Meridian

Meridian (Lat. meridies, 'mid-day'), the name given to the great circle of the celestial sphere which passes through both poles of the heavens, and also through the zenith and nadir of any place on the earth's surface. Every place on the earth's surface has consequently its own meridian. The meridian is divided by the polar axis into two equal portions, which stretch from pole to pole, one on each side of the earth. It is mid-day at any place on the earth's surface when the centre of the sun comes upon the meridian of that place; at the same instant it is mid-day at all places under the same half of that meridian, and midnight at all places under the opposite half. All places under the same meridian have therefore the same longitude (see LATITUDE AND LONGITUDE, where the question of the First Meridian is discussed). Stars attain their greatest altitude when they come upon the meridian; the same thing is true approximately of the sun and planets; and as at this point the effect of refraction upon these bodies is at a minimum, and their apparent motion is also more uniform, astronomers prefer to make their observations when the body is on the meridian. The instruments used for this purpose are called meridian circles. See MURAL CIRCLE.

Meridian Measurement.—Two stations, having nearly the same longitude, are chosen; their latitude and longitude are accurately determined (the error of a second in latitude introduces a considerable error into the result), and the direction of the meridian to be measured ascertained; then a base line is measured with the greatest accuracy, as an error here generally becomes increased at every subsequent step; and then, by the method known as Triangulation (q.v.), the length of the arc of the meridian contained between the parallels of latitude of the two stations is ascertained. As the previously found latitudes of its two extremities give the number of degrees it contains, the average length of a degree of this arc can be at once determined. This operation of meridian measurement has been performed at different times on a great many arcs lying between 68° N. lat. and 38° S. lat., and the results show a steady though irregular increase in the length of the degree of latitude as the latitude increases. On the supposition that this law of increase holds good to the poles, the length of every tenth degree of latitude in English feet is as follows:

This result shows that the earth is not spherical, as in that case the length of all degrees of latitude would be alike, but of spheroidal form—its curvature becomes less as we go from the extremity of its greater or equatorial diameter to the pole. See EARTH.