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.

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.

So, influenced by these advisors and this hope, I have at length allowed my friends to publish the work, as they had long besought me to do.

Not only the phenomena of the others followed from this, but also it so bound together both the order and magnitude of all the planets and the spheres and the heaven itself, that in no single part could one thing be altered without confusion among the other parts and in all the universe.

For it is the duty of an astronomer to compose the history of the celestial motions through careful and expert study.

Pouring forth its seas everywhere, then, the ocean envelops the earth and fills its deeper chasms.

In so many and such important ways, then, do the planets bear witness to the earth's mobility.