wrote:
[snip]
Ioannis responded:


It's going to be very messy. ...
Since the celestial dome appears "spherical", be prepared to be
bombarded with thousands of sin's and cos's.


Yikes! You make this sound like a major undertaking; it's actually
a problem in elementary trigonometry that any kid in a high-school
pre-calculus class should be able to solve in 5 minutes.

Conversion from RA/Dec to rectangular coordinates requires two sines
and two cosines -- or two and three, depending how you count.

My only question is about the meaning of the phrase "relative not to
Earth but to a person at 0,0,0". I've been searching for 0,0,0 all
my life, and I still haven't found it!

My impression was that he wanted a conversion for a person situated on
the surface of the Earth for EACH such position.

Since there are going to be so many such positions (taken as 0,0,0), I
thought he wanted a model that would simultaneously provide for every
position, as the Earth rotates.

For example, what are the rectangular coordinates of Alnitak with 0,0,0
taken at my position in Spring in Athens and at your position in Winter?

If this wasn't the case, I seem to have misunderstood him.

- Tony Flanders
--
I. N. G. --- http://users.forthnet.gr/ath/jgal/