Celestial sphere

celestialcelestial hemispherehemispheresouthern skybackgroundcelestial domehemispheresnorthern skyskyspherical background
In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth.wikipedia
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Sky

celestialskiesheavens
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.
In the field of astronomy, the sky is also called the celestial sphere.

Spherical astronomy

positional astronomyastronomie de positionastronomy
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.
Spherical astronomy or positional astronomy is the branch of astronomy that is used to determine the location of objects on the celestial sphere, as seen at a particular date, time, and location on Earth.

Celestial equator

equatorialequatorial planeequatorial sky
In the equatorial coordinate system, the celestial equator divides the celestial sphere into two halves: the northern and southern celestial hemispheres. At their intersections with the celestial sphere, these form the celestial equator, the north and south celestial poles, and the ecliptic, respectively. 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 celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth.

Celestial coordinate system

celestial longitudecelestial coordinatesaltitude
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.
Coordinate systems can specify an object's position in three-dimensional space or plot merely its direction on a celestial sphere, if the object's distance is unknown or trivial.

Geocentric model

geocentricPtolemaic systemPtolemaic
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.
The celestial sphere, still used for teaching purposes and sometimes for navigation, is also based on a geocentric system which in effect ignores parallax.

Southern celestial hemisphere

southern skysouthernsouthern hemisphere
In the equatorial coordinate system, the celestial equator divides the celestial sphere into two halves: the northern and southern celestial hemispheres.
The southern celestial hemisphere, also called the Southern Sky, is the southern half of the celestial sphere; that is, it lies south of the celestial equator.

Northern celestial hemisphere

northern skynorthernnorth
In the equatorial coordinate system, the celestial equator divides the celestial sphere into two halves: the northern and southern celestial hemispheres.
The northern celestial hemisphere, also called the Northern Sky, is the northern half of the celestial sphere; that is, it lies north of the celestial equator.

Equatorial coordinate system

equatorial coordinatesequatorialCoordinates
In the equatorial coordinate system, the celestial equator divides the celestial sphere into two halves: the northern and southern celestial hemispheres. 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 6 h 57 m 48.86 s, 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. 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.
It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fundamental plane consisting of the projection of Earth's equator onto the celestial sphere (forming the celestial equator), a primary direction towards the vernal equinox, and a right-handed convention.

Declination

DecDec.declinations
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 6 h 57 m 48.86 s, 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. 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.
In astronomy, declination (abbreviated dec; symbol δ) is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle.

Fixed stars

fixed starfixedstars
All celestial objects seem equally far away, as if fixed onto the inside of a sphere with a large but unknown radius,
Other models of the planetary system show a celestial sphere containing fixed stars on the outer most part of the universe.

Parallax

trigonometric parallaxsolar parallaxmotion parallax
This effect, known as parallax, can be represented as a small offset from a mean position.
It is clear from Euclid's geometry that the effect would be undetectable if the stars were far enough away, but for various reasons such gigantic distances involved seemed entirely implausible: it was one of Tycho's principal objections to Copernican heliocentrism that in order for it to be compatible with the lack of observable stellar parallax, there would have to be an enormous and unlikely void between the orbit of Saturn (then the most distant known planet) and the eighth sphere (the fixed stars).

Celestial pole

North Celestial Polesouth celestial polecelestial north pole
At their intersections with the celestial sphere, these form the celestial equator, the north and south celestial poles, and the ecliptic, respectively. 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 north and south celestial poles are the two imaginary points in the sky where the Earth's axis of rotation, indefinitely extended, intersects the celestial sphere.

Right ascension

RAR.A.α
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 6 h 57 m 48.86 s, 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. 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.
When paired with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system.

Diurnal motion

diurnalapparent motionappears to move
which appears to rotate westward overhead; meanwhile, Earth underfoot seems to remain still.
The daily arc path of an object on the celestial sphere, including the possible part below the horizon, has a length proportional to the cosine of the declination.

Equator

equatorial planeThe Equator
These include the Earth's equator, axis, and orbit.
The plane of Earth's equator, when projected outwards to the celestial sphere, defines the celestial equator.

Earth

Earth's surfaceterrestrialworld
which appears to rotate westward overhead; meanwhile, Earth underfoot seems to remain still. In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth.

Longitude

WestlongitudinalE
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 vertical north–south plane still intersects the plane of the Greenwich meridian at some angle; that angle is the astronomical longitude, calculated from star observations.

Classical planet

classical planetsplanetsnaked eye planets
The Eudoxan planetary model, on which the Aristotelian and Ptolemaic models were based, was the first geometric explanation for the "wandering" of the classical planets.
The word planet comes from two related Greek words, πλάνης planēs (whence πλάνητες ἀστέρες planētes asteres "wandering stars, planets") and πλανήτης planētēs, both with the original meaning of "wanderer", expressing the fact that these objects move across the celestial sphere relative to the fixed stars.

Great circle

Great Circle Routegreat circlesarcs of great circle
All parallel planes will seem to intersect the sphere in a coincident great circle
Some examples of great circles on the celestial sphere include the celestial horizon, the celestial equator, and the ecliptic.

Ecliptic

ecliptical orbitsecliptic planeplane of the ecliptic
At their intersections with the celestial sphere, these form the celestial equator, the north and south celestial poles, and the ecliptic, respectively.
If the equator is projected outward to the celestial sphere, forming the celestial equator, it crosses the ecliptic at two points known as the equinoxes.

Zenith

zenith angledirectly overheadup
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere.

Farnese Atlas

The Farnese Atlas
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.
It is the oldest extant statue of the Titan of Greek mythology, who is represented in earlier vase-painting, and more importantly, the oldest known representation of the celestial sphere.

Zodiac

signs of the zodiactropical zodiaczodiacal signs
The zodiac is an area of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year.

Equinox (celestial coordinates)

equinoxvernal equinoxMarch equinox
In astronomy, equinox is either of two places on the celestial sphere at which the ecliptic intersects the celestial equator.

Orbital plane (astronomy)

orbital planeorbital planesplane of its orbit
The ecliptic coordinate system specifies positions relative to the ecliptic (Earth's orbit), using ecliptic longitude and latitude.
By definition, the reference plane for the Solar System is usually considered to be Earth's orbital plane, which defines the ecliptic, the circular path on the celestial sphere that the Sun appears to follow over the course of a year.