centre by the arc of the Mt:nu:sue intercepted between the place and the equator. This angle is equal to the altitude of the polo of the heavens at the place ; and the determination of the altitude of the pole is the method usually resorted to for determining the latitude. But the earth not being precisely a sphere, but a spheroid
[Geonesv], the zenith line (which is a perpendicular to the tangent plane) does not pass exactly through the centre, and the altitude of the polo IA not precisely the angle subtended at the centre by the arc of the meridian. Still, however, the altitude of the pole is called the latitude of the place; and it must be distinctly understood that a lati tude, astronomically determined, is the angle made by a line which is vertical at the place with its projection on the equator. The angle subtended at the centre of the earth by the arc of the meridian is less than the altitude of the pole by a number of seconds, equal to e sin twice the latitude, sin 1" where e is the Kuarrscrre. Assuming this at the above is such a proportion of 114' as the sine of twice the latitude is of unity.
The reason why the preceding is not of more importance in the con struction of maps lies in this, that when a large portion of the earth is mapped, the scale is necessarily too small to make such en error of any consequence; and when a small portion of the earth is taken, the error is nearly the same in every part of the map, and relative positions are not sensibly affected.
The method of finding longitudes and latitudes is given in the next article. The history of this problem, or rather of that of finding the longitude in particular, divides itself into two portions : the first, or the account of the real progress of the problem, is so mixed up with the history of astronomy and horology, that it would be useless to attempt it within any limits which we could afford; the second is that of the speculators who have misunderstood the problem, and is not worth the recital. Since, however, there are still persons who imagine that some mysterious method is yet attainable, by which the longitude is to be found, and since the conductors of the newspaper press are not all sufficiently aware of the state of the problem to prevent the insertion from time to time of paragraphs which create a most erroneous, impression, we shall briefly point out the source of the fallacy which has misled so many persons.