<P> Early in the sixteenth century Nicolaus Copernicus drastically reformed the model of astronomy by displacing the Earth from its central place in favour of the Sun, yet he called his great work De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres). Although Copernicus does not treat the physical nature of the spheres in detail, his few allusions make it clear that, like many of his predecessors, he accepted non-solid celestial spheres . Copernicus rejected the ninth and tenth spheres, placed the orb of the Moon around the Earth and moved the Sun from its orb to the center of the world . The planetary orbs circled the center of the world in the order Mercury, Venus, the great orb containing the Earth and the orb of the Moon, then the orbs of Mars, Jupiter, and Saturn . Finally he retained the eighth starry sphere, which he held to be unmoving . </P> <P> The English almanac maker, Thomas Digges, delineated the spheres of the new cosmological system in his Perfit Description of the Caelestiall Orbes...(1576). Here he arranged the "orbes" in the new Copernican order, expanding one sphere to carry "the globe of mortalitye", the Earth, the four elements, and the Moon; and expanding the starry sphere infinitely upward to encompass all the stars, and also to serve as "the court of the Great God, the habitacle of the elect, and of the coelestiall angelles ." </P> <P> In the course of the sixteenth century, a number of philosophers, theologians, and astronomers--among them Francesco Patrizi, Andrea Cisalpino, Peter Ramus, Robert Bellarmine, Giordano Bruno, Jerónimo Muñoz, Michael Neander, Jean Pena, and Christoph Rothmann--abandoned the concept of celestial spheres . Rothmann argued from the observations of the comet of 1585 that the lack of observed parallax indicated that the comet was beyond Saturn, while the absence of observed refraction indicated the celestial region was of the same material as air, hence there were no planetary spheres . </P> <P> Tycho Brahe's investigations of a series of comets from 1577 to 1585, aided by Rothmann's discussion of the comet of 1585 and Michael Maestlin's tabulated distances of the comet of 1577, which passed through the planetary orbs, led Tycho to conclude that "the structure of the heavens was very fluid and simple ." Tycho opposed his view to that of "very many modern philosophers" who divided the heavens into "various orbs made of hard and impervious matter ." Edward Grant found relatively few believers in hard celestial spheres before Copernicus, and concluded that the idea first became common sometime between the publication of Copernicus's De revolutionibus in 1542 and Tycho Brahe's publication of his cometary research in 1588 . </P>

Where are the planets mostly located in the celestial sphere