[fitswcs] Helioecliptic coords

[David Berry]

FITS-WCS paper 2 explicitly includes CTYPE codes for helioecliptic coords.
Showing my ignorance here, but can someone tell me what the definition of
helioecliptic coords is? I'm guessing it is a spherical system centred
on the centre of the earth at some date, with the equator parallel to
the mean ecliptic at the same date, and the zero longitude meridian
passing through the centre of the sun. [My main doubt is about the
position of the centre of the spherical system - is it at the
centre of the earth, or at the observer, or at the projection of the
centre of the earth onto the mean ecliptic, or somewhere else?]

If my above guess is basically correct, then my next question is how is
the "date" specified in a FITS header? Is it given by DATE-OBS/MJD-OBS?

David