Celestial coordinates in catalogues

Most astronomical catalogues contain columns of celestial coordinates of some sort: usually Right Ascension and Declination for some equinox and epoch, or perhaps Galactic or ecliptic coordinates. The storage, manipulation and presentation for display of celestial coordinates in the computer-readable version of astronomical catalogues is something of a vexed topic which has caused a deal of confusion and difficulty, much of it, in principle, unnecessary. Celestial coordinates are angles. The basic conundrum in storing and processing them is as follows:

• for use in computations inside a program the only sensible way to represent coordinates is as DOUBLE PRECISION or REAL numbers expressed in radians,

• for display to a user the coordinates should almost invariably be expressed in units of hours or degrees and formatted as a sexagesimal value, with subdivisions into minutes and seconds.

For preexisting catalogues the format of the celestial coordinates will already be fixed and they must simply be used in whatever way is possible. For example, many catalogues contain the hours, or degrees, minutes and seconds which comprise a coordinate as separate columns; a form which is singularly inconvenient for further processing. However, CAT has some special facilities for processing and displaying angles which conveniently and automatically provide for:

• representation inside a program as a DOUBLE PRECISION or REAL number expressed in radians,

• representation for display to a user as a CHARACTER string expressed in hours or degrees and formatted as a sexagesimal value.

This facility works as follows:

• when the value for a field in a column containing angles is obtained using
  CAT_EGT0D or CAT_EGT0R it is returned as a DOUBLE PRECISION or REAL value in radians,

• when the value for a field in a column containing angles is obtained formatted for display using CAT_EGT0F it is returned as a CHARACTER string containing the value expressed in hours or degrees and formatted as a sexagesimal value.

In order to use this facility CAT must know that the column contains an angle and the units (hours or degrees) and format to use when formatting the angle for display. The prescription for creating a column of angles in a CAT catalogue is as follows.

1. Create the column with data type DOUBLE PRECISION.

2. The UNITS attribute should start with the string RADIANS to indicate that the column contains an angle in radians and this should be followed by a specifier enclosed in curly brackets `{}' indicating how it is to be formatted for display. For example:

   RADIANS{HOURS}

   if it is to be displayed in hours,

   RADIANS{DEGREES}

   if it is to be displayed in degrees.

   Specifying `HOURS' will cause the angle to be displayed in hours, minutes and seconds, with the seconds displayed to one place of decimals; `DEGREES' will cause the angle to be displayed in degrees, minutes and seconds, with the seconds displayed as whole numbers⁶.

3. When writing the table of values for the catalogue, express the angles in radians as DOUBLE PRECISION variables and write them to the catalogue using CAT_PUT0D.

An example program is available which illustrates the creation of columns containing angular celestial coordinates. It creates a catalogue containing (fake) equatorial coordinates and B magnitudes for a set of stars. It is called EXAMPLE_ANGLES and the source code is available in file:

/star/share/cat/example_angles.f

The overall structure is very similar to the previous example of writing a catalogue,
EXAMPLE_WRITE, but note the way that the three points outlined above are followed in order to create the angular columns.