Equatorial coordinate system

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so-called rotating equatorial coordinate system
So-called fixed equatorial coordinate system

The equatorial coordinate system refers to two geocentric coordinate systems used in spherical astronomy on the celestial sphere , the rotating and the stationary equatorial coordinate system. The reference plane is in both cases the level of the celestial equator . The point of reference on the celestial equator is the spring equinox for the rotating system , and the southern intersection between the celestial equator and the meridian of the observation location for the stationary system .


The position of an object in the sky, i.e. on the imaginary celestial sphere in the middle of which the earth is located, is indicated by two polar coordinates from the earth:

1. Spring point as a reference point: For observers on earth, the starry sky rotates around the earth once a day with the spring point. That is why we speak of a rotating system .

  • Declination ( angular distance from the equatorial plane; positive sign for the northern hemisphere )
  • Right ascension (angular distance from the vernal equinox; a star with a larger right ascension travels later through the meridian than one with a smaller right ascension)

2. Meridional section of the observer as a reference point: so-called stationary system (as long as the observer is stationary)

  • Declination (angular distance from the equatorial plane)
  • Hourly angle (angular distance from the meridian, increasing from 0 ° to the west, decreasing from 360 ° to the east)

See also


  1. Since the earth is infinitely small compared to the celestial sphere, every point on the earth's surface can be viewed as the center of the celestial sphere.
  2. The spring equinox shifts slowly against the starry sky due to the precession , which must be taken into account when using it away from the present.