SEP for the re-implementation of `sunpy.wcs` as `sunpy.coordinates`

Putting in the PR so that people can form their opinions on what I've written so far.
Fire away.
I don't understand this.  I thought the example

sc = SkyCoord(230*u.arcsec, 120*u.arcsec, 1*u.au, frame='helioprojective')


specified a point on the Sun?  If that is true then the *1*u.au* part is
very strange.  If it is not true, then I am guessing that the *1*u.au* part
specifies only the distance of the observer from the Sun.  It does not
fully specify the location of the observer in space.



On Fri, Sep 19, 2014 at 7:07 AM, Stuart Mumford <notifications@github.com>
wrote:

> In SEP-0005.md:
>
> > +system, where it is unusual to have information on the third spatial
> > +dimension of a coordinate, some assumptions are normally made when
> > +transforming from helioprojective to heliocentric coordinates.
> > +
> > +This assumption (as currently implemented in `sunpy.wcs`) is that
> > +during transformation, if no helioprojective radial coordinate was specified
> > +then the radial coordinate is calculated as if the coordinate point
> > +is on the solar surface.
> > +
> > +In the new coordinates submodule this assumption happens at
> > +construction time of a `HelioProjective` frame. This means that all
> > +`HelioProjective` frames are explicitly three dimensional.
> > +
> > +The third coordinate can be specified explicitly upon construction:
> > +
> > +    sc = SkyCoord(230*u.arcsec, 120*u.arcsec, 1*u.au, frame='helioprojective')
>
> The third coordinate is distance, i.e. radius from the origin (observer)
> to the point. There is the ability to specify zeta which is Dsun - distance
>
> --
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> <https://github.com/sunpy/sunpy-SEP/pull/11/files#r17779113>.
>