"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"

Nicolaus Copernicus was a 16th century astronomer who proposed the heliocentric model of deep space, which positioned the sun at the center of the universe rather of the Earth. In this quote, he is reviewing the motion of the heavenly bodies, which he thinks to be circular. He is suggesting that the motion of the celestial bodies is an outcome of the shape of the sphere they live in. He is implying that the movement of the heavenly bodies is an outcome of the rotation of the sphere they live in, which is a circular motion.

This quote is significant since it is among the very first circumstances of Copernicus proposing his heliocentric design of the universe. He was one of the very first astronomers to recommend that the Earth was not the center of the universe, and that the sun was instead. This quote is likewise significant since it is one of the first circumstances of Copernicus proposing that the movement of the celestial bodies was an outcome of the shape of the sphere they populate. This was an advanced idea at the time, and it laid the structure for contemporary astronomy.