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In astronomy, declination (abbreviated dec; symbol δ) is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. The declination angle is measured north (positive) or south (negative) of the celestial equator, along the hour circle passing through the point in question. [1]
In the star chart to the right, declination is shown by the radial coordinate, starting at 90° north in the center and decreasing to 30° north at the outer edge. Sidereal hour angle is shown as the angular coordinate, starting at 0° at the left of the chart, and increasing counter-clockwise.
Diagram illustrating the definition of the hour circle of a star In astronomy , the hour circle is the great circle through a given object and the two celestial poles . [ 1 ] Together with declination and distance (from the planet's centre of mass ), it determines the location of any celestial object .
Star position is the apparent angular position of any given star in the sky, which seems fixed onto an arbitrary sphere centered on Earth. The location is defined by a pair of angular coordinates relative to the celestial equator: right ascension (α) and declination (δ). This pair based the equatorial coordinate system.
Ian Ridpath's Star Tales: Constellation Mythology and History – Ian Ridpath's Star Tales. VizieR – CDS's archive of constellation boundaries. The text file constbnd.dat gives the 1875.0 coordinates of the vertices of the constellation regions, together with the constellations adjacent to each boundary segment.
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 .
Chart of the southern constellations from declination –40° to the south celestial pole by the Jesuit missionary François Noël published in Acta Eruditorum, 1711.. From the South Pole, in good visibility conditions, the Southern Sky features over 2,000 fixed stars that are easily visible to the naked eye, while about 20,000 to 40,000 with the aided eye.
The first star catalog to use right ascension and declination was John Flamsteed's Historia Coelestis Britannica (1712, 1725). The entire sky, divided into two halves. Right ascension (blue) begins at the March equinox (at right, at the intersection of the ecliptic (red) and the equator (green)) and increases eastward (towards the left).