The Discovery of a World in the Moone/Chapter 5

by John Wilkins

Proposition 4.

That the Moone is a solid, compacted, opacous body.

I Shall not need to stand long in the proofe of this proposition, since it is a truth already agreed on by the generall consent of the most and the best Philosophers.

1. It is solid in opposition to fluid, as is the ayre, for how otherwise could it beate backe the light which it receives from the Sunne?

But here it may be questioned, whether or no the Moone bestow her light upon us by the reflection of the Sunne-beames from the superficies of her body, or else by her owne illumination. Some there are who affirme this latter part. So Averroes, Cælius Rhodiginus, Iulius Cæsar, &c. and their reason is because this light is discerned in many places,^[1][2][3] whereas those bodies which give light by reflexion can there onely be perceived where the angle of reflexion is equall to the angle of incidence, and this is onely in one place, as in a looking-glasse those beames which are reflected from it cannot bee perceived in every place where you may see the glasse, but onely there where your eye is placed on the same line whereon the beames are reflected.

But to this I answere, that the argument will not hold of such bodies, whose superficies is full of unequall parts and gibbosities as the Moone is. Wherefore it is as well the more probable as the more common opinion, that her light proceedes from both these causes, from reflexion and illumination; nor doth it herein differ from our earth, since that also hath some light by illumination: for how otherwise would the parts about us in a Sunne-shine day appeare so bright, when as all the rayes of reflexion cannot enter into our eye?

2. It is compact, and not a spungie and porous substance. But this is denied by Diogenes, Vitellio, and Reinoldus,^[4][5][6] and some others, who held the Moone to bee of the same kind of nature as a Pumice-stone, and this, say they, is the reason why in the Suns eclipses there appeares within her a duskish ruddy colour, because the Sunne-beames being refracted in passing through the pores of her body, must necessarily be represented under such a colour.

But I reply, if this be the cause of her rednesse, then why doth she not appeare under the same forme when she is about a sextile aspect, and the darkned part of her body is discernable? for then also doe the same rayes passe through her, and therefore in all likelihood should produce the same effect, and notwithstanding those beames are then diverted from us, that they cannot enter into our eyes by a streight line, yet must the colour still remaine visible in her body,^[7] and besides according to this opinion, the spots would not alwaies be the same, but divers, as the various distance of the Sunne requires. Againe, if the Sunne-beames did passe through her, why then hath she not a taile as the Comets? why doth she appeare in such an exact round? and not rather attended with a long flame, since it is meerely this penetration of the Sunne beames that is usually attributed to be the cause of beards in blazing starres.

3. It is opacous, not transparent or diaphanous like Chrystall or glasse,^[8] as Empedocles thought, who held the Moone to bee a globe of pure congealed aire, like haile inclosed in a spheare of fire, for then.

1. Why does shee not alwaies appeare in the full? since the light is dispersed through all her body?

2. How can the interposition of her body so darken the Sun, or cause such great eclipses as have turned day into night,^[9][10] that have discovered the stars, and frighted the birds with such a sudden darknesse, that they fell downe upon the earth, as it is related in divers Histories? And therefore Herodotus telling of an Eclipse which fell in Xerxes time, describes it thus:^[11] ὁ ἥλιος ἐκλιπὼν τὴν ἐκ τοῦ οὐρανοῦ ἕδρην ἀφανὴς ἦν. The Sunne leaving his wonted seate in the heavens, vanished away: all which argues such a great darknesse, as could not have beene, if her body had beene perspicuous. Yet some there are who interpret all these relations to bee hyperbolicall expressions, and the noble Tycho thinkes it naturally impossible, that any eclipse should cause such darknesse, because the body of the Moone can never totally cover the Sunne; however, in this he is singular, all other Astronomers (if I may believe Keplar) being on the contrary opinion, by reason the Diameter of the Moone does for the most part appeare bigger to us then the Diameter of the Sunne.

But here Julius Cæsar^[12] once more, puts in to hinder our passage. The Moone (saith he) is not altogether opacous, because ’tis still of the same nature with the Heavens, which are incapable of totall opacity: and his reason is, because perspicuity is an inseparable accident of those purer bodies, and this hee thinkes must necessarily bee granted, for hee stops there, and proves no further; but to this I shall deferre an answere, till hee hath made up his argument.

We may frequently see, that her body does so eclipse the Sunne, as our earth doth the Moone; since then the like interposition of them both, doth produce the like effect, they must necessarily be of the like natures, that is a like opacous, which is the thing to be shewed; and this was the reason (as the Interpreters guesse) why Aristotle^[13] affirmed the Moone to be of the earths nature, because of their agreement in opacity, whereas all the other elements save that, are in some measure perspicuous.

But the greatest difference which may seeme to make our earth altogether unlike the Moone, is, because the one is a bright body, and hath light of its owne, and the other a grosse dark body which cannot shine at all. 'Tis requisite therefore, that in the next place I cleare this doubt, and shew that the Moone hath no more light of her owne than our earth.

---

1. De cœlo. l. 2. com. 49.
2. Ant. lection. l. 20. c. 4.
3. De phænom. lunæ. c. 11.
4. Plut. de pla. phil. l. 2. c. 13.
5. Opt. l. 4.
6. Com. Purbac. Theo. p. 164.
7. Scaliger exercit. 80. § 13.
8. Plut. de fa lunæ.
9. Thucid. Livii.
10. Plut. de fa. Lunæ.
11. Herodot. l. 7 c. 37.
12. De phænom. Lunæ. c. 11.
13. In lib. de animalib.