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Celestial Sphere

 
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Celestial sphere
 
 
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In astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
 and navigation
Navigation

Navigation is the process of reading, and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks....
, the celestial sphere is an imaginary
Imagination

Imagination is the faculty of imagining, or of forming mental images or concepts of what is not actually present to the senses, and the action or process of forming such images or concepts....
 rotating
Rotation

A rotation is a movement of an object in a circular motion. A two-dimensional object rotates around a center of rotation. A Three-dimensional space object rotates around a line called an axis....
 sphere
Sphere

A sphere is a symmetrical geometrical object. In non-mathematical usage, the term is used to refer either to a round ball or to its two-dimensional surface....
 of "gigantic radius
RADIUS

Remote Authentication Dial In User Service is a networking protocol that provides centralized access, authorization and accounting management for people or computers to connect and use a network service....
", concentric and coaxial
Coaxial

In geometry, coaxial means that two or more forms share a common Coordinate axis; it is the three-dimensional linear analog of "concentric".Coaxial cable, as a common example, has a wire Conductor in the center a circumferential outer conductor and an insulating medium called the dielectric separating these two conductors....
 with the Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
. All objects in the sky
Sky

The sky is the part of the atmosphere or of outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons....
 can be thought of as lying upon the sphere. Projected from their corresponding geographic equivalents are the celestial equator
Celestial equator

The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space....
 and the celestial pole
Celestial pole

The north and south celestial poles are the two imaginary points in the sky where the Earth axis of rotation, "infinitely extended", intersects the imaginary rotating sphere of stars called the celestial sphere....
s.






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Celestial Sphere


In astronomy
Astronomy

Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology....
 and navigation
Navigation

Navigation is the process of reading, and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks....
, the celestial sphere is an imaginary
Imagination

Imagination is the faculty of imagining, or of forming mental images or concepts of what is not actually present to the senses, and the action or process of forming such images or concepts....
 rotating
Rotation

A rotation is a movement of an object in a circular motion. A two-dimensional object rotates around a center of rotation. A Three-dimensional space object rotates around a line called an axis....
 sphere
Sphere

A sphere is a symmetrical geometrical object. In non-mathematical usage, the term is used to refer either to a round ball or to its two-dimensional surface....
 of "gigantic radius
RADIUS

Remote Authentication Dial In User Service is a networking protocol that provides centralized access, authorization and accounting management for people or computers to connect and use a network service....
", concentric and coaxial
Coaxial

In geometry, coaxial means that two or more forms share a common Coordinate axis; it is the three-dimensional linear analog of "concentric".Coaxial cable, as a common example, has a wire Conductor in the center a circumferential outer conductor and an insulating medium called the dielectric separating these two conductors....
 with the Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
. All objects in the sky
Sky

The sky is the part of the atmosphere or of outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons....
 can be thought of as lying upon the sphere. Projected from their corresponding geographic equivalents are the celestial equator
Celestial equator

The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space....
 and the celestial pole
Celestial pole

The north and south celestial poles are the two imaginary points in the sky where the Earth axis of rotation, "infinitely extended", intersects the imaginary rotating sphere of stars called the celestial sphere....
s. The celestial sphere projection is a very practical tool for positional astronomy.

The celestial sphere can be used geocentrically and topocentrically. The former means that it is centred upon an imaginary observer in the centre of the Earth, and no parallax
Parallax

Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines....
 effects need to be taken into account. In the latter case it is centred upon an observer on the surface of the Earth and then horizontal
Horizontal plane

In astronomy, geography, geometry and related sciences and contexts, a Plane is said to be horizontal at a given point if it is locally perpendicular to the gradient of the Gravitation Field , i.e., with the direction of the gravitational force at that point....
 parallax
Parallax

Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines....
 cannot always be ignored; especially not for the Moon.

In the Aristotelic
Aristotle

Aristotle was a Greeks philosopher, a student of Plato and teacher of Alexander the Great. He wrote on many subjects, including physics, metaphysics, Poetics , theater, music, logic, rhetoric, politics, government, ethics, biology and zoology....
 and Ptolemaic
Ptolemaic

Ptolemaic is the adjective formed from the name Ptolemy.This adjective is usually used in one of three ways:*To describe the Egyptian dynasty founded in 305 BC by Ptolemy I Soter...
 models, the celestial sphere was imagined as a physical reality rather than a geometrical projection (see Celestial spheres
Celestial spheres

The celestial spheres, or celestial orbs, were the fundamental celestial entities of the cosmological celestial mechanics first invented by Eudoxus, and developed by Aristotle, Ptolemy, Copernicus and others....
).

The celestial sphere is divided by projecting the equator
Equator

The equator is the intersection of the Earth's surface with the Plane perpendicular to the Earth's rotation and containing the Earth's center of mass....
 into space. This divides the sphere into the north celestial hemisphere and the south celestial hemisphere. Likewise, one can locate the Celestial Tropic of Cancer, Celestial Tropic of Capricorn, North Celestial Pole, and South Celestial Pole. The directions toward various objects in the sky can be quantified by constructing a celestial coordinate system
Celestial coordinate system

In astronomy, a celestial coordinate system is a coordinate system for mapping positions in the sky.There are different celestial coordinate systems each using a system of spherical coordinates projected on the celestial sphere, in analogy to the geographic coordinate system used on the surface of the Earth....
.

As the Earth rotates from west
West

West is most commonly a noun, adjective, or adverb indicating Direction or geography.West is one of the four cardinal directions or compass points....
 to east
East

East is a Direction in geography. It is one of the four cardinal directions or compass points, opposite of west and at right angles to north and south....
 around its axis once every 23 hours 56 minutes, the celestial sphere and all objects on it appear to rotate from east
East

East is a Direction in geography. It is one of the four cardinal directions or compass points, opposite of west and at right angles to north and south....
 to west
West

West is most commonly a noun, adjective, or adverb indicating Direction or geography.West is one of the four cardinal directions or compass points....
 around the celestial pole
Celestial pole

The north and south celestial poles are the two imaginary points in the sky where the Earth axis of rotation, "infinitely extended", intersects the imaginary rotating sphere of stars called the celestial sphere....
s in the same time. This is the diurnal motion
Diurnal motion

Diurnal motion is an astronomy term referring to the apparent daily motion of stars around the Earth, or more precisely around the two celestial poles....
. Therefore stars will rise in the east, culminate on the north-south line (meridian
Meridian (astronomy)

This article is about the astronomical concept. For other uses of the word, see meridian .In the sky, a meridian is an imaginary great circle on the celestial sphere....
) and set in the west, (unless a star is circumpolar
Circumpolar star

A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets , due to its proximity to one of the celestial poles. Circumpolar stars are therefore visible for the entire night on every night of the year ....
). On the next night a particular star will rise again, but with our normal clocks running a 24 hour 0 minutes cycle, it will do so 4 minutes earlier. By the following night the difference will be 8 minutes, and so forth with every following night (or day).

The reason for this apparent misadjustment of our clocks is that the Sun is not standing still on the celestial sphere, as the stars do, but moves about 1° per day eastwards over a great circle
Great circle

A great circle of a sphere is a circle that runs along the surface of that sphere so as to cut it into two equal halves. The great circle therefore has both the same circumference and the same center as the sphere....
 known as the ecliptic
Ecliptic

The ecliptic is the apparent path that the Sun traces out in the sky during the year. As it appears to move in the sky in relation to the stars, the apparent path aligns with the planets throughout the course of the year....
 (which is 360° or a full circle in one year, the annual motion of the Sun). As an angle of 1° corresponds to 4 minutes in time (360° = 24 hours), we need therefore 4 extra minutes of diurnal motion to see the Sun back on (for example) the meridian again, making the duration of one rotation just 24 hours exactly (on the average, ignoring small seasonal variations, see equation of time
Equation of time

The equation of time is the difference over the course of a year between time as read from a sundial and time as read from a clock, measured in an ideal situation ....
)

Normal clocks therefore indicate solar time
Solar time

Solar times are measures of the apparent position of the Sun on the celestial sphere. They are not actually the physical time, but rather hour angles, that is, angles expressed in time units....
. Astronomers studying the movements of stars may want clocks indicating sidereal time
Sidereal time

Sidereal time is a measure of the position of the Earth in its rotation around its axis, or time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical to, the motion of stars....
, going around once in 23h56m (solar time units).

A celestial sphere can also refer to a physical model of the celestial sphere. Also known as a star globe, this sort of celestial sphere will indicate which constellations are visible at a given time and place.

See also


External links