heliographic longitudeThe angular distance from a standard meridian on the Sun (taken to be the meridian that passed through the ascending node of the Sun's equator on the ecliptic on 1854 Jan 1 at 12h 00m UT and calculated for the present day assuming a uniform sidereal period of rotation of 25.38 days; see Sun), measured from east to west (0° to 360°) along the equator. The position of a point on the Sun's disk may be defined in terms of this and its heliographic latitude.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
heliographic longitude[‚hē·lē·ə′graf·ik ′lan·jə‚tüd]
On the sun, angular distance east or west from given point on the equator of the sun.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.