WCS question

[Patrick Wallace]

On Mon, 9 Nov 1998, Arnold Rots wrote:

> Be careful!  There are three elements to the definition of a reference
> frame: the epoch (2000), J or B, and the reference frame itself (FK5,
> ICRS).

No longer true for ICRF.

> However, "J2000" does not imply FK5 (as you imply above).

Maybe not, strictly speaking, but in practice "J2000" is always taken
to mean "in accordance with the IAU 1976 recommendations", which, as
well as introducing J2000 as the fundamental epoch, brought in a
specific precession model (Lieske's) and frame (FK5).

> Instead, the IAU recommendation is to move from J2000-FK5 to J2000-ICRS.

The J2000 is superfluous in the case of ICRS.  You only need epochs
when the system depends on a model of precession.  ICRS has no ecliptic
or equinox.  It is fixed for all time in the extragalactic background,
liberated from considerations of the Earth's orientation or orbit.  If
you wrote J2050-ICRS it wouldn't mean anything.


Patrick Wallace
____________________________________________________________________________
Starlink Project Manager                        Internet:  ptw at star.rl.ac.uk
Rutherford Appleton Laboratory                       Tel:    +44-1235-445372
Chilton, Didcot, Oxon OX11 0QX, UK                   Fax:    +44-1235-446667
____________________________________________________________________________