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Right ascension is the celestial equivalent of terrestrial longitude. Both right ascension and longitude measure an angle from a primary direction (a zero point) on an equator. Right ascension is measured from the Sun at the March equinox i.e. the First Point of Aries, which is the place on the celestial sphere where the Sun crosses the ...
As seen from above the Earth's north pole, a star's local hour angle (LHA) for an observer near New York (red dot). Also depicted are the star's right ascension and Greenwich hour angle (GHA), the local mean sidereal time (LMST) and Greenwich mean sidereal time (GMST). The symbol ʏ identifies the March equinox direction.
Model of the equatorial coordinate system. Declination (vertical arcs, degrees) and hour angle (horizontal arcs, hours) is shown. For hour angle, right ascension (horizontal arcs, degrees) can be used as an alternative. The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects.
Proper motion may alternatively be defined by the angular changes per year in the star's right ascension ( μα) and declination ( μδ) with respect to a constant epoch . The components of proper motion by convention are arrived at as follows. Suppose an object moves from coordinates (α 1, δ 1) to coordinates (α 2, δ 2) in a time Δ t.
A star's direction remains nearly fixed due to its vast distance, but its right ascension and declination do change gradually due to precession of the equinoxes and proper motion, and cyclically due to annual parallax. The declinations of Solar System objects change very rapidly compared to those of stars, due to orbital motion and close proximity.
Polaris is the brightest star in the constellation of Ursa Minor (upper right). Big Dipper and Ursa Minor in relation to Polaris. A view of Polaris in a small telescope. Polaris B is separated by 18 arc seconds from the primary star, Polaris A. A 4 day time lapse of Polaris illustrating its Cepheid type variability.
Capella is the brightest star in the constellation of Auriga (upper left). α Aurigae ( Latinised to Alpha Aurigae) is the star system's Bayer designation. It also has the Flamsteed designation 13 Aurigae. It is listed in several multiple star catalogues as ADS 3841, CCDM J05168+4559, and WDS J05167+4600. As a relatively nearby star system ...
Astronomical coordinate systems. A star 's galactic, ecliptic, and equatorial coordinates, as projected on the celestial sphere. Ecliptic and equatorial coordinates share the March equinox as the primary direction, and galactic coordinates are referred to the galactic center. The origin of coordinates (the "center of the sphere") is ambiguous ...