Page:Astronomy for Everybody.djvu/158

marked out on the celestial sphere, let us suppose that we also mark out the orbit of the moon during the course of its monthly period. We should then find the orbit of the moon crossing that of the sun in two opposite points, at the very small angle of five degrees. These points of crossing are called nodes. At one node the moon passes from below, or south of the ecliptic, to the north of it. This is called the ascending node. At the other the moon passes from north to south of the ecliptic. This is called the descending node. The terms ascending and descending are applied to the node, because to us in the northern hemisphere, the north side of the ecliptic and equator seem to be above the south side.

At the points halfway between the nodes the centre of the moon is above the ecliptic by about one twelfth its distance from us, that is, by about twenty thousand miles. The sun being larger than the earth, the shadow of the latter gradually grows smaller away from the earth. At the distance of the moon its diameter is about three fourths that of the earth, that is about six thousand miles. Its centre being in the plane of the ecliptic, it extends only about three thousand miles above and below that plane. Hence it is that the moon will pass through it only when near the nodes.

The line joining the sun and moon of course turns round as the earth moves around the sun. It therefore crosses the moon's nodes twice in the course of a year. That is to say if we supose the nodes to be marked in the