![]() ![]() Copyright © 2023, Columbia University Press. The Columbia Electronic Encyclopedia, 6th ed. 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. 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. The right ascension of the sigma point is equal to the observer's local sidereal time. By definition, the vernal equinox is located at right ascension 0 h and declination 0°.Īnother useful reference point is the sigma point, the point where the observer's celestial meridian intersects the celestial equator. Because a star's position may change slightly (see proper motion and precession of the equinoxes), such tables must be revised at regular intervals. The right ascensions and declinations of many stars are listed in various reference tables published for astronomers and navigators. Right ascension and declination together determine the location of a star on the celestial sphere. 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. ![]() ![]() Next the observer measures along the star's hour circle the angle between the celestial equator and the position of the star. (There are 360 degrees or 24 hours in a full circle.) The right ascension is always measured eastward from the vernal equinox. ![]() 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. To astronomers, Local Sidereal Time is useful because it corresponds to the coordinate of Right Ascension of celestial bodies that are presently on the local. The astronomer then measures the angle between the vernal equinox and the point where the hour circle intersects the celestial equator. This is the star's hour circle, analogous to a meridian of longitude on earth. 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. The proper motion vector is, right ascension, declination, position angle. An important reference point on the celestial equator is the vernal equinox, the point at which the sun crosses the celestial equator in March. North declination is considered positive and. the angular distance of the hour circle of a celestial body from the vernal equinox, measured eastward along the celestial equator and expressed in degrees (from 0 to 360) or, more commonly, in hours (from 0 to 24), minutes, and seconds: it is used with declination (sense 6) to find an exact position in the sky: abbrev. The Greenwich Meridian was defined as the zero of longitude for. It divides the celestial sphere into the northern and southern skies. Declination and right ascension, an east-west coordinate, together define the position of an object in the sky. And by analogy with longitude, we need to define a point as the zero of Right Ascension. 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. Dec measures north and south on the celestial sphere and is like latitude on the Earth. RA measures east and west on the celestial sphere and is like longitude on the Earth. 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. RA (right ascension) and Dec (declination) are the coordinates on the sky that correspond to longitude and latitude on Earth. It represents the entire sky all celestial objects other than the earth are imagined as being located on its inside surface. The celestial sphere is an imaginary sphere with the observer at its center. Equatorial coordinate system, the most commonly used astronomical coordinate system for indicating the positions of stars or other celestial objects on the celestial sphere. declination, in astronomy, the angular distance of a body north or south of the celestial equator. ![]()
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