Celestial sphere

Earth rotating within a relatively small-radius geocentric celestial sphere. Shown here are stars (white), the ecliptic (red), and the lines of right ascension and circles of declination (cyan) of the equatorial coordinate system.

In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer. If centered on the observer, half of the sphere would resemble a hemispherical screen over the observing location.

The celestial sphere is a practical tool for spherical astronomy, allowing astronomers to specify the apparent positions of objects in the sky if their distances are unknown or irrelevant. In the equatorial coordinate system, the celestial equator divides the celestial sphere into two halves: the northern and southern celestial hemispheres.

Introduction

Celestial Sphere, 18th century. Brooklyn Museum.

Because astronomical objects are at such remote distances, casual observation of the sky offers no information on their actual distances. 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 purposes of spherical astronomy, which is concerned only with the directions to celestial objects, it makes no difference if this is actually the case or if it is Earth that is rotating while the celestial sphere is stationary.

The celestial sphere can be considered to be infinite in radius. This means any point within it, including that occupied by the observer, can be considered the center. It also means that all parallel lines, be they millimetres 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 graphical perspective.[2] All parallel planes will seem to intersect the sphere in a coincident great circle[3] (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 Earth's center, the Sun's center, or any other convenient location, and offsets from positions referred to these centers can be calculated.[4]

In this way, astronomers can predict geocentric or 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.

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 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. 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.[5] This greatly abbreviates the amount of detail necessary in such almanacs, as each observer can handle their own specific circumstances.

Other Languages
العربية: قبة سماوية
asturianu: Esfera celeste
বাংলা: খ-গোলক
Bân-lâm-gú: Thian-kiû
беларуская: Нябесная сфера
беларуская (тарашкевіца)‎: Нябесная сфэра
български: Небесна сфера
čeština: Nebeská sféra
Deutsch: Himmelskugel
Ελληνικά: Ουράνια σφαίρα
español: Esfera celeste
Esperanto: Ĉielosfero
euskara: Zeru-esfera
فارسی: کره آسمان
français: Sphère céleste
한국어: 천구
հայերեն: Երկնոլորտ
hrvatski: Nebeska sfera
Bahasa Indonesia: Bola langit
italiano: Sfera celeste
ಕನ್ನಡ: ಖಗೋಳ
ქართული: ცის სფერო
latviešu: Debess sfēra
Lëtzebuergesch: Himmelskugel
lietuvių: Dangaus sfera
македонски: Небесна сфера
മലയാളം: ഖഗോളം
Bahasa Melayu: Sfera cakerawala
Nederlands: Hemelbol
日本語: 天球
norsk nynorsk: Himmelkvelving
oʻzbekcha/ўзбекча: Osmon sferasi
português: Esfera celeste
Simple English: Celestial sphere
slovenčina: Nebeská sféra
slovenščina: Nebesna krogla
српски / srpski: Небеска сфера
srpskohrvatski / српскохрватски: Nebeska sfera
татарча/tatarça: Күк йөзе
Türkçe: Gökküre
українська: Небесна сфера
Tiếng Việt: Thiên cầu
中文: 天球