celestial equator

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equatorial coordinate system

equatorial coordinate system, the most commonly used astronomical coordinate system for indicating the positions of stars or other celestial objects on the celestial sphere. 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, 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, 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 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 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 and precession of the equinoxes), 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 meridian intersects the celestial equator. The right ascension of the sigma point is equal to the observer's local sidereal time. The angular distance from the sigma point to a star's hour circle is called its hour angle; 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.

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celestial equator

The great circle in which the extension of the Earth's equatorial plane cuts the celestial sphere. The plane of the celestial (and hence terrestrial) equator is perpendicular to the celestial axis and is the reference plane for the equatorial coordinates right ascension and declination. The orientation of the celestial equator is slowly changing as a result of the precession of the Earth's axis.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006

Celestial Equator

(religion, spiritualism, and occult)

The celestial equator, also termed the equinoctial, is the terrestrial equator imaginarily projected outward from Earth and onto the background of the stars (i.e., against the backdrop of the celestial sphere). Because of the tilt of the Earth on its axis, the celestial equator intersects the ecliptic at an angle of 23½°. Similarly, the celestial poles are the north and south poles projected outward against the backdrop of the celestial sphere.

The Astrology Book, Second Edition © 2003 Visible Ink Press®. All rights reserved.

celestial equator

[sə′les·chəl i′kwād·ər]
The primary great circle of the celestial sphere in the equatorial system, everywhere 90° from the celestial poles; the intersection of the extended plane of the equator and the celestial sphere. Also known as equinoctial.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

celestial equator

celestial equator
The great circle of the celestial sphere, in which all points are 90° from the poles. It is the plane of the earth's equator projected onto the celestial sphere. Also called equinoctial.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved

celestial equator

the great circle lying on the celestial sphere the plane of which is perpendicular to the line joining the north and south celestial poles
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005