Editing Celestial sphere (section)

== Greek history on celestial spheres ==
Celestial spheres (or celestial orbs) were envisioned to be perfect and divine entities initially from Greek astronomers such as [[Aristotle]]. 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".<ref>Arthur Berry (1898) [https://archive.org/details/shorthistoryofas025511mbp A Short History of Astronomy], page 38</ref> With his adoption of [[Eudoxus of Cnidus]]' theory, Aristotle had described celestial bodies within the Celestial sphere to be filled with pureness, perfect and quintessence (the fifth element that was known to be divine and purity according to Aristotle). Aristotle deemed the Sun, Moon, planets and the fixed stars to be perfectly concentric spheres in a superlunary region above the [[sublunary sphere]]. 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. Corruptible elements were only contained in the sublunary region and incorruptible elements were in the superlunary region of Aristotle's geocentric model. Aristotle had the notion that celestial orbs must exhibit celestial motion (a perfect circular motion) that goes on for eternity. He also argued that the behavior and property follows strictly to a principle of natural place where the quintessential  element moves freely of divine will, while other elements, fire, air, water and earth, are corruptible, subject to change and imperfection. 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.

Numerous discoveries from Aristotle and Eudoxus (approximately 395 B.C. to 337 B.C.) have sparked differences in both of their models and sharing similar properties simultaneously. Aristotle and Eudoxus claimed two different counts of spheres in the heavens. According to Eudoxus, there were only 27 spheres in the heavens, while there are 55 spheres in Aristotle's model. Eudoxus attempted to construct his model mathematically from a treatise known as ''On Speeds'' ({{langx|grc|Περί Ταχών}}) and asserted the shape of the hippopede or [[lemniscate]] was associated with [[Apparent retrograde motion|planetary retrogression]]. Aristotle emphasized that the speed of the celestial orbs is unchanging, like the heavens, while Eudoxus emphasized that the orbs are in a perfect geometrical shape. Eudoxus's spheres would produce undesirable motions to the lower region of the planets, while Aristotle introduced unrollers between each set of active spheres to counteract the motions of the outer set, or else the outer motions will be transferred to the outer planets. Aristotle would later observe "...the motions of the planets by using the combinations of nested spheres and circular motions in creative ways, but further observations kept undoing their work".<ref>[[Margaret J. Osler]] (2010) ''Reconfiguring the World'', [[Johns Hopkins University Press]] page 15 {{ISBN|0-8018-9656-8}}</ref>

Aside from Aristotle and Eudoxus, [[Empedocles]] gave an explanation that the motion of the heavens, moving about it at divine (relatively high) speed, puts the Earth in a stationary position due to the [[circular motion]] preventing the downward movement from natural causes. Aristotle criticized Empedocles's model, arguing that all heavy objects go towards the Earth and not the whirl itself coming to Earth. He ridiculed it and claimed that Empedocles's statement was extremely absurd. Anything that defied the motion of natural place and the unchanging heavens (including the celestial spheres) was criticized immediately by Aristotle.

{{DEFAULTSORT:}}==Celestial coordinate systems==
These concepts are important for understanding [[celestial coordinate system]]s, frameworks for measuring the positions of [[astronomical object|objects in the sky]]. Certain reference lines and [[plane of reference|planes]] on Earth, when projected onto the celestial sphere, form the bases of the reference systems. These include the Earth's [[equator]], [[Earth's rotation|axis]], and [[Earth's orbit|orbit]]. At their intersections with the celestial sphere, these form the [[celestial equator]], the north and south [[celestial pole]]s, and the [[ecliptic]], respectively.<ref>Newcomb (1906), p. 92–93.</ref> As the celestial sphere is considered arbitrary or infinite in radius, all observers see the celestial equator, celestial poles, and ecliptic at the same place against the [[fixed stars|background stars]].

From these bases, directions toward objects in the sky can be quantified by constructing celestial coordinate systems. Similar to geographic [[longitude]] and [[latitude]], the [[equatorial coordinate system]] specifies positions relative to the celestial equator and celestial poles, using right ascension and declination. The [[ecliptic coordinate system]] specifies positions relative to the ecliptic (Earth's [[orbital plane|orbit]]), using [[ecliptic coordinate system#Spherical coordinates|ecliptic longitude and latitude]]. Besides the equatorial and ecliptic systems, some other celestial coordinate systems, like the [[galactic coordinate system]], are more appropriate for particular purposes.