| .. _astropy-coordinates-definitions: |
|
|
| Important Definitions |
| ********************* |
|
|
| For reference, below, we define some key terms as they are used in |
| `~astropy.coordinates`, due to some ambiguities that exist in the |
| colloquial use of these terms. Chief among these terms is the concept |
| of a "coordinate system." To some members of the community, "coordinate |
| system" means the *representation* of a point in space (e.g., "Cartesian |
| coordinate system" is different from "Spherical polar coordinate |
| system"). Another use of "coordinate system" is to mean a unique |
| reference frame with a particular set of reference points (e.g., "the |
| ICRS coordinate system" or the "J2000 coordinate system"). This second |
| meaning is further complicated by the fact that such systems use quite |
| different ways of defining a frame. |
|
|
| Because of the likelihood of confusion between these meanings of |
| "coordinate system," `~astropy.coordinates` avoids this term wherever |
| possible, and instead adopts the following terms (loosely inspired by |
| the IAU2000 resolutions on celestial coordinate systems): |
|
|
| * A "Coordinate Representation" is a particular way of describing a unique |
| point in a vector space. (Here, this means three-dimensional space, but future |
| extensions might have different dimensionality, particularly if relativistic |
| effects are desired.) Examples include Cartesian coordinates, cylindrical |
| polar, or latitude/longitude spherical polar coordinates. Note that this term |
| applies to the positions, *not* their velocities or other derivatives (which |
| are represented as "differential" classes). |
|
|
| * A "Reference System" is a scheme for orienting points in a space and |
| describing how they transforms to other systems. Examples include the ICRS, |
| equatorial coordinates with mean equinox, or the WGS84 geoid for |
| latitude/longitude on the Earth. |
|
|
| * A "Coordinate Frame," "Reference Frame," or just "Frame" is a specific |
| realization of a reference system (e.g., the ICRF, or J2000 equatorial |
| coordinates). For some systems, there may be only one meaningful frame, while |
| others may have many different frames (differentiated by something like a |
| different equinox, or a different set of reference points). |
|
|
| * A "Coordinate" is a combination of all of the above that specifies a unique |
| point. |
|
|
