Celestial Sphere: the Apparent Motions of the Sun, Moon, Planets, and Stars
The celestial sphere is an imaginary projection of the Sun, Moon, planets, stars, and all astronomical bodies upon an imaginary sphere surrounding Earth. The celestial sphere is a useful mapping and tracking remnant of the geocentric theory of the ancient Greek astronomers.
Although originally developed as part of the ancient Greek concept of an Earth-centered universe (i.e., a geocentric model of the Universe), the hypothetical celestial sphere provides an important tool to astronomers for fixing the location and plotting movements of celestial objects. The celestial sphere describes an extension of the lines of latitude and longitude, and the plotting of all visible celestial objects on a hypothetical sphere surrounding the earth.
The ancient Greek astronomers actually envisioned concentric crystalline spheres, centered around Earth, upon which the Sun, Moon, planets, and stars moved. Although heliocentric (Sun-centered) models of the universe were also proposed by the Greeks, they were disregarded as "counter-intuitive" to the apparent motions of celestial bodies across the sky.
Early in the sixteenth century, Polish astronomer Nicolaus Copernicus (1473–1543) reasserted the heliocentric theory abandoned by the Ancient Greeks. Although sparking a revolution in astronomy, Copernicus' system was deeply flawed by the fact that the Sun is certainly not the center of the universe, and Copernicus insisted that planetary orbits were circular.
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