The longitude of any place on the earth is i:s distance from the first meridian, measured on the equator ; or it is the arch of the equator intercepted between the first me ridian and the meridian of the place. It is called cast or west longitude, according as the place lies to the east or west of the first meridian. Longitude on the earth cor responds, not to longitude, but to right ascension, on the celestial sphere. As the longitude of places on the earth is reckoned in two directions, it never can exceed 180°, whereas the right ascension oPa heavenly body may be any thing between 0 and 360°, being always reckoned in one direction only, viz. in the order of the signs.
The difference of longitude between two places, is the dis tance between them measured on the equator ; or it is the arch of the equator intercepted between the meridians of the places. When the places are on different sides of the first meridian, the sum of their longitudes gives their dif ference of longitude.
As the sun in his apparent diurnal revolution round the earth moves over in 24 hours, or 15° in one hour, he is on the meridian of any given place, or it is noon at that place one hour sooner that at any other place I5° far ther west. Hence longitude, and difference of longitude, may be expressed in time, allowing to one hour, or 1° to four minutes. As the degree and hour are similarly subdivided, any number of degrees, minutes, and seconds, divided by 15. will give the corresponding number of hours, minutes, and seconds ; and, on the contrary, hours, minutes, and seconds of time multiplied by 15, give the correspond ing number of degrees, minutes, and seconds.
The latitude of a place on the earth is its distance from the equator measured on a meridian, or it is the arch of a meridian intercepted between the equator, and a parallel of latitude passing through the place. It is called north or south latitude, according as the place lies to the north or south of the equator. The distance of the place from the nearest pole measured on the meridian is called the co-latitude of the place, or the complement of the latitude. Latitude on the earth corresponds to declination on the celestial sphere.
The difference of latitude between two places is the dis tance between them measured on a meridian ; or it is the arch of a meridian intercepted between the parallels of latitude passing through the places. If they lie on oppo site sides of the equator, tlre":,um of their latitudes gives their difference of latitude.
4 zone is any portion of the earth's surface included be tween two parallels of latitude. There are, however, usually reckoned live zones ; the Torrid, the two Tempe rate, and the two Frigid.
The torrid zone is the space included between the two tropics: the temperate zones extend from the tropics to the polar circles ; and the frigid from the polar circles to the poles. To explain this division of the globe mathema tically, we may observe, that whatever be the position of the sun, he always illuminates one half of the terrestrial sphere at once, and that the great circle which separates the light from the dark hemisphere, and which is called the circle of illumination, has for its pole that point on the sur face of the earth to which the sun is vertical. Now, as
the sun appears to be always in some point or other of the ecliptic, moving over EL, (Plate CCLXV. Fig. 1.), between the middle of winter and the middle of summer, and from L to E on the opposite side of the sphere, in the next half year, the phenomenon is precisely the same as ii he vibrat ed among the arch DL, viz. from D to L during the first of these periods, and from L to D during the next ; the earth, in the mean time, revolving daily on its axis. When the sun is at Q, he is vertical to q ; that is, the circle of illu mination corresponds with a meridian p 8; and every point of the earth's surface is successively 12 hours above and 12 hours below that circle. When the sun advances 1° towards L, he becomes vertical to a point 1° to the north of q, or the circle of illumination falls 1° below p towards pi, and rises 1° above s towards r ; that is, while the sun's declination is 1° north, the circular space about the north pole to the distance of 1° from it, never sinks below the cir cle of illumination, aad a similar space around the south pole never rises above it. As the sun approaches L. a greater space about P continues to be permanently within the circle of illumination, and a corresponding space about 8 to be permanently without it. When the sun reaches L, or is vertical to 1, the circle of illumination falls 23° 28' below P, and rises as far above s, and may be represented by ht g; that is, on the day of the summer solstice, the whole of the north frigid zone is within the circle of illumi nation, and the whole of the south frigid zone is without it for 24 hours. From this period the sun again returns towards the south, when the circle of illumination also begins to move backwards, till at the autumnal equinox, three months after the summer solstice, it again occupies the position p 8, so that for six months the north pole is above, and the south pole is helow the circle of illumination. The sun still continuing his motion southward, reaches D by the middle of winter, when the circle of illumination oc cupies the position b f, and the circumstances of the fri gid zones are just reversed. Hence, to every place within the torrid zone, the sun is vertical twice a year, which it never is to any other part of the earth. In the temperate zones, no place is either above or below the circle of illumination for 24 hours together. And in the frigid zones, a place may be in the dark or light hemisphere for any pe riod, between 24 hours and six months. The zones might also be distinguished from each other physically, by the difference in the intensity of the sun's rays, as indicated in the difference of mean temperature in different latitudes. But for the investigation of this subject, we refer to Pity SICAL Gcogralihy.