Page:Astronomy for Everybody.djvu/43

We all know that a line running through any place on the earth in a north and south direction, is called the meridian of that particular place. More exactly, a meridian of the earth's surface is a semicircle passing from the north to the south pole. Such semicircles pass in every direction from the north pole, and one may be drawn so as to pass through any place. The meridian of the Royal Observatory at Greenwich is now adopted by most nations, our own included, as the one from which longitudes are measured, and by which in the United States and a considerable part of Europe the clocks are set.

Corresponding to the terrestrial meridian of a place is a celestial meridian which passes from the north celestial pole through the zenith, intersects the horizon at its south point, and continues to the south pole. As the earth revolves on its axis it carries the celestial as well as the terrestrial meridian with it, so that the former, in the course of a day sweeps over the whole celestial sphere. The appearance to us is that every point of the celestial sphere crosses the meridian in the course of a day.

Noon is the moment at which the sun passes the meridian. Before the introduction of railways, people used