# Page History

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Check here for recent discussions:
https://wiki.ivoa.net/twiki/bin/view/IVOA/UCDList_1-3_RFM

Coordinates -
The above page includes a proposal to add pos.bodycentric & pos.bodygraphic (which are not standard terms) to cover both the planets and the Sun (but not the Earth)

With description:

• Discussion : In body-centric coordinates, longitudes are defined from sub-observer point, while body-graphic coordinates are defined from a standard body reference frame

and a comment:
TODO: VESPA team to check consistency between heliocentric and planetocentric definitions

First, the discussion is inadequate and instead relates to rotating frames. This has to be corrected along these lines (from IAU WGCCRE 2015 report)

-centric= a right-hand spherical coordinate system in which latitude is defined as the angle between a vector passing through the origin of the spherical coordinate system and the equator, and longitude is the angle between the vector projected onto the X Y plane and the positive X axis (the projection of the prime meridian on the X Y plan) measured in an eastern direction.
-graphic= The planetographic latitude of a point on the reference surface is the angle between the equatorial plane and the normal to the reference surface at the point. W longitudes (measured positively to the West) are used when the rotation is direct and E longitudes are used when the rotation is retrograde.
(we may want to simplify this a bit…)
+ notice that this only applies to planets and satellites (not to small bodies, and other conventions are mentioned for the Sun and Earth).

Second, enlarging the concept to the Sun surface is certainly useful, although the term “heliocentric" is not used in WGCCRE reports and traditionally refers to something different (just like “geocentric“).
Instead, these systems are defined in Thompson A&A 2006 - which states that “there is no distinction between planetographic and planetocentric latitudes and longitudes for the Sun“ and recommends to use the standard term “heliographic“ in this context.
So… shouldn’t we keep the widely used and unambiguous pos.planetocentric / pos.planetographic, and add pos.heliographic to these?

Use cases:

• UCD to be associated with coordinates, e.g. in a VOtable, for use in tools.
• Can also be used for c1min, etc with body fixed frames in EPN-TAP: pos.bodyrc.lat;pos.planetocentric (this is the reason to have a global term I guess).

But in both cases our spatial_coordinate_description parameter is more detailed (to be finalized)

SE

IVOA semantics
(sent Oct 2019)

• Add date of query in return VOtable from Dachs (or ask how to add it)SEMarkusQueried to ~identify version of evolving services
• In HiPS metadata: hips_frame should accommodate planetary coordinate systems (when ready) in addition to celestial onesSEPierre FerniqueIdentify body and reference  frame
• Clarify conditions to reuse HiPS in external sotware (issue with CNES and possibly other environments)SEPierre Fernique

2020