De revolutionibus orbium coelestium divided into six "books" (sections or parts), following closely the layout of Ptolemy's "Almagest" which it updated and replaced: Copernicus argued that the universe comprised eight spheres. The outermost consisted of motionless, fixed stars, with the Sun motionless at the center. The known planets revolved about the Sun, each in its own sphere, in the order: Mercury, Venus, Earth, Mars, Jupiter, Saturn. The Moon, however, revolved in its sphere around the Earth. What appeared to be the daily revolution of the Sun and fixed stars around the Earth was actually the Earth's daily rotation on its own axis. Copernicus adhered
History of the center of the Universe southern constellations rise higher above the horizon, and the shadow of Earth on the Moon during a lunar eclipse is round, and spheres cast circular shadows while discs generally do not. This understanding was accompanied by models of the Universe that depicted the Sun, Moon, stars, and naked eye planets circling the spherical Earth, including the noteworthy models of Aristotle (see Aristotelian physics) and Ptolemy. This geocentric model was the dominant model from the 4th century BCE until the 17th century CE. Heliocentrism, or heliocentricism, is the astronomical model in which the Earth and planets revolve around a relatively stationary