Clarifications on decimal years representations

[tomdonaldson]

(This suggestion was first made on the Applications mail list.)

In section 3.5:
Usage of besselian or julian years for timestamps apart from making timeorigin un-useful (and then forbidden) also implies some inference on the timescale used (see the TIME WCS paper for references and discussion). It has been suggested to add the the following sentence at the end of timeorigin explanation: "When using calendar epochs written in yr or Ba, note that conventionally Julian years are tied to the TDB timescale and Besselian years to ET (written here as TT) (Rots and Bunclark et al., 2015)."

On a related topic, in 3.4 (COOSYS):
We don't explicitly constrain the representation of time in equinox and epoch attributes. This however clear in the xml schema. The suggestion is to add after "... the epoch of the positions if necessary", the following sentence: " Both equinox and epoch MUST be expressed in the Julian or Besselian calendar".