2. Heliographic Inertial Coordinate System (HGI)
The HGI coordinates are Sun-centered and inertially fixed with respect to an X-axis directed along the intersection line of the ecliptic and solar equatorial planes, and defines zero of the longitude, HGI_LONG. The solar equator plane is inclined at 7.25 degrees from the ecliptic. This direction was towards ecliptic longitude of 74.367 deg on 1 January 1900 at 12:00 UT; because of the precession of the Earth's equator, this longitude increases by 1.4 deg/century. The Z-axis is directed perpendicular to and northward of the solar equator, and the Y-axis completes the right-handed set. The longitude, HGI_LONG increase from zero in the X-direction towards Y-direction.The latitude HG_LAT increases to +90 deg towards the north pole, and to -90 deg towards south pole.
3. Heliographic (rotating) Coordinate System (HG)
HG, differes from HGI only in the sense that the zero of the longitude, HG_LONG is fixed on the Sun and (by convention) rotates at the fixed period of 25.38 days. The zero longitude is defined as the longitude that passed through the ascending node of the solar equator on the ecliptic plane on 1 January, 1854 at 12 UT (Julian day = 2398220.0). The longitudes are also known as Carrington longitudes.The latitude is HG_LAT.
See http://cohoweb.gsfc.nasa.gov/helios/epoch_des.html for discussion of Equinox Epochs