WCS question

[Patrick Wallace]

On Mon, 9 Nov 1998, Arnold Rots wrote:

> ...However, in practice (for observing), one
> may want to precess the ICRS position to a different epoch.  Doesn't
> it then become an ICRS position at epoch 1998.85?

Proper motion apart, an "ICRS position at epoch 1998.85" is precisely the
same as an ICRS position at any other epoch, because the ICRS-defined
axes don't move.  There is no such thing as mean place any more.

Putting it another way, precession isn't involved until you want to
point a telescope.  Even then, in the new deal there is never any reason
to apply precession alone;  only the orientation of the Earth matters, and
the artificial separation into secular/long-period terms (precession) and
short-period terms (nutation) is avoided except perhaps as a computing
convenience.

Patrick Wallace
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Starlink Project Manager                        Internet:  ptw at star.rl.ac.uk
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