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Definition:
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Physical features on the surface of the Sun are located in Heliographic coordinates (Expl.Suppl., 1961,11.B). Heliographic latitude is measured from the solar equator positive towards North, Heliographic longitude
is defined westward (i.e. in the direction of planetary motion) from the solar prime meridian which passed through the ascending node on the ecliptic of date on 1854 Jan 1, noon (JD 239 8220.0). Heliographic
longitude is sometimes identified with Carrington longitude, but this usage should be avoided since there have been different definitions of the later term over time.
XY-plane: Solar equator of date
+X-axis: ascending node on 1854 Jan 1, noon (JD 239 8220.0)
Angles: Heliographic latitude Y and longitude F right handed.
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