Even when viewed ideally at sea level, the earth's surface is not equally far from the center of the earth at all points, since the earth is not a geometric sphere . Rather, the earth figure can be described much better approximately by means of an ellipsoid of revolution. The diameter of the earth, which is over 12,700 kilometers, is therefore about 42 km less between the poles than the diameter at the equator . The earth figure is described even more precisely by the geoid .
- topocentric : related to the point of view of the observer ( Greek τόπος, "the place"), i.e. ultimately for all measurements and sensory impressions,
- barycentric : related to the earth-moon center of gravity ; it partly also means the center of gravity of the planetary system , e.g. B. for the atomic time scale TAB ( temps atomique barymetrique )
- Heliocentric : related to the center of the sun , which differs from the center of gravity of the planetary system by 0.5 to 1.7 solar radii.
- galactic ( galactocentric ): related to the center of gravity of the Milky Way (approx. 35,000 light years away from us).
- inertial : related to a non-accelerated, non-rotating coordinate system in one of these 5 possible centers.
In the geocentric coordinate system measured data include:
- all calculations of satellite orbits , rockets and near-earth space probes ,
- most calculations of star coordinates ("apparent star locations "),
- geocentric coordinates of a few thousand global survey points (also called Cartesian coordinates )
- geocentric latitudes that differ from geographic latitudes by up to 0.2 ° or 20 km.
The geocentric and bary- or heliocentric directions to celestial bodies differ depending on their distance. In the case of planets , values can be up to 180 ° (e.g. during a solar eclipse or Venus passage ), in the case of nearby stars up to 0.7 ″ ( parallax of Alpha Centauri ).
The geocentric worldview
The " geocentric worldview " concerns another aspect of the word. It is the interpretation of astronomical phenomena from the point of view of an observer on earth, the size of which is considered to be almost zero compared to the sphere ( celestial sphere ). Therefore this system can be both geocentric in the above sense and topocentric .
The geocentric worldview was widespread from antiquity to the Renaissance and was replaced by the heliocentric worldview in the 17th century.