Editing Celestial sphere (section)

==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.