| Term: | HG |
| Definition: |
Heliographic - A heliocentric rotatingcoordinate system where the Z axis is normal to the solar equatorial plane,positive northward. X, Y axes rotate with a 25.38 day period. The zerolongitude (X axis) is defined as the longitude that passed through theascending node of the solar equator on the ecliptic plane on 1 January, 1854at 12 UT. See http://nssdc.gsfc.nasa.gov/space/helios/coor_des.html |