As here represented, there are formed complete shadows, called umbras; and partial shadows, called penumbras.
When an eclipse of the sun occurs, it appears totally eclipsed to those persons who are within the moon's umbra, and partially eclipsed, to those situated within the penumbra.
When an eclipse of the moon occurs, it appears totally eclipsed, if entirely within the earth's umbra, and partially eclipsed, if partially within it.
The length of the moon's umbra is usually greater than the distance of the moon from the earth.
The moon, therefore, crosses the sun's path twice in each revolution.
If, at new moon, it always crossed exactly in that part where the sun is, there would evidently be an eclipse of the sun; and it would recross in the opposite part and pass through the earth's umbra.
But the moon does not always cross the ecliptic where the sun is, nor uniformly in the same part.
Its crossing-place is different at each succeeding revolution.