I declare that the moon is the cause of the eclipse of the sun, because its sphere is the lowest of the heavenly spheres, and the sphere of the sun is the fourth sphere above that, and, accordingly, every course it makes is beneath the sun. When it arrives at the head or the tail of the Dragon, in exactly the same degree as the sun, without inclining to the south or to the north, it deprives us of the light of the sun, and that darkness is an eclipse. When, however, it the moon inclines to its right or left side, and does not move exactly in the head or in the tail of the Dragon beneath the sun in the same degree as it, it avoids producing an eclipse. That darkness which is seen on the sun, when there is an eclipse, is the body of the moon. Therefore, it is evident that an eclipse never occurs, except when the moon is exactly beneath the sun in the same degree as it. It always begins to the west side of the sun and finishes in the east. When the moon moves outside that exact degree of the
p.176sun, it sometimes obscures a portion of the sun from us. Therefore, an eclipse of the sun is of two kinds as is an eclipse of the moon, i.e., total and partial129.
The eclipse varies in various lands, for when there is an eclipse, it is not visible to the same extent in every land, for there is one land in which it is visible, and another in which it is not, and one land in which it is more visible, and another in which it is less visible; in this wise If the sun were in the straight line up over our heads and the moon in the same line beneath it, it would necessitate an eclipse for us. If a person were at the same time in the east of the world, looking at the sun, he would imagine he saw it in the west of the world, and if there was another person at the same time in the west of the world beholding the sun, he would imagine he saw it in the east of the world, but neither of them see the eclipse of the sun because the sight of each would pass exactly between the sun and the moon, and, consequently, the moon, or any dark body, even though it were in the straight line beneath the sun would not deprive them of the sun's light, because of their distance from that line. At the location in which the sun is thus obscured, the stars are visible in the daytime, so great is the extent of the darkness.
An eclipse of the sun is of shorter duration than an eclipse of the moon on account of the rapidity with which each passes the other, but not so is an eclipse of the moon, which is caused by the earth. There is nothing interfering with it but the course of the moon above whilst the earth is stationary.
If anyone opposed me in this by saying that the moon is not the cause of the eclipse of the sun, and that if it were as I said, would it not also be fitting for it to be eclipsed by Venus and Mercury when they are in the straight line beneath the sun, I answer him thus: When Venus and Mercury are beneath the sun in the same direction as it, they are (as always) far away (from us) and (hence) in that position, they are much less than the sun in (apparent) size and breadth.130
The same thing often happens to the moon, for it is often beneath the sun in the same direction as it, while it is far from being as broad as it.131
In the same way an eclipse of the sun is not caused by the stars of less magnitude than the moon, which are nearer to it, because, when a small body is placed under a large body near it, the nearer it is to it, the less of it it conceals, and when it is placed far away from the large body, and near the sight which is looking at both, the further it recedes from the large body, and the nearer it approaches the sight, the more does it conceal the large body; so that in this manner a wild apple would conceal the body of the sun from the sight.
To explain this, I will make a figure132 here below in which I will make a large circle, like the orbit of the sun, and will place the sun up at the top of it, and make another circle inside that like the orbit of the moon, and place the moon itself up at the top of it in the straight line under the sun, and I will make the third circle inside that in the form of the earth and place the letter E in the centre of it and assume it to be the point in the earth whence men behold the sun, and put A in the east and B in the west of the sphere of the sun, and place the human sight in the straight line from E and from A and from B towards the sun.
Therefore the people in E see the eclipse of the sun clearly, and the same company that are in A and B see the sun clearly without an eclipse because the moon is too small compared to the sun. It does not conceal the sun from them, and consequently, on account of their distance from the sun their sight passes easily above the moon towards the sun.