SIA 2.0 POS parameters

[Rob Seaman]

Howdy,

On Jul 9, 2014, at 12:09 AM, Markus Demleitner wrote:

> On Tue, Jul 08, 2014 at 11:04:29AM -0700, Walter Landry wrote:
> 
>> Some datasets (e.g. planetary) can not be transformed to ICRS.  So we
>> are going to have to handle different coordinate systems anyway.
> 
> Well, *if* you consider planetary images in scope for SIAv2 (which,
> as far as I know, is not in the CSP's requirments), that's an issue.
> 
> But even if we chose to support, say, planetary surfaces, it would be
> a completely different use case.  There's no sensible transformation
> between coordinates on Mars and those on Jupiter.

I'm not expressing any opinion on the SIAv2 questions under debate.  However, the planetary use cases include innumerable resonances and synodic effects that can tie together diverse coordinate systems, for instance through their implications on timekeeping on different bodies but also more generally.

Consider that Charon and Pluto are each tidally locked to the other.  As with our Moon each presents the same side to viewers on the other world, but in addition the other remains at a stationary azimuth and elevation in the sky.  If the axis passing through the planetary centers marks the prime meridian on Pluto then the equivalent of the international date line is always visible on Charon.  (A solar clock on Pluto would be 12 hours off from the matching clock visible straight overhead on Charon.)

Or consider the Laplace resonance of the three innermost Galilean satellites of Jupiter:

	http://commons.wikimedia.org/wiki/File:Galilean_moon_Laplace_resonance_animation.gif

Each is tidally looked to Jupiter and at closest approach the same sides will be facing each time.  This is expressed in terms of an explicit relationship between their mean longitudes:

	http://en.wikipedia.org/wiki/Orbital_resonance

and that will in turn tie together the coordinates on their surfaces.

These resonances apply to non-planetary surfaces as well:

	http://saturn.jpl.nasa.gov/video/videodetails/?videoID=189

Daphnis is tidally locked to Saturn and the waves in the rings will remain in view, stationary in the sky, their amplitude waxing and waning as Daphnis dives above and below the plane of the rings each orbit.

These are real places with real maps, and hence, useful coordinate systems:

	http://pubs.usgs.gov/sim/3168/

To engage with expert (or even casual) practitioners in various fields, IVOA will have to address esoteric topical use cases.  Complex effects like precession, libration and nutation will have to be describable.  Proofs of concept exist as to how to go about doing so in web services, e.g.:

	http://ssd.jpl.nasa.gov/?horizons

The tutorial at ADASS XVIII in Québec (http://adass2008.artisan.net) was a face-off between WorldWide Telescope and Google Sky.  Attendees will remember the brouhaha over the comparative ability (or inability) of each to represent coordinates at the poles.  Simplifying protocols is a good goal, and as I said I am expressing no opinion on SIAv2 issues.  But there are thresholds of simplification.  Eventually IVOA will have to step across the threshold.

Rob Seaman
NOAO