The primary direction (the x axis) is the vernal equinox. The fundamental plane is the plane of the Earth's equator. Setting circles in conjunction with a star chart or ephemeris allow the telescope to be easily pointed at known objects on the celestial sphere. Telescopes equipped with equatorial mounts and setting circles employ the equatorial coordinate system to find objects. In contrast, in the horizontal coordinate system, a star's position differs from observer to observer based on their positions on the Earth's surface, and is continuously changing with the Earth's rotation. The direction of sufficiently distant objects is the same for all observers, and it is convenient to specify this direction with the same coordinates for all. Spherical coordinates Use in astronomy Ī star's spherical coordinates are often expressed as a pair, right ascension and declination, without a distance coordinate. Note that there is no "mean ecliptic", as the ecliptic is not subject to small periodic oscillations. This is the actual intersection of the two planes at any particular moment, with all motions accounted for.Ī position in the equatorial coordinate system is thus typically specified true equinox and equator of date, mean equinox and equator of J2000.0, or similar.
#Right ascension definition astronomy plus#
True equinox of date is the intersection of the ecliptic of "date" with the true equator (that is, the mean equator plus nutation). Commonly used in planetary orbit calculation.
#Right ascension definition astronomy free#
Mean equinox of date is the intersection of the ecliptic of "date" (that is, the ecliptic in its position at "date") with the mean equator (that is, the equator rotated by precession to its position at "date", but free from the small periodic oscillations of nutation).
Mean equinox of a standard epoch (usually J2000.0, but may include B1950.0, B1900.0, etc.) is a fixed standard direction, allowing positions established at various dates to be compared directly. In order to fix the exact primary direction, these motions necessitate the specification of the equinox of a particular date, known as an epoch, when giving a position. Superimposed on this is a smaller motion of the ecliptic, and a small oscillation of the Earth's axis, nutation. A slow motion of Earth's axis, precession, causes a slow, continuous turning of the coordinate system westward about the poles of the ecliptic, completing one circuit in about 26,000 years. This description of the orientation of the reference frame is somewhat simplified the orientation is not quite fixed. See also: Axial precession and Astronomical nutation
A right-handed convention means that coordinates increase northward from and eastward around the fundamental plane. The fundamental plane and the primary direction mean that the coordinate system, while aligned with Earth's equator and pole, does not rotate with the Earth, but remains relatively fixed against the background stars. The origin at the centre of Earth means the coordinates are geocentric, that is, as seen from the centre of Earth as if it were transparent. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fundamental plane consisting of the projection of Earth's equator onto the celestial sphere (forming the celestial equator), a primary direction towards the vernal equinox, and a right-handed convention. The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. (Two such coordinate pairs are shown here.) Projections of the Earth's north and south geographic poles form the north and south celestial poles, respectively. Right ascension is measured eastward along the celestial equator from the equinox, and declination is measured positive northward from the celestial equator. The primary direction is established by projecting Earth's orbit onto the celestial sphere, forming the ecliptic, and setting up the ascending node of the ecliptic on the celestial equator, forming the vernal equinox. The fundamental plane is formed by projection of Earth's equator onto the celestial sphere, forming the celestial equator. The equatorial coordinate system using spherical coordinates.
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