Editing Celestial sphere

{{short description|Imaginary sphere of arbitrarily large radius, concentric with the observer}}
{{for multi|the ancient cosmological model|Celestial spheres|other uses|Celestial (disambiguation)}}
[[File:Celestial Sphere - Eq w Label figures.png|thumb|Visualization of a celestial sphere]]
In [[astronomy]] and [[navigation]], the '''celestial sphere''' is an [[abstraction|abstract]] [[sphere]] that has an arbitrarily large [[radius]] and is [[concentric objects|concentric]] to [[Earth]]. All objects in the [[sky]] can be conceived as being [[projective geometry|projected]] upon the inner surface of the celestial sphere, which may be [[geocentric model|centered on Earth]] or the observer. If centered on the observer, half of the sphere would resemble a [[Sphere#Hemisphere|hemispherical]] [[projection screen|screen]] over the observing location.

The celestial sphere is a conceptual tool used in [[spherical astronomy]] to [[celestial coordinate system|specify]] the position of an [[astronomical object|object]] in the sky without consideration of its linear distance from the observer. The [[celestial equator]] divides the celestial sphere into [[northern celestial hemisphere|northern]] and [[southern celestial hemisphere|southern]] hemispheres.

==Description==
Because [[astronomical object]]s are at such remote distances, casual observation of the [[sky]] offers no information on their actual distances. All celestial objects seem [[equidistant|equally far away]], as if [[fixed stars|fixed]] onto the inside of a [[sphere]] with a large but unknown radius,<ref>
{{cite book
|title = Astronomy
|date = 1890
|last1=Newcomb
|first1=Simon
|last2=Holden
|first2=Edward S.
|publisher=Henry Holt and Co., New York
|url=https://books.google.com/books?id=gek3AAAAMAAJ&q=astronomy+newcomb
}}, p. 14</ref> which [[diurnal motion|appears to rotate]] westward overhead; meanwhile, [[Earth]] underfoot seems to remain still. For purposes of [[spherical astronomy]], which is concerned only with the [[Orientation (geometry)|directions]] to celestial objects, it makes no difference if this is actually the case or if it is Earth that is [[Earth's rotation|rotating]] while the celestial sphere is stationary.

The celestial sphere can be considered to be [[infinity|infinite]] in [[radius]]. This means any [[point (geometry)|point]] within it, including that occupied by the observer, can be considered the [[centre (geometry)|center]]. It also means that all [[parallel (geometry)|parallel]] [[line (geometry)|lines]], be they [[millimetre]]s apart or across the [[Solar System]] from each other, will seem to intersect the sphere at a single point, analogous to the [[vanishing point]] of [[perspective (graphical)|graphical perspective]].<ref>
{{cite book
| last          = Chauvenet
| first         = William
| title         = A Manual of Spherical and Practical Astronomy
| url           = https://archive.org/details/amanualspherica09chaugoog
| quote         = chauvenet spherical astronomy.
| date          = 1900
| publisher     = J.B. Lippincott Co., Philadelphia
}}, p. 19, at Google books.</ref> All parallel [[plane (geometry)|planes]] will seem to intersect the sphere in a coincident [[great circle]]<ref>
{{cite book
| last          = Newcomb
| first         = Simon
| title         = A Compendium of Spherical Astronomy
| url           = https://archive.org/details/acompendiumsphe00newcgoog
| date          = 1906
| publisher     = Macmillan Co., New York
}}, p. 90, at Google books.</ref> (a "vanishing circle").

Conversely, observers looking toward the same point on an infinite-radius celestial sphere will be looking along parallel lines, and observers looking toward the same great circle, along parallel planes. On an infinite-radius celestial sphere, all observers see the same things in the same direction.

For some objects, this is over-simplified. Objects which are relatively near to the observer (for instance, the [[Moon]]) will seem to change position against the distant celestial sphere if the observer moves far enough, say, from one side of planet Earth to the other. This effect, known as [[parallax]], can be represented as a small offset from a mean position. The celestial sphere can be considered to be centered at the [[geocentric model|Earth's center]], the [[heliocentrism|Sun's center]], or any other convenient location, and offsets from positions referred to these centers can be calculated.<ref>
{{cite book 
 | author = U.S. Naval Observatory Nautical Almanac Office
 | first =Nautical Almanac Office
 | author2 = U.K. Hydrographic Office, H.M. Nautical Almanac Office
 | title = The Astronomical Almanac for the Year 2010
 | publisher = U.S. Govt. Printing Office
 | date = 2008
 | isbn = 978-0-7077-4082-9}}
, p. M3-M4</ref>

In this way, [[astronomer]]s can predict [[geocentric coordinates|geocentric]] or [[Heliocentrism#Modern use of geocentric and heliocentric|heliocentric]] positions of objects on the celestial sphere, without the need to calculate the individual [[geometry]] of any particular observer, and the utility of the celestial sphere is maintained. Individual observers can work out their own small offsets from the mean positions, if necessary. In many cases in astronomy, the offsets are insignificant.

=== Determining location of objects ===
The celestial sphere can thus be thought of as a kind of astronomical [[shorthand]], and is applied very frequently by astronomers. 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 coordinate system|equatorial coordinates]], as [[right ascension]] 6<sup>h</sup> 57<sup>m</sup> 48.86<sup>s</sup>, [[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. For many rough uses (e.g. calculating an approximate phase of the Moon), this position, as seen from the Earth's center, is adequate.

For applications requiring precision (e.g. calculating the shadow path of an [[eclipse]]), the ''Almanac'' gives formulae and methods for calculating the ''topocentric'' coordinates, that is, as seen from a particular place on the Earth's surface, based on the geocentric position.<ref>''Astronomical Almanac 2010'', sec. D</ref> This greatly abbreviates the amount of detail necessary in such almanacs, as each observer can handle their own specific circumstances.

== 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.

==History==
{{main|Cosmic pluralism}}
{{see|History of astronomy}}
The ancient Greeks assumed the literal truth of stars attached to a celestial sphere, revolving about the Earth in one day, and a fixed Earth.<ref>
{{cite book
| last          = Seares
| first         = Frederick H.
| title         = Practical Astronomy for Engineers
| url           = https://archive.org/details/ost-engineering-practicalastrono00searuoft
| quote         = practical astronomy.
| date          = 1909
| publisher     = [[E.W. Stephens Publishing Company]], Columbia, MO
| bibcode = 1909pafe.book.....S
}}, art. 2, p. 5, at Google books.</ref> 
The [[Eudoxus of Cnidus#Eudoxan planetary models|Eudoxan planetary model]], on which the Aristotelian and [[Ptolemy|Ptolemaic]] models were based, was the first geometric explanation for the "wandering" of the [[classical planets]].<ref>{{cite web |first=Henry |last=Mendell |url=http://www.calstatela.edu/faculty/hmendel/Ancient%20Mathematics/Eudoxus/Astronomy/EudoxusHomocentricSpheres.htm |title=Eudoxus of Cnidus: Astronomy and Homocentric Spheres |series=Vignettes of Ancient Mathematics |date=16 September 2009 |url-status=dead |archive-url=https://web.archive.org/web/20110516145329/http://www.calstatela.edu/faculty/hmendel/Ancient%20Mathematics/Eudoxus/Astronomy/EudoxusHomocentricSpheres.htm |archive-date=16 May 2011 }}</ref> The outermost of these [[Celestial spheres|"crystal spheres"]] was thought to carry the [[fixed stars]]. Eudoxus used 27 concentric spherical solids to answer [[Plato|Plato's]] challenge: "By the assumption of what uniform and orderly motions can the apparent motions of the planets be accounted for?"<ref>{{cite book |last=Lloyd |first=Geoffrey Ernest Richard |date=1970 |title=Early Greek Science: Thales to Aristotle |isbn=978-0-393-00583-7 |location=New York, NY |publisher=[[W. W. Norton & Co.]] |page=84 |url=https://books.google.com/books?id=Mym7xTLfNfIC }}</ref>
[[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. Similar ideas were expressed by [[Aristarchus of Samos]]. However, they did not enter mainstream European and Islamic astronomy of the late ancient and medieval period.
Copernican [[heliocentrism]] did away with the planetary spheres, but it did not necessarily preclude the existence of a sphere for the fixed stars. 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). This idea was among the charges, albeit not in a prominent position, brought against him by the Inquisition.
The idea became mainstream in the later 17th century, especially following the publication of ''[[Conversations on the Plurality of Worlds]]'' by [[Bernard Le Bovier de Fontenelle]] (1686), and by the early 18th century it was the default working assumptions in stellar astronomy.

==Star globe==
[[File:JostBurgi-MechanisedCelestialGlobe1594.jpg|thumb|upright=.8|Celestial globe by [[Jost Bürgi]] (1594)]]
{{main|Star chart|Armillary sphere|Celestial globe}}
A celestial sphere can also refer to a physical model of the celestial sphere or celestial globe.
Such globes map the constellations on the ''outside'' of a sphere, resulting in a mirror image of the constellations as seen from Earth. 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. 120 BCE) work.

==Bodies other than Earth==
{{see also|International Celestial Reference System}}
Observers on other worlds would, of course, see objects in that sky under much the same conditions – as if projected onto a dome. Coordinate systems based on the sky of that world could be constructed. 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.

==See also==
{{Portal|Astronomy
}}{{div col|colwidth=35em}}
* [[Horizontal coordinate system]]
* [[Equatorial coordinate system]]
** [[Hour angle]]
** [[Pole star]]
** [[Polar alignment]]
** [[Equatorial mount]]
* [[Equinox (celestial coordinates)]]
* [[Spherical astronomy]]
* [[Ecliptic]]
* [[Zodiac]]
* [[Orbital pole]]
* [[Stellar parallax]], a type of short-term motion of distant stars
* [[Proper motion]], a type of longer-term motion of distant stars
* [[Firmament]]
* [[Fixed stars]], about the old concept of the celestial sphere to be a material, physical entity.
{{div col end}}

==Notes==
{{Reflist}}

==References==
*{{cite book
  | first = Nathaniel
  | last = Bowditch
  | author-link = Nathaniel Bowditch
  | title = The American Practical Navigator
  | publisher = [[National Imagery and Mapping Agency]]
  | url = http://www.irbs.com/bowditch/
  | location = Bethesda, MD
  | date = 2002
  | isbn = 0-939837-54-4
  | url-status = dead
  | archive-url = https://web.archive.org/web/20070624193729/http://www.irbs.com/bowditch/
  | archive-date = 2007-06-24
  }}
*{{cite book |author1=MacEwen, William A. |author2=William Hayler |author3=Turpin, Edward A. |title=Merchant Marine officers' handbook: based on the original edition by Edward A. Turpin and William A. MacEwen |publisher=[[Cornell Maritime Press]] |location=Cambridge, Md |date=1989 |pages=46–51 |edition= 5th|isbn=0-87033-379-8 }}Bibliography (References) for Wikipedia assignment on Celestial Sphere. (APA6 format). Crowe, M. J. (2001). ''Theories of the world from antiquity to the Copernican revolution''. Mineola, NY: Dover Publications. 

==External links==
{{commons category|Celestial spheres}}
* [http://stars.astro.illinois.edu/celsph.html MEASURING THE SKY A Quick Guide to the Celestial Sphere] – Jim Kaler, University of Illinois
* [[Wikibooks:General Astronomy/The Celestial Sphere|General Astronomy/The Celestial Sphere]] – Wikibooks
* [http://astro.unl.edu/naap/motion2/animations/ce_hc.html Rotating Sky Explorer] – University of Nebraska-Lincoln
* {{webarchive |url=http://webarchive.loc.gov/all/20050613050209/http://skyandtelescope.com/observing/skychart/ |title=Interactive Sky Chart – SkyandTelescope.com |date=2005-06-13}}
* [http://astroclub.tau.ac.il/skymaps/monthly/ Monthly skymaps] {{Webarchive|url=https://web.archive.org/web/20070913163917/http://astroclub.tau.ac.il/skymaps/monthly/ |date=2007-09-13 }} – for every location on Earth

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