# Topocentric

Systems of reference are called **topocentric** (Greek τοπος *topos* , place) if their origin (or another point) is tied to a place that is mostly on the surface of a celestial body . Typically two of the three coordinate axes are oriented horizontally (see Astronomical Coordinate **Systems** ), and the location selected as reference, the **topocenter** , is also the location of an observer or a measuring instrument.

Topocentric reference systems are relative coordinate systems and are conceptually *in opposition* to

- the coordinate systems that are tied to a focal point ( geocentric , heliocentric, etc.)
- the free falling inertial systems
- the vehicle coordinate systems .

If you choose straight lines for the horizontal axes , then there is an approximate correspondence with the surface of the assigned celestial body only for short distances. If, on the other hand, one chooses a curved axis course (often an ellipse and thus finite axis lengths), the result is a surface or space description in the manner of a geodetic datum .

In particular, objects that are further away are often only described in the direction of azimuth (cardinal direction along the plane) and elevation angle (vertical angle ) in the sense of a local celestial hemisphere .

If the goal is to observe the sky , then the unobstructed view that can actually be achieved may depend heavily on the selected location, i.e. H. mostly somewhat restricted, but in a few special cases it can be much more than a hemisphere.

By repeating angle measurements at different, known times, object distances for quasi-static objects in space can also be determined in practice for a known movement and rotation of the topocenter in space. It is also possible to use this, for. B. to determine planetary orbits and many other astronomical quantities.

The dominance of the topocentric view, which is imposed on every observer of the sky (including astronomy , cf. astronomical coordinate systems ), is called **topocentrism** . For example, we speak of sunrise and sunset .

Examples of technical terms relating to topocentric coordinates are elevation angle , sidereal time, and geomagnetic inclination .