3 Input Formats

The input and output coordinate systems require, variously, equinoxes and epochs. The timescale is TDB, which for most uses of COCO can be regarded as the same as UTC.

Equinoxes are Besselian or Julian epochs. They can be preceded by a B or J as appropriate; in default, epochs before 1984.0 are assumed to be Besselian, while epochs 1984.0 and after are assumed to be Julian (the distinction is usually unimportant in this context). Valid examples are B1950.0, J2000, 1975.

Epochs may either be expressed as Besselian or Julian epochs, or as year,month,day in the Gregorian calendar. Valid examples are J1984.3296 and 1985 2 13.2439. Calendar dates have to have valid years and months, but a days value outside the conventional range is permissible (e.g. 1992 12 32)

In the mean $\left[\alpha ,\delta \right]$ systems, the equinox defines the coordinate system while the epoch defines the date of observation. In the two cases where the reference frame is inertial – FK5 and galactic – the epoch is required merely to allow the proper motion to be calculated.

Each input coordinate system has its own data format, as follows.

3.1 The three sorts of Mean RA,Dec

For $\alpha$ in hours the following formats are accepted:

The first two, four, five, or six numeric fields, representing $\alpha$ and $\delta$, are mandatory. The seventh, eighth, ninth and tenth, representing proper motion, parallax and radial velocity, are optional, with the proviso that both proper motions are required if present at all. The $\left[\alpha ,\delta \right]$ fields are permitted to exceed the conventional ranges; this is to remain consistent with many existing tabulations (for example $\alpha$ 24 00 01.063 would be accepted and correctly interpreted). The scope for data validation is correspondingly limited.

Note that the proper motions are per year rather than per century. Note also that east-west proper motion is normally seconds of time (of $\alpha$) rather than arcseconds (on the sky). However, COCO will accept east-west proper motion expressed in arcseconds on the sky if the $\alpha$ proper motion is followed by the double-quote character fstring". Star data from catalogues which express proper motions in other ways (for example per century or in terms of position-angle) have to be transformed before they can be used by COCO.

In the case of FK4 coordinates (with or without E-terms), omitting proper motions is interpreted as meaning that the proper motions are assumed negligible in an inertial frame. If a star has zero proper motion in the FK4 system, then zeroes must be entered explicitly; such a star will have a real proper motion of up to 0.5 arcsec per century (just as distant sources, galaxies for example, have a fictitious proper motion in the FK4 system). In addition, because the FK4 system is rotating relative to an inertial frame, if the proper motions are omitted it is important to specify the epoch at which the position was correct.

In the case of FK5 coordinates, omission of proper motions simply implies zero proper motion (FK5 is presumed not to be rotating).

The parallax and radial velocity both default to zero. (Even when the parallax is not zero, the $\left[\alpha ,\delta \right]$ is barycentric – see Section 2, Note 6.)

For $\alpha$ in degrees (selected by the command “F D"), the following format is accepted:

Both numbers are required. There is provision for neither sexagesimal notation nor proper motion.

3.2 Geocentric Apparent

For $\alpha$ in hours the following formats are accepted:

Four, five, or six numbers are required.

For $\alpha$ in degrees (selected by the command “F D"), the following format is accepted:

Both numbers are required.

3.3 Ecliptic

Both numbers are required.

3.4 Galactic

Both numbers are required.