The discovery of a world in the moone

by John Wilkins

If any should aske whether the other Planets lend any light to the Moone; I answer they doe not.

True indeed, the noble Tycho l. 20. c. 5.[6] discussing the reason of this light attributes it to the Planet Uenus, and I grant that this may convey some light to the Moone; but that it is not the cause of this whereof wee now discourse, is of itselfe sufficiently plaine, because Uenus is sometimes over the Moone, when as shee cannot convey any light to that part which is turned from her.

It doth not proceede from the fixed starres, for then it would retaine the same light in eclipses, whereas the light at such times is more ruddy and dull. Then also the light of the Moone would not be greater or lesser, according to its distance from the edge of the earths shadow, since it did at all times equally participate this light of the starres.

Now because there is no other body in the whole Universe, save the earth, it remaines that this light must necessarily be caused by that which with a just gratitude repaies to the Moone, such illumination as it receives from her.

And as loving friends equally participate of the same joy and griefe, so doe these mutually partake of the same light from the Sunne, and the same darkenesse from the eclipses, being also severally helped by one another in their greatest wants: For when the Moone is in conjunction with the Sunne, and her upper part receives all the light, then her lower Hemispheare (which would otherwise be altogether darke) is enlightened by the reflexion of the Sunne beames from the earth. When these two planets are in opposition, then that part of the earth which could not receive any light from the Sunne beames, is most enlightened by the Moone, being then in her full; and as she doth most illuminate the earth when the Sunne beames cannot, so the gratefull earth returnes to her as great, nay greater light when shee most wants it; so that alwaies that visible part of the Moone which receives nothing from the Sunne, is enlightened by the earth, as is proved by Galilæus, with many more arguments, in that Treatise which he calls Systema mundi. True indeed, when the Moone comes to a quartile, then you can neither discerne this light, nor yet the darker part of her body, but the reason is, because of the exuperancy of the light in the other parts. Quippe illustratum medium speciem recipit valentiorem, Scal. exerc. 62. the clearer brightnesse involves the weaker, it being with the species of sight, as it is with those of sound, and as the greater noise drownes the lesse, so the brighter object hides that which is more obscure. But they doe alwaies in their mutuall vicissitudes participate of one anothers light; so also doe they partake of the same defects and darknings, for when our Moone is eclipsed, then is their Sunne darkened, and when our Sunne is eclipsed, then is their Moone deprived of its light, as you may see affirmed by Mæslin. Epit. Astro. l. 4. part. 2. Quod si terram nobis ex alto liceret intueri, quemadmodum deficientem lunam ex longinquo spectare possumus, videremus tempore eclipsis solis terræ aliquam partem lumine solis deficere, eodem planè modo sicut ex opposito luna deficit,

If wee might behold this globe of earth at the same distance as we doe the Moone in her defects, wee might discerne some part of it darkened in the Sunnes eclipses, just so as the Moone is in hers.

For as our Moone is eclipsed by the interposition of our earth, so is their Moone eclipsed by the interposition of theirs. The manner of this mutuall illumination betwixt these two you may plainly discerne in this Figure following.

Where A represents the Sun, B the Earth, and C the Moone; Now suppose the Moone C to be in a sextile of increase, when there is onely one small part of her body enlightened, then the earth B will have such a part of its visible Hemispheare darkened, as is proportionable to that part of the Moone which is enlightened; and as for so much of the Moone, as the Sun beames cannot reach unto, it receives light from a proportionall part of the earth which shines upon it, as you may plainly perceive by the Figure.

You see then that agreement and similitude which there is betwixt our earth and the Moone. Now the greatest difference which makes them unlike, is this, that the Moone enlightens our earth round about, whereas our earth gives light onely to that Hemispheare of the Moone which is visible unto us, as may be certainly gathered from the constant appearance of the same spots, which could not thus come to passe, if the Moone had such a diurnall motion about its own axis, as perhaps our earth hath. And though some suppose her to move in an epicycle, yet this doth not so turne her body round, that we may discerne both Hemispheares, for according to that hypothesis, the motion of her eccentrick, doth turne her face towards us, as much as the other doth from us.