Finally we shall place the Sun himself at the center of the Universe.
Near the sun is the center of the universe.
The earth also is spherical, since it presses upon its center from every direction.
Moreover, since the sun remains stationary, whatever appears as a motion of the sun is really due rather to the motion of the earth.
At rest, however, in the middle of everything is the sun.
First of all, we must note that the universe is spherical.
We regard it as a certainty that the earth, enclosed between poles, is bounded by a spherical surface.
Pouring forth its seas everywhere, then, the ocean envelops the earth and fills its deeper chasms.
For it is the duty of an astronomer to compose the history of the celestial motions through careful and expert study.
The massive bulk of the earth does indeed shrink to insignificance in comparison with the size of the heavens.
Of all things visible, the highest is the heaven of the fixed stars.
Moreover, there is no difference between the earth's centers of gravity and magnitude.
Yet if anyone believes that the earth rotates, surely he will hold that its motion is natural, not violent.
Every observed change of place is caused by a motion of either the observed object or the observer or, of course, by an unequal displacement of each.
Therefore, having obtained the opportunity from these sources, I too began to consider the mobility of the earth.
Although all the good arts serve to draw man's mind away from vices and lead it toward better things, this function can be more fully performed by this art, which also provides extraordinary intellectual pleasure.
To know that we know what we know, and to know that we do not know what we do not know, that is true knowledge.
I shall now recall to mind that the motion of the heavenly bodies is circular, since the motion appropriate to a sphere is rotation in a circle.
Those things which I am saying now may be obscure, yet they will be made clearer in their proper place.
Therefore, in the course of the work I have followed this plan: I describe in the first book all the positions of the orbits together with the movements which I ascribe to the Earth, in order that this book might contain, as it were, the general scheme of the universe.
Therefore I would not have it unknown to Your Holiness, the the only thing which induced me to look for another way of reckoning the movements of the heavenly bodies was that I knew that mathematicians by no means agree in their investigation thereof.
In so many and such important ways, then, do the planets bear witness to the earth's mobility.
Those who know that the consensus of many centuries has sanctioned the conception that the earth remains at rest in the middle of the heavens as its center, would, I reflected, regard it as an insane pronouncement if I made the opposite assertion that the earth moves.
Not a few other very eminent and scholarly men made the same request, urging that I should no longer through fear refuse to give out my work for the common benefit of students of Mathematics.
Mathematics is written for mathematicians.