The Discovery of a World in the Moone/Chapter 11
Proposition 10.
AS that part of our aire which is neerest to the earth, is of a thicker substance than the other, by reason tis alwaies mixed with some vapours, which are continually exhaled into it. So is it equally requisite, that if there be a world in the Moone, that the aire about that should be alike qualified with ours. Now, that there is such an orbe of grosse aire, was first of all (for ought I can reade) observed by Meslin[1], afterwards assented unto by Keplar and Galilæus, and since by Baptistæ Cisatus, Sheiner with others, all of them confirming it by the same arguments which I shall onely cite, and then leave this Proposition.
1. 'Tis observed, that so much of the Moone as is enlightened, is alwaies part of a bigger circle then that which is darker. Their frequent experience hath proved this, and an easie observation may quickely confirme it. But now this cannot proceede from any other cause so probable, as from this orbe of aire, especially when we consider how that planet shining with a borrowed light, doth not send forth any such rayes as may make her appearance bigger then her body,
2. 'Tis oberved in the Solary eclipses, that there is a great trepidation about the body of the Moone, from which we may likewise argue an Atmo-sphæra, since we cannot well conceive what so probable a cause there should be of such an appearance as this, Quod radii Solares á vaporibus Lunam ambientibus fuerint intercisi,[2] that the Sun beames were broken and refracted by the vapours that encompassed the Moone.
3. I may adde the like argument taken from another observation which will be easily tried and granted. When the Sunne is eclipsed, wee discerne the Moone as shee is in her owne naturall bignesse, but then she appeares somewhat lesse then when shee is in the full, though she be in the same place of her supposed excentrick and epicycle, and therefore Tycho hath calculated a Table for the Diameter of the divers new Moones. But now there is no reason so probable to salve this appearance, as to place an orbe of thicker aire, neare the body of that Planet, which may be enlightened by the reflected beames, and through which the direct raies may easily penetrate.
But some may object that this will not consist with that which was before delivered, where I said, that the thinnest parts had least light.
If this were true, how comes it to passe then, that this aire should be as bright as any of the other parts, when as tis the thinnest of all?
I answer, if the light be received by reflection, then the thickest body hath most, because it is best able to beate backe the raies, but if the light be received by illumination[3] (especially if there be an opacous body behinde, which may double the beames by reflexion) as it is here, then I deny not but a thinne body may retaine much light, and perhaps, some of those appearances which wee take for fiery comets, are nothing else but a bright cloud enlightened, so that probable it is, there may be such aire without the Moone, and hence it comes to passe, that the greater spots are onely visible towards her middle parts, and none neere the circumference, not but that there are some as well in those parts as else where, but they are not there perceiveable, by reason of those brighter vapours which hide them.
- ↑ Vide Euseb. Nierem. de Nat. Hist. l. 2. c. 11..
- ↑ Scheiner Ros. Urs. l. 4. pars 2. c. 27.
- ↑ Hist l. 1. c. 7. § 11.