Merid′ian, from meridies, midday, noon, is the great circle passing through the earth's surface and the celestial sphere, which passes through both poles of the heavens and the zenith and nadir of any place on the earth's surface. Every place, therefore, on the earth's surface has its own meridian. When the center of the sun comes upon the meridian of any place it is midday or noon there. But, as it is midday at all places directly under that meridian, it is midnight at all places directly opposite upon the other side of the globe. All places under the same meridian therefore have the same longitude. Stars are measured as to their distance from the celestial meridian. In making a map some place is arbitrarily chosen, as Greenwich or Washington, from which longitude is computed by measuring the distances in degrees of their meridians from each other. Since the vast development of railways in the United States it has become more and more important to have all watches mark the same time within certain geographical limits. In consequence, certain meridians have been chosen by the railway authorities as standards of time; and all watches between such meridians, one hour of the sun's journey apart, are set alike. When the distance between two such standard meridians has been traversed, timepieces are so reset as take up or strike off an hour.