Page:Astronomy for Everybody.djvu/50

being continued in both directions through space, marks the north and south poles of the celestial sphere.

We know that the earth's equator passes around it at an equal distance from the two poles. In the same way we have an equator on the celestial sphere which passes around it at a distance of ninety degrees from either celestial pole. If it could be painted on the sky we should always see it, by day or night, in one fixed position. We can imagine exactly how it would look. It intersects the horizon in the east and west points, and is in fact the line which the sun seems to mark out in the sky by its diurnal course during the twelve hours that it is above the horizon, in March or September. In our northernmost States, it passes about halfway between the zenith and the south horizon, but passes nearer the zenith the farther south we are.

As we have circles of latitude parallel to the equator passing around the earth both north and south of the equator, so we have on the celestial sphere circles parallel to the celestial equator, and therefore having one or the other of the celestial poles as a centre. As the parallels of latitude on the earth grow smaller and smaller toward the pole, so do these celestial circles grow smaller toward the celestial poles.

We know that longitude on the earth is measured by the position of a meridian passing from the north to the south pole through the place whose position is to be defined. The angle which this meridian makes with that through the Greenwich Observatory is the longitude of the place.