linguistic:paraphrasing
Contemporary cosmological beliefs

In the first half of the 15th century, Nicholas of Cusa challenged the then widely accepted philosophies of Aristotelianism, envisioning instead an infinite universe whose center was everywhere and circumference nowhere, and moreover teeming with countless stars. He also predicted that neither were the rotational orbits circular nor were their movements uniform.

In the second half of the 16th century, the theories of Copernicus (1473–1543) began diffusing through Europe. Copernicus conserved the idea of planets fixed to solid spheres, but considered the apparent motion of the stars to be an illusion caused by the rotation of the Earth on its axis; he also preserved the notion of an immobile center, but it was the Sun rather than the Earth. Copernicus also argued the Earth was a planet orbiting the Sun once every year. However he maintained the Ptolemaic hypothesis that the orbits of the planets were composed of perfect circles— deferents and epicycles —and that the stars were fixed on a stationary outer sphere.

Despite the widespread publication of Copernicus' work De revolutionibus orbium coelestium, during Bruno's time most educated Catholics subscribed to the Aristotelian geocentric view that the Earth was the center of the universe, and that all heavenly bodies revolved around it. The ultimate limit of the universe was the primum mobile, whose diurnal rotation was conferred upon it by a transcendental God, not part of the universe (although, as the kingdom of heaven, adjacent to it), a motionless prime mover and first cause. The fixed stars were part of this celestial sphere, all at the same fixed distance from the immobile Earth at the center of the sphere. Ptolemy had numbered these at 1,022, grouped into 48 constellations. The planets were each fixed to a transparent sphere.

Few astronomers of Bruno's time accepted Copernicus's heliocentric model. Among those who did were the Germans Michael Maestlin (1550–1631), Christoph Rothmann, Johannes Kepler (1571–1630); the Englishman Thomas Digges (c. 1546–1595), author of A Perfit Description of the Caelestial Orbes; and the Italian Galileo Galilei (1564–1642).