MERIDIAN, in astronomy, a great cir cle passing through the poles of the world, and both the zenith and nadir, crosses the equinoctial at right angles, and divides the sphere into two hemis pheres, the eastern and western ; it has its poles in the east and west points of the horizon. It is called meridian, because, when the sun cometh to the south part of this circle, it is then midday; and then the sun has his greatest altitude for that day. These meridians are various, and change according to the longitudes of places ; so that they may be said to be in finite in number, for all places from east to west have their several meridians : but there is (or should be) one fixed, which is called the first meridian. Ptolemy chose to make that the first meridian which passes near the Fortunate islands, at about the distance of one degree from them; and reckons from thence to the east through Africa and Asia; eitoosi ng to begin at a place inhabited, and which was then the boundsand limits of the known part of the earth to the west, and to the end at the eastern shore of Scain in Asia ; but America being discovered not many ages ago, and long after Ptolemy's time, the first meridian was removed more to the west. Some made that the first meridian which passes through the isle of St. Ni cholas, which is one of those near Cape Verd; and Hondius chose the isle of St. James to be the first in his map.
Otherschose that which passes through the isle del Corvo, one of the Azores, because the needle was found not to de cline from the north there and in the ad jacent seas, but to lie in the meridian line; and this beginning Mercator chooses. But seeing there are other places where the needle points to the north, and it doth not so in every part of that meridian, geographers thought this not a sufficient reason ; some fixing it at the shore of Brasil, that runs out into the sea. Later geographers choose tobegin at the moun tain Teneriffe, in the Fortunate or Cana ry islands, which is counted one of the highest on the earth; tad the rather, be. cause.they thought some remarkable place should be chosen, that might be most known to future ages; and so Pto lemy's first meridian, though long observ ed, was not laid aside without good rea son. The French, since the year 1634,
have taken that which goes through the west part of the isle of Faro, one of the Canaries. Astronomers also have takendi vcrs places for the first meridian ; the fol lowers of Tycho fix it at UL.,.linirg, an isl and in the Danish streights, and calculate the celestial motions to that place, and from thence accommodate them to the rest. Others choose other places, accord ing to the authors of the ephemeris they use, who calculate the ephemeris, and the planets' places for the meridian of their own place ; as Riccioli, who fixed his first meridian at Bologna ; Mr. Flam steed, at the Royal Observatory at Green wich ; and the French, at the Observato ry at Paris. See Osszavassiar. But without regard to any of these rules, our geographers and map-makers frequently assume the meridian of the place, or the capital of the country, for the first meri dian ; and thence reckon the longitudes of their places.
In the PhilosophicalTransactions, there is a suggestion that the meridians vary in time. This seems very probable, from the old meridian line in the church of St. Petronio at Bologna, which is found to vary no less than eight degrees from the true meridian of that place at this time ; and from that of Tycho Brahe at Urani burg, which M. Picart observes varies eighteen minutes from the modern meri dian. If there be any thing of truth in this hint, Dr. Wallis says, the change must arise from a change of the terrestrial poles (here on earth, of the earth's diurnal ma tion), not of their pointing to this or that of the fixed stars ; for if the poles of the diurnal motion remain fixed to the same place on the earth, the meridians which pass throtigh these poles must be the same. But this notion of the changes of the meridian seems overthrown by an observation of M. Chazelles, of the French academy of sciences, who when in Egypt, found that the four sides of a pyramid, built 3000 years ago, still look ed very exactly to the four cardinal points; a position which could never be looked on as fortuitous.