Editing Celestial spheres (section)

== Context ==

In modern thought, the [[Orbit#Planetary orbits|orbits of the planets]] are viewed as the paths of those planets through mostly empty space. Ancient and medieval thinkers, however, considered the celestial orbs to be thick spheres of rarefied matter nested one within the other, each one in complete contact with the sphere above it and the sphere below.<ref name="Lindberg, p. 251">Lindberg, ''Beginnings of Western Science'', p. 251.</ref> When scholars applied Ptolemy's [[epicycles]], they presumed that each planetary sphere was exactly thick enough to accommodate them.<ref name="Lindberg, p. 251"/> By combining this nested sphere model with astronomical observations, scholars calculated what became generally accepted values at the time for the distances to the Sun: about {{convert|4|e6mi|e6km|abbr=off}}, to the other planets, and to the edge of the universe: about {{convert|73|e6mi|e6km|abbr=off}}.<ref>Van Helden, ''Measuring the Universe'', pp. 28–40.</ref> The nested sphere model's distances to the Sun and planets differ significantly from modern measurements of the distances,<ref>Grant, ''Planets, Stars, and Orbs'', pp. 437–8.</ref> and the [[size of the universe]] is now known to be inconceivably large and continuously [[Metric expansion of space|expanding]].<ref>Van Helden, ''Measuring the Universe'', p. 3.</ref>

Albert Van Helden has suggested that from about 1250 until the 17th century, virtually all educated Europeans were familiar with the Ptolemaic model of "nesting spheres and the cosmic dimensions derived from it".<ref>Van Helden, ''Measuring the Universe'', pp. 37, 40.</ref> Even following the adoption of Copernicus's [[heliocentric model]] of the universe, new versions of the celestial sphere model were introduced, with the planetary spheres following this sequence from the Sun at the centre: Mercury, Venus, Earth-Moon, Mars, Jupiter and Saturn.

Mainstream belief in the theory of celestial spheres did not survive the [[Scientific Revolution]]. In the early 1600s, [[Johannes Kepler|Kepler]] continued to discuss celestial spheres, although he did not consider that the planets were carried by the spheres but held that they moved in elliptical paths described by [[Kepler's laws of planetary motion]]. In the late 1600s, Greek and medieval theories concerning the motion of terrestrial and celestial objects were replaced by [[Newton's law of universal gravitation]] and [[Newtonian mechanics]], which explain how Kepler's laws arise from the gravitational attraction between bodies.