00 What is the Equatorial Coordinate System?

Equatorial coordinate system – Wikipedia

In spherical astronomy, the equatorial coordinate system is two geocentric coordinate systems on the celestial sphere, each of which has the plane of the celestial equator as its reference plane.

The position of an object in the sky, thus on the imaginary celestial sphere, in whose center the earth is, is indicated in each case by two polar coordinates from the earth.

Rotating equatorial coordinate system

The rotating equatorial coordinate system has the vernal point on the celestial equator as reference point. For observers on Earth, the starry sky with the vernal equinox rotates once around the Earth every day, therefore it is called a rotating system.

Coordinates of this system are:

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

Stationary equatorial coordinate system

The stationary equatorial coordinate system has as reference point the southern intersection of the celestial equator and the meridian of the observing site.

Coordinates of this system are:

  • Declination: angular distance from the equatorial plane.
  • Hour angle: angular distance from the meridian, increasing to the west from 0°, decreasing to the east from 360°.