Celestial sphere
The celestial sphere is a conceptual tool used in spherical astronomy to specify the position of an object in the sky without consideration of its linear distance from the observer.
All celestial objects seem equally far away, as if fixed onto the inside of a sphere with a large but unknown radius,[1] which appears to rotate westward overhead; meanwhile, Earth underfoot seems to remain still.
For instance, the Astronomical Almanac for 2010 lists the apparent geocentric position of the Moon on January 1 at 00:00:00.00 Terrestrial Time, in equatorial coordinates, as right ascension 6h 57m 48.86s, declination +23° 30' 05.5".
Implied in this position is that it is as projected onto the celestial sphere; any observer at any location looking in that direction would see the "geocentric Moon" in the same place against the stars.
He composed a set of principles called Aristotelian physics that outlined the natural order and structure of the world.
Like other Greek astronomers, Aristotle also thought the "...celestial sphere as the frame of reference for their geometric theories of the motions of the heavenly bodies".
Aristotle had asserted that these bodies (in the superlunary region) are perfect and cannot be corrupted by any of the classical elements: fire, water, air, and earth.
Aristotle's key concepts rely on the nature of the five elements distinguishing the Earth and the Heavens in the astronomical reality, taking Eudoxus's model of separate spheres.
Eudoxus attempted to construct his model mathematically from a treatise known as On Speeds (Ancient Greek: Περί Ταχών) and asserted the shape of the hippopede or lemniscate was associated with planetary retrogression.
Anything that defied the motion of natural place and the unchanging heavens (including the celestial spheres) was criticized immediately by Aristotle.
These concepts are important for understanding celestial coordinate systems, frameworks for measuring the positions of objects in the sky.
"[11] Anaxagoras in the mid 5th century BC was the first known philosopher to suggest that the stars were "fiery stones" too far away for their heat to be felt.
The first astronomer of the European Renaissance to suggest that the stars were distant suns was Giordano Bruno in his De l'infinito universo et mondi (1584).
The oldest surviving example of such an artifact is the globe of the Farnese Atlas sculpture, a 2nd-century copy of an older (Hellenistic period, ca.
These could be based on the equivalent "ecliptic", poles and equator, although the reasons for building a system that way are as much historic as technical.
