# equatorial coordinate system

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## equatorial coordinate system,

the most commonly used astronomical coordinate system**astronomical 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.

**.....**Click the link for more information. for indicating the positions of stars or other celestial objects on the celestial sphere

**celestial 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

**.....**Click the link for more information. . 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 equinox

**equinox**

, 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.

**.....**Click the link for more information. , 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 circle**hour 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.**.....** Click the link for more information. , 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 ascension**right 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.**.....** Click the link for more information. 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 declination**declination,**

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.**.....** Click the link for more information. 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 motion**proper 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.**.....** Click the link for more information. and precession of the equinoxes**precession 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**.....** Click the link for more information. ), such tables must be revised at regular intervals. By definition, the vernal equinox is located at right ascension 0^{h} and declination 0°.

Another useful reference point is the sigma point, the point where the observer's celestial meridian**celestial meridian,**

vertical circle passing through the north celestial pole and an observer's zenith. It is an axis in the altazimuth coordinate system.**.....** Click the link for more information. intersects the celestial equator. The right ascension of the sigma point is equal to the observer's local sidereal time**sidereal 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.**.....** Click the link for more information. . The angular distance from the sigma point to a star's hour circle is called its hour angle**hour 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**.....** Click the link for more information. ; 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

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.