astronomical coordinate systems

(astronomy) Any system of spherical coordinates serving to locate astronomical objects on the celestial sphere.

Schemes for locating astronomical objects in space. To an observer on the Earth's surface, the stars of the night sky appear to be placed upon a spherical shell of infinite radius with the observer at the center. Celestial objects appear to move with respect to the stars, and at any given time their position on this imaginary sphere, called the celestial sphere, can be specified by two angles, called celestial coordinates, whose values depend upon what coordinate system is used. See also Celestial sphere.

Each coordinate system is defined by a fundamental plane and a principal axis. For example, on the Earth's surface, longitude and latitude coordinates are used to determine positions. In this system, which is analogous to astronomical coordinate systems, the fundamental plane is that of the Earth's Equator, and the principal axis is defined by a line running from the Earth's center to a point on the Equator at the longitude of Greenwich, England. See also Equatorial currents; Latitude and longitude.

Horizon system

The boundary between the hemisphere of the sky that is visible and the hemisphere which is hidden from view by the Earth is called the horizon. The observer is located at the center of the horizon system, the pole directly overhead is termed the zenith, and the opposite pole, the nadir. These pole directions are aligned with a plumb line, which is determined by the observer's local gravity. The fundamental horizon plane is 90° from the poles, and for astronomical applications the principal axis is most often taken to pass through the north point. Great circles that pass through the zenith and nadir are termed vertical circles; the one passing through the east and west points is termed the prime vertical, and that passing through the north and south points is called the celestial meridian. The longitudinal coordinate of a celestial object is termed its azimuth and is most often measured eastward from the north point to the object's vertical circle; and the latitudinal coordinate, termed its altitude, is measured along the object's vertical circle, north or south from the horizon plane to the object. See also Horizon; Zenith.

Equatorial system

The fundamental plane of the equatorial coordinate system can be visualized by imagining that the Earth's equatorial plane is extended to intersect the celestial sphere. An alternate fundamental plane, the ecliptic plane, is the extension of the Earth's mean orbital plane onto the celestial sphere (see illustration). These planes intersect at two points, called equinoxes, with the angle between them ε being termed the obliquity of the ecliptic. This angle is about 23.4°. See also Ecliptic; Equinox.

Equatorial system of astronomical coordinates.

Due to the Earth's motion about the Sun, observers on Earth see an apparent motion of the Sun along the ecliptic plane. The point where the Sun's annual apparent motion takes it northward across the equatorial plane is called the vernal equinox Υ, and the line between the Earth's center and this point defines the principal axis for both the equatorial and ecliptic coordinate systems. The apparent passage of the Sun through the vernal equinox, on about March 21, marks the beginning of spring in the Northern Hemisphere. Because of disturbing effects of the Sun and Moon on the Earth's figure, the Earth's rotation axis precesses, causing the celestial pole to describe an approximate circular motion about the ecliptic pole once every 26,000 years. This causes the location of the vernal equinox to drift westward along the ecliptic about 50 arc-seconds each year. Hence for an inertial coordinate system, where the principal axis is not moving, an epoch must be specified at which time the coordinate system is held fixed. In practice, the beginning of the year 2000 is most often used as an epoch. See also Earth rotation and orbital motion; Precession of equinoxes.

The north and south celestial poles represent the extension of the Earth's North and South poles onto the celestial sphere. For a celestial object (for example, object X in the illustration), the longitudinal coordinate is termed the right ascension α and is measured eastward along the celestial equator from the vernal equinox Υ to the great circle passing through the object and the north and south celestial poles. The latitudinal coordinate, called the declination δ, is then measured along the object's great circle, north or south from the Equator to the object.

Ecliptic system

The ecliptic coordinate system is often used when representing the orbital motions of the planets, asteroids, and comets. The fundamental plane is that of the ecliptic, and as in the equatorial system, the principal axis is the line extending from the Earth's center to the vernal equinox.

Galactic system

Astronomers working with stars and other objects within the Milky Way Galaxy often find it convenient to use the galactic disk as the fundamental plane of their coordinate system, and the line extending from the galactic center to the Sun's location as the principal axis. See also Milky Way Galaxy.