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Astronomical Coordinates

Astronomical Coordinates

The official classical sequence of [RA,Dec] coordinate systems, using J2000.0 as an example,is:

Astronomical Coordinates Full

Neglecting proper motion and various sub-arcsecond effects, the sequence can be simplified to:

Astronomical Coordinates Simple

  1. The term "JNow", unknown in professional circles, is confusing, because the "J" implies that JNow is the "today" equivalent of "J2000". It isn't - even at exactly midday on 1st January 2000 (the J2000 origin date/time), stars weren't at their J2000.0 positions, but up to 30 arcseconds away, because of aberration and nutation.

    For this reason, use of the term "JNow" is not recommended. If you do come across it, it will most likely refer to observed coordinates.

  2. When discussing coordinates, the term "Local Topocentric" is sometimes seen. The intention of the phrase is to refer to an object's coordinates at the current date/time, however, the term "Topocentric" already carries this meaning and prefixing it with "Local" adds no further information.

    For this reason, use of the term "Local Topocentric" is not recommended, "Topocentric" does the job more concisely.

    Historically, the ASCOM EquatorialCoordinate Type enumeration contained the member "equLocalTopocentric" and this name has now been deprecated in favour of the new member "equTopocentric". This is a non-breaking change and continued use of equLocalTopocentric will just show up as a warning when compiling ASCOM drivers and applications.

  1. When designing your driver you need to choose which type of coordinates you will accept and the ITelescopeV3 interface provides the DoesRefraction and EquatorialSystem properties for this purpose. The full range of coordinate systems recognised are described here: EquatorialCoordinateType. Usually you would accept one or more of J2000, Topocentric or Observed coordinates at your discretion. To specify a particular coordinate type you must set the properties as below:

    If you wish the user to be able to select the coordinate type that your driver will receive, you can implement Set methods for the EquatorialSystem and DoesRefraction properties. Your driver / mount should then process the coordinates as described above.

  2. "Apparent place" differs from topocentric because of diurnal aberration, a sub-arcsecond effect.

  3. Modern "ICRS" coordinates are the same as J2000.0 mean place to better than 0.025 arcsec.

See Also