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Book II, Chapter 7-8. Of the Eclipses of the Sun and the Moon and the Magnitude of the Stars
For it is evident that the sun is hid by the intervention of the moon, and the moon by the opposition of the earth, and that these changes are mutual, the moon, by her interposition, taking the rays of the sun from the earth, and the earth from the moon. As she advances darkness is suddenly produced, and again the sun is obscured by her shade; for night is nothing more than the shade of the earth. The figure of this shade is like that of a pyramid or an inverted top; and the moon enters it only near its point, and it does not exceed the height of the moon, for there is no other star which is obscured in the same manner, while a figure of this kind always terminates in a point. The flight of birds, when very lofty, shows that shadows do not extend beyond a certain distance; their limit appears to be the termination of the air and the commencement of the æther. Above the moon everything is pure and full of an eternal light. The stars are visible to us in the night, in the same way that other luminous bodies are seen in the dark. It is from these causes that the moon is eclipsed during the night. The two kinds of eclipses are not, however, at the stated monthly periods, on account of the obliquity of the zodiac, and the irregularly wandering course of the moon, as stated above; besides that the motions of these stars do not always occur exactly at the same points.
This kind of reasoning carries the human mind to the heavens, and by contemplating the world as it were from thence, it discloses to us the magnitude of the three greatest bodies in nature. For the sun could not be entirely concealed from the earth, by the intervention of the moon, if the earth were greater than the moon. And the vast size of the third body, the sun, is manifest from that of the other two, so that it is not necessary to scrutinize its size, by arguing from its visible appearance, or from any conjectures of the mind; it must be immense, because the shadows of rows of trees, extending for any number of miles, are disposed in right lines, as if the sun were in the middle of space. Also, because, at the equinox, he is vertical to all the inhabitants of the southern districts at the same time; also, because the shadows of all the people who live on this side of the tropic fall, at noon, towards the north, and, at sunrise, point to the west. But this could not be the case unless the sun were much greater than the earth; nor, unless it much exceeded Mount Ida in breadth, could he be seen when he rises, passing considerably beyond it to the right and to the left, especially, considering that it is separated by so great an interval.
The eclipse of the moon affords an undoubted argument of the sun’s magnitude, as it also does of the small size of the earth. For there are shadows of three figures, and it is evident, that if the body which produces the shadow be equal to the light, then it will be thrown off in the form of a pillar, and have no termination. If the body be greater than the light, the shadow will be in the form of an inverted cone the bottom being the narrowest part, and being, at the same time, of an infinite length. If the body be less than the light, then we shall have the figure of a pyramid, terminating in a point. Now of this last kind is the shadow which produces the eclipse of the moon, and this is so manifest that there can be no doubt remaining, that the earth is exceeded in magnitude by the sun, a circumstance which is indeed indicated by the silent declaration of nature herself. For why does he recede from us at the winter half of the year? That by the darkness of the nights the earth may be refreshed, which otherwise would be burned up, as indeed it is in certain parts; so great is his size.
To prove your understanding of this lesson, draw a diagram that illustrates the causes of a solar eclipse and another to illustrate a lunar eclipse.