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., the component of motion across the line of sight to the star; it does not include the component of motion toward or away from the sun. The most distant stars show the least proper motion. Barnard's Star, one of the closest stars, has the largest measured proper motion, 10.27 sec of arc per year. The average proper motion of the stars that can be seen with the naked eye is 0.1" per year.

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.

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.

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. A star's hour circle is used in determining its right ascension and declination.

Galactic coordinate system

Galactic coordinate system, astronomical coordinate system in which the principal axis is the galactic equator (the intersection of the plane of the Milky Way with the celestial sphere) and the reference points are the north galactic pole and the zero point on the galactic equator; the coordinates of a celestial body are its galactic longitude and galactic latitude. In the IAU galactic coordinate system, introduced in 1958 by the International Astronomical Union, the zero point on the galactic equator has the equatorial coordinates R.A. 17h39.3m and Dec. −28°55'; this lies in the direction of the center of our galaxy, the Milky Way.

Ecliptic coordinate system

Ecliptic coordinate system, an astronomical coordinate system in which the principal coordinate axis is the ecliptic, the apparent path of the sun through the heavens. The ecliptic poles are the two points at which a line perpendicular to the plane of the ecliptic through the center of the earth strikes the surface of the celestial sphere. The north ecliptic pole lies in the constellation Draco.

Celestial horizon

Celestial horizon, one axis of the altazimuth coordinate system. It is the great circle on the celestial sphere midway between the observer's zenith and nadir; it divides the celestial sphere into two equal hemispheres. The observer may be unable to see all the stars that lie above his celestial horizon because of obstructions such as buildings, trees, or mountains; he may be able to see some stars that lie below his celestial horizon because of atmospheric refraction.