# Confusing?

Roy Williams roy at caltech.edu
Wed Nov 24 07:57:59 PST 2004

```>>On p.7, the paragraph beginning "The order in which words are arranged..."
>>seems like circular reasoning to me.  It says the order matters only if the
>>order matters.  I think the standard should be clear on this -- does the
>>order matter or not?  The text elsewhere suggests yes.

My understanding is that the first word matters because it is the primary type. Each
subsequent word modifies what goes before. Think of a ucd "A; B; C; D" as "((((A) B) C)
D)". For example:

stat.mean; phys.temperature; arith.diff; stat.max
The maximum of the difference of two mean temperatures

stat.mean; phys.temperature; stat.max; arith.diff
The difference of the two maximum temperatures

>>I find Section 3.4 confusing...does anyone else?  phot.color is a difference
>>of two magnitudes, but does NOT have the associated word arith.diff.

In this case there is a special semantic concept for the difference of magnitudes, which
is called color. It would be equally correct to say "arith.diff; phot.magnitude", but it
is more efficient to use one word than two.

>> A temperature ratio is not a temperature, so phys.temperature;arith.ratio seems
>> backwards to me.

I agree. This should be "arith.ratio: phys.temperature"

>> If phot.color;em.opt.B;em.opt.V means B-V and phot.color;em.opt.V;em.opt.B
>>means V-B, then order is clearly important.  The concern this all raises for
>>me is that the construction rules for new UCDs are not very clear, and that
>>the existing UCD1+s have a certain (large) amount of arbitrariness.

Yes that is true. But not because of the UCD construction, rather because of the word
"color". Perhaps I can ask those mor knowledgeable than myself how exactly color is
defined? If I say "BV color" or "VB color", do they have opposite sign? Or do I rather
subtract the highest wavelength magnitude from the lowest wavelength?

Roy

```