The two chiefe motions in the World, which are more especially remarkable above the rest, are the Diurnall, and Annuall.
The Diurnall, which makes the difference betwixt night and day, is caused by the revolution of our Earth vpon it’s owne Axis, in the space of foure and twenty houres.
The Annuall, which makes the difference betwixt Winter and Summer, is likewise caused by the Earth, when being carried through the Eclipticke in it’s owne Orbe, it finishes it’s course in a yeare.
The first is usually stiled, Motus revolutionis: The second Motus circumlationis: There is likewise a third, which Copernicus calls Motus inclinationis: but this being throughly considered, cannot properly be stiled a motion, but rather an immutabilitie, it being that whereby the Axis of the Earth do’s alwaies keep paralell to it selfe, from which scituation, it is not his annuall course that do’s make it in the least manner to decline.
As for the difficulties which concern the second of these, they have been alreadie handled in the sixth Proposition, where the Earth’s eccentricitie was maintained.
So that the chiefe businesse of this Chapter, is to defend the Earth’s diurnall motion, against the objections of our adversaries. Sundry of which objections, to speak (as the truth is) do beare in them a great shew of probabilitie, and such too (as it seemes) was very efficacious, since Aristotle and Ptolomy, &c. men of excellent parts and deep judgements, did ground upon them, as being of infallible and necessarie consequence.
I shall reckon them up severally, and set downe such answers unto each, as may yeeld some satisfaction to every indifferent seeker of truth.
1 First then, ‘tis objected from our sences; If the Earth did move, we should perceive it. The Westerne mountaines would then appeare to ascend towards the Starres, rather than the Stars to descend below them.
I answer: The sight judges of motion according as any thing do’s desert the plane whereon it selfe is seated: which plane every where keeping the same scituation and distance, in respect of the eye, do’s therefore seem immovable unto it, and the motion will appeare in those Starres and parts of the Heaven; through which the verticall Line do’s passe.
The reason of such deceit may be this: Motion being not a proper object of the sight, nor belonging to any other peculiar sence, must therefore be judged of by the sensus communis, which is liable to mistake in this respect; because it apprehends the eye it self to rest immovable, whilest it do’s not feel any effects of this motion in the body: As it is when a man is carried in a Ship; so that sence it but an ill judge of naturall secrets. ‘Tis a good rule of Plato, […]: A Philosopher must not bee carried away by the bare appearance of things to sight, but must examine them by reason. If this were a good consequence, The Earth do’s not move, because it do’s not appeare so to us, we might then as well argue, that it do’s move when we goe upon the water, according to the verse: Provehimur portu, terraeque, verbesque recedunt.
Or if such Arguments would hold, it were an easie matter to prove the Sunne and Moone not so big as a Hat, or the fixed stars as a Candle.
Yea,1 but if the motions of the Heavens bee onely apparant, and not reall, then the motion of the clouds will be so too, since the eye may bee as well deceived in the one as the other.
I answer: ‘Tis all one, as if he should inferre, that the sence was mistaken in every thing, because it was so in one thing: and this would be an excellent Argument to prove that opinion of Anaxagoras, that the Snow was blacke.
The reason why that motion which is caused by the Earth, do’s appeare as if it were in the Heavens, is, because the sensus communis in judging of it, do’s conceive the eye to be it selfe immovable (as was said before) there being no sence that do’s discerne the effects of any motion in the body; and therefore, it do’s conclude every thing to move, which it do’s perceive to change it’s distance from it: So that the clouds do not seem to move sometimes, when as notwithstanding they are every where carried about with our Earth, by such a swift revolution; yet this can be no hinderance at all, why wee may not judge aright of their other particular motions, for which there is not the same reason. Though to a man in a Ship, the Trees and Bankes may seeme to move; yet it would be but a weak Argument, to conclude from hence, that therefore such a one could not tell whether his friend do’s really stirre, whom he sees to walke up and downe in the Ship: or that hee might as well bee deceived in judging the Oares to move, when they doe not.