The Conjunction and Comparison of the Moon with Our Sphere

Abstract

The moon is similar to the earth:

• (1) In its shape which necessarily must be that of a sphere in view of its perfectly round appearance and its manner of receiving the light of the sun. For if the surface of the moon were a plane, at one and the same moment, it would be lighted in its entirety and at another, would be absolutely without light, for the parts turned directly towards the sun would not be successively illuminated.

• (2) In that the moon is no less dark in itself than the earth and is adapted to receive and reflect light.

• (3) In its matter which is dense and solid as well as

• (4) Its surface which is irregular like that of the earth, as is evidenced by its many heights and cavities which are revealed by the telescope. We have discussed this aspect of the moon in the preceding Chapters 17 and 18.

• (5) Just as the regions of the land and sea on our earth are distinct from one another, so we see a varied surface on the moon’s disc, some parts of which reflect more light than others. The appearance of the earth would undoubtedly be similar to that of the moon if anyone could look down upon it from the moon or any other body at a comparable distance when it is lighted by the sun. For the surfaces of the watery areas would appear darker because they absorb the light while those of the land would appear brighter because they reflect it.

• (6) Just as, from our position on the earth we at one time see the moon shining with a full orb, at another as a half moon etc., and again being completely removed from our sight when it is hidden from the rays of the sun, so necessarily must the earth appear to us from the moon, provided that there be a distinction in the periods of illumination, for what takes place on the surface of the moon in one month occurs on the earth within 24 hours.

• (7) From this it follows that that which happens in one full day to those dwelling on the earth would befall, in the space of a month, the inhabitants of the moon (if there were any), but with this difference, however, that whoever was on the moon, specifically, in the lower hemisphere, (for if a person were located on the other side of the moon, invisible to us, or on its upper hemisphere, he would never be able to see the earth), he would observe the whole surface of the earth revolve every day in the space of twenty-four hours. We here on earth, however, always see only one half of the moon since it does not turn about on itself, a condition which must necessarily prevail if it were to be observed in its entirety.

• (8) Just as the moon acts as substitute for the absent light of the sun a good deal of the time and by its reflection of this light makes dark nights days for us, so does the earth send out light to the moon by reflecting the rays of the sun, as we have described in the preceding Chapter 17. Furthermore, because the earth is much larger than the moon, the earth’s illumination of the moon is much more effective than that of the moon on earth.

• (9) Just as these bodies mutually light one another, so do they occasionally bring darkness to one another, when, for example, the moon becomes eclipsed, being deprived of light because the earth comes between it and the sun. In the same way the moon from time to time comes between the earth and sun and deprives the former of the sun’s light. Although frequently and for a longer time the moon remains completely immersed in the earth’s shadow, never is the earth in its entirety darkened for as long a period of time by the moon, although it does occasionally cause an eclipse on the earth as one might expect when one body deprives another of the light of the sun.

• (10) The moon is an eternal and inseparable companion of the earth, for the latter always carries the moon with it in its ecliptic. In the same way as a woman accompanies a man, so does the moon accompany the earth and they proceed around the sun in the same annual forward motion. We do not regard the moon’s presence relative to the earth any differently from that of Jupiter’s satellites relative to Jupiter, for just as Jupiter has four satellites or companions, so has the earth one.