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 $[\alpha,\delta ]$ 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.