sexagesimal

[Rob Seaman]

I think the nonchalant way this discussion has proceeded reflects how  
few people take spherical trig as undergraduates.  Anybody who has  
ever cracked open Wm. Smart's book knows enough to be afraid, very  
afraid :–)

On Sep 18, 2006, at 2:25 AM, Gilles DUVERT wrote:

> Physically, this things are angles, and already, In the  
> international system MKSA,  angles are expressed in RADIANS...

RA (or longitude) is an angle in a funny way that includes the cosine  
of the declination.  The radian is a perfectly lovely unit for some  
purposes – and is extremely unintuitive for other purposes.   
Standards exist for people, not the other way around.

But yes, even time-of-day can be interpreted as an angle and  
sexagesimal notation is just another way to write a (double  
precision) floating point number.  One can mix in the pi/180  
conversion factor as simply as the factor of 15 or the cosine of the  
"other" angle – meaning that it's all starting to sound very  
complicated.

> Anyway, enforcing a better use of the sexagesimal notation for  
> coordinates and _times_ is a good idea, although I do not think  
> today's data producer amuse themselves to write "borderline"  
> sexagesimal coordinates/times.

It's the IVOA that the data producers are amusing – or vice versa,  
perhaps :–)  Is "enforce" even the right word?  I look forward to  
seeing the uniforms of the enforcement arm of the IVOA.  The kiddies  
love bagpipes on parade.

> In fact, since the whole planet use sexagesimal notation for time,  
> it has been de facto fixed by ISO 8601 standard, and it would only  
> complicate things if VO standards were markedly different from ISO  
> ones.

ISO standards are proprietary and are subject to the whims of  
international communities quite distant from astronomy.  The full  
range of time notations supported by ISO 8601 dwarfs the few paltry  
sexagesimal examples we've encountered in the current discussion.

> For me, a sexagesimal notation of a real number is  3 numerical  
> fields separated by ONE "separator type".

In particular, ISO 8601 supports sexagesimal notation with no field  
separators at all:

	12:34:56 –> 123456

> Note that the "null" separator is allowed because 6435 is 64:35 and  
> not 643:5 since the last fields have two digits or none.

Yeah, but is 12345 to be interpreted as 1:23:45 or 123:45?  (Or  
twelve-thousand-three-hundred-forty-five degrees, for that matter.)   
The interpretation is not inherent in the notation, it has to be  
specifically invoked using a format string.

> ... But recommending the form "(+)64:35:26.546" would be a good idea.

No argument here, but what is the force of a "recommendation"?  It  
would be better to phrase such a document as "IVOA will interpret a  
sexagesimal expression using the following rules...", rather than  
"you should write sexagesimal numbers using these rules..."

> And recommending that angles be expressed as floats, not  
> sexagesimal, and perhaps even in radians (after all, nobody's going  
> to look at the XML, all values will be mediated by data converters  
> in the user's preferences), would be a still better idea.

Sexagesimal notation is universally used to express base-60 fractions  
of angles only in degrees (or hours).  Astronomers have long since  
standardized their human-readable angles in degrees, whether decimal  
or sexagesimal.  You may well run into radians when manipulating  
standard coordinates in the tangent plane, but there is no advantage  
to the IVOA's specifying radians.  In particular, astronomers  
recognize celestial coordinates as old friends.  By requiring  
radians, you would also be tossing out the easy conversion from RA to  
hour angle to sidereal time.

I think this discussion is a question of data entry and  
classification, not of mandating specific formats.

Rob