MERIDIAN, in geography, a great circle, as PAQD, passing through the poles of the ear th P and o, and anv given place at z. So that the plane of the terrestrial meridian is in the plane of the celestial one.
hence, 1. As the meridian invests the whole earth, there are several places situated under the same meridian. And, 2. As it is noon-tide whenever the centre of the sun is in the meridian of the heavens, and as the meridian of the earth is in the plane of the former, it follows, that it is noon at the same time, in all places situate under the same meridian.. 3. There are as many meridians on the earth as there are points conceived in the equator. In et vet, the meridians always change, as the longitude of the place is varied ; and may be said to be infinite ; each respective place, from east to west, having its respective meridian.
MEamtAs, First, is that from which the rest arc ac counted, reckoning from west to cast. The first meridian is the beginning of longitude.
The of the first meridian is merely arbitrary ; and hence different persons. nations, and ages, have fixed it differently ; w hence some conffision has arisen in geography. The rule among the ancients was, to make it pass through the place thrthest to the west that was known. But the moderns, knowing that there is no such place in the earth as can be esteemed the most westerly, the way of computing the longitudes of places from one fixed point is much laid aside.
But without touch regard to any of these rules, our geo graphers and map-nmkers frequently assuine the meridian of the place where they live, or the capital of their country, for a first meridian ; and thence reckon the longitudes of their places.
The astronomers. in their calculations, usually choose the meridian of the place where their observations are made, for their first meridian ; as Ptolemy, at Alexandria ; Tycho Brahe, at Uranibourg; Uiccioli, at Bologna ; Flamsteed, at the Royal Observator) at Greenwich ; and the French, at the Observatory at Paris.
MERmiAs LINE, an arc, or part of the meridian of the place, terminated each way by the horizon. Or, a meridian line is the intersection of the plane of the meridian of the place with the plane of the horizon, vulgarly called a north and south line, because its direction is from one pole towards the other.
The use of a meridian line in astronomy. geography, dial ing, &c., is very great, and on its exactness all depends; whence infinite pains have been taken by divers astronomers to fix it with the utmost precision. Cassini has distin
guished himself by a meridian line drawn on the pavement of the church of S. Pctronio, at Bologna, the largest and most accurate in the world ; being 1'20 feet in length. In the roof of this church, 1,000 inches above the pavement, is a little hole, through which the sun's rays, when in the meri dian, fhlling upon the line, mark his progress all the year. When finished, M. Cassini, by a public writing, infmned the mathematicians of Europe of a new oracle of Apollo, or the Sun established in a temple, which might be consulted, with entire confidence, as to all difficulties in astronomy.
To draw a meridian line.
On the horizontal plane, from the sante centre, c, draw several arcs of circles, B A, b a, &c., and on the same centre, c, erect a style, or gnomon, perpendicular to the plane A C B, a foot, or half a foot lung. About the 21st of June, between the hours of nine and eleven in the morning, and between one and three in the afternoon, observe the points a, b. &e., A, a, wherein the shadow of the style terminates. Bisect the arcs A B, a b, ill n, d, &c. If then the same right line, D E, bisect all the arcs, A 11, a b, &c., it will be the meri dian line sought.
As it is difficult to determine the extremity of the shadow exactly, it is best to have the style flat at top, and to drill a little hole, noting the lucid spot projected by it on the arcs A B and a b, instead of the extremity of the shadow. Other wise the circles may be made with yellow, instead of black, &c.
If the meridian line be bisected by a right line, o v, drawn perpendicularly through the point c, o v will be the intersec tion of the meridian, and first vertical ; and, consequently, o will show the east point, and v the west.
Lastly,—If a style be erected perpendicularly in any other horizontal plane, and a signal be given when the shadow of the st, he covers the meridian line drawn in another plane, noting the apex, or extremity, of the shadow projected by the style. a line drawn front that point through that wherein the style is raised, will be a meridian line.
MninniAN LINE, on a dial, is a right line arising from the intersection of the meridian of the place with the plane of the dial. This is the line of twelve o'clock, and from hence the division of the hour-line begins.