# equatorial coordinate system

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Related to Equatorial Coordinates: Celestial coordinates

## equatorial coordinate system,

the most commonly used astronomical coordinate systemastronomical coordinate systems.
A coordinate system is a method of indicating positions. Each coordinate is a quantity measured from some starting point along some line or curve, called a coordinate axis.
for indicating the positions of stars or other celestial objects on the celestial spherecelestial sphere,
imaginary sphere of infinite radius with the earth at its center. It is used for describing the positions and motions of stars and other objects. For these purposes, any astronomical object can be thought of as being located at the point where the line of sight
. The celestial sphere is an imaginary sphere with the observer at its center. It represents the entire sky; all celestial objects other than the earth are imagined as being located on its inside surface. If the earth's axis is extended, the points where it intersects the celestial sphere are called the celestial poles; the north celestial pole is directly above the earth's North Pole, and the south celestial pole directly above the earth's South Pole. The great circle on the celestial sphere halfway between the celestial poles is called the celestial equator; it can be thought of as the earth's equator projected onto the celestial sphere. It divides the celestial sphere into the northern and southern skies. An important reference point on the celestial equator is the vernal equinoxequinox
, either of two points on the celestial sphere where the ecliptic and the celestial equator intersect. The vernal equinox, also known as "the first point of Aries," is the point at which the sun appears to cross the celestial equator from south to north.
, the point at which the sun crosses the celestial equator in March.

To designate the position of a star, the astronomer considers an imaginary great circle passing through the celestial poles and through the star in question. This is the star's hour circlehour circle,
in astronomy, a secondary axis in the equatorial coordinate system. The hour circle of a celestial body is the great circle on the celestial sphere that passes through both the body and the north celestial pole.
, analogous to a meridian of longitude on earth. The astronomer then measures the angle between the vernal equinox and the point where the hour circle intersects the celestial equator. This angle is called the star's right ascensionright ascension,
in astronomy, one of the coordinates in the equatorial coordinate system. The right ascension of a celestial body is the angular distance measured eastward from the vernal equinox along the celestial equator to its intersection with the body's hour circle.
and is measured in hours, minutes, and seconds rather than in the more familiar degrees, minutes, and seconds. (There are 360 degrees or 24 hours in a full circle.) The right ascension is always measured eastward from the vernal equinox. Next the observer measures along the star's hour circle the angle between the celestial equator and the position of the star. This angle is called the declinationdeclination,
in astronomy, one of the coordinates in the equatorial coordinate system. The declination of a celestial body is its angular distance north or south of the celestial equator measured along its hour circle.
of the star and is measured in degrees, minutes, and seconds north or south of the celestial equator, analogous to latitude on the earth. Right ascension and declination together determine the location of a star on the celestial sphere. The right ascensions and declinations of many stars are listed in various reference tables published for astronomers and navigators. Because a star's position may change slightly (see proper motionproper motion,
in astronomy, apparent movement of a star on the celestial sphere, usually measured as seconds of arc per year; it is due both to the actual relative motions of the sun and the star through space. Proper motion reflects only transverse motion, i.e.
and precession of the equinoxesprecession of the equinoxes,
westward motion of the equinoxes along the ecliptic. This motion was first noted by Hipparchus c.120 B.C. The precession is due to the gravitational attraction of the moon and sun on the equatorial bulge of the earth, which causes the earth's axis to
), such tables must be revised at regular intervals. By definition, the vernal equinox is located at right ascension 0h and declination 0°.

Another useful reference point is the sigma point, the point where the observer's celestial meridiancelestial meridian,
vertical circle passing through the north celestial pole and an observer's zenith. It is an axis in the altazimuth coordinate system.
intersects the celestial equator. The right ascension of the sigma point is equal to the observer's local sidereal timesidereal time
(ST), time measured relative to the fixed stars; thus, the sidereal day is the period during which the earth completes one rotation on its axis so that some chosen star appears twice on the observer's celestial meridian.
. The angular distance from the sigma point to a star's hour circle is called its hour anglehour angle,
in astronomy, a coordinate in the equatorial coordinate system. The hour angle of a celestial body is the angular distance, expressed in hours, minutes, and seconds (one hour equals 15 degrees), measured westward along the celestial equator from the observer's
; it is equal to the star's right ascension minus the local sidereal time. Because the vernal equinox is not always visible in the night sky (especially in the spring), whereas the sigma point is always visible, the hour angle is used in actually locating a body in the sky.

equatorial coordinate system

## equatorial coordinate system

The most widely used astronomical coordinate system in which the fundamental reference circle is the celestial equator and the zero point is, strictly, a catalog equinox, or less formally the vernal (or dynamical) equinox. The coordinates are right ascension (α) and declination (δ), which are measured along directions equivalent to those of terrestrial longitude and latitude, respectively (see illustration). Sidereal hour angle is sometimes used instead of right ascension and north or south polar distance instead of declination. Because of the slow westerly drift of the dynamical equinox around the ecliptic the coordinates right ascension and declination are normally referred to the mean equator and equinox for a standard epoch, which in present use is 2000.0.

References in periodicals archive ?
We can't imagine that the measurement of equatorial coordinates used by ancient Inca Astronomers was much simpler.
The second way of specifying star positions is the equatorial coordinate system.
The Incas adopted the equatorial coordinate system, or the right ascension and the declination to show position of the 7 brightest stars in the Pleiades cluster on the celestial sphere.
Like the Mayas, the adoption of the equatorial coordinate system was a unique contribution of the Incas to pre-Columbian astronomy, as ancient astronomers all used the ecliptic coordinate system to mark the position of fixed stars.
3D Cartesian geocentric equatorial coordinates are tied to the geodetic coordinate system.
The object's 3D location determined in the horizontal coordinate system is tied to the applied Cartesian geocentric equatorial coordinates of the geodetic coordinate system.
The horizontal coordinates of the object to be determined are transformed into the 3D Cartesian geocentric equatorial coordinate system (Jakucionis et al.
Finally, the apparent equatorial coordinates are calculated from the following relations:
Equatorial coordinates [alpha] and [delta] used in (2) can be determined on the basis of apparent coordinates [alpha]CIRS and [[delta].
where [alpha] and [delta] are the catalogue star positions in the equatorial coordinate system.
equatorial coordinates of centre of rotation ([[alpha].
applying the inclination corrections to obtain zenith equatorial coordinates ([[alpha].

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