Corpus of Electronic Texts Edition
An Irish Astronomical Tract (Author: [unknown])

Chapter 22

Concerning the cause of the eclipse of the moon:


Having proved by forcible arguments and geometrical figures that it is from the sun that the moon and all the stars receive light, we shall now show whence comes the natural darkness upon the moon which is called an eclipse and I declare, approaching that subject, since the moon receives its light from the sun, and there is nothing else to deprive it of that light except the earth, it is the shadow of the earth, which is exactly between the sun and the moon, which envelopes the moon and deprives it of the sun's light; and that obscuration of the moon by the shadow of the earth is an eclipse. Now this would occur without fail, if every time the moon meets the head or tail of the Dragon, it were in a straight line, right opposite to the sun and the earth; it then would penetrate the shadow of the earth, and be totally obscured89. When the


moon advances to the south or north of that shadow, it avoids the total eclipse, and on whatever side it meets that shadow, the portion of it which is beneath that shadow is eclipsed.

Therefore, there are two kinds of eclipse, i.e., eclipsis universalis, i.e., a general eclipse, and eclipsis particularis, i.e., a partial eclipse. Wherefore it does not begin at the same time in every place, and is not of equal size in every place, since it is not of the same size in the east as in the west, and neither is it the same in the south of the world as in the north. On that account it the eclipse does not appear the same to people in the countries of the world, and they on whom day rises do not behold it, though it is present naturally at that time. Afterwards, I will speak of the eclipse of the sun in its own place.

The eclipse of the other planets: One of the planets themselves is the cause of the eclipse of the others, for the lower planet obscures the higher. In the same way, the moon obscures them all, each at its own time. To explain better what I have said of eclipses, I will make a geometrical figure here below, from which the, eclipse will be clearly understood.