In astronomy , the declination δ ( Latin for “deviation, diffraction”) is one of the two spherical coordinates . They are used to indicate the position of celestial bodies on the imaginary celestial sphere . For a complete position specification, the right ascension or the hour angle is also required as the second spherical coordinate .
In the equatorial coordinate system , the declination is the elevation angle above the celestial equator - with which it corresponds to the geographical latitude φ north or south of the earth's equator . The values of δ north of the celestial equator are given as positive, south of the celestial equator as negative; the two celestial poles are + 90 ° and −90 °.
The circle parallel to the celestial equator on which the respective celestial body is located is called the declination circle (also celestial latitude ). One can imagine the declination circles as a projection of the latitudes of the earth's surface onto the imaginary celestial sphere.
The declination values for fixed stars are essentially constant and independent of the observation location. However, they change in the long term due to the proper movement of the stars and the changes in the celestial equator due to the precession of the earth's axis . Therefore, the equinox must also be given for precise information .
The declination of the sun fluctuates over the course of the year, due to the inclination of the earth's axis (or inclination of the ecliptic against the celestial equator) between (as of December 2014 or 2015) + 23.43 ° (at the summer solstice in June) and −23.43 ° (at the winter solstice in December). The same applies to the moon and the planets , which are also approximately in the plane of the ecliptic.