Heliocentric Inertial (HCI) Frame :
* The solar rotation axis is the primary vector: the Z axis points in the solar north direction;
* The solar ascending node on the ecliptic of J2000 forms the X axis.
* The Y axis is Z cross X, completing the right-handed reference frame.
Heliocentric Earth Ecliptic (HEE) Frame :
* The position of the earth relative to the sun is the primary vector: the X axis points from the sun to the earth.
* The northern surface normal to the mean ecliptic of date is the secondary vector: the Z axis is the component of this vector orthogonal to the X axis.
* The Y axis is Z cross X, completing the right-handed reference frame.
Solar longitude (Ls) :

• The solar longitude Ls is the Planet-Sun angle, measured from the Northern Hemisphere spring equinox where Ls=0.
  Ls=90 thus corresponds to northern summer solstice, just as Ls=180 marks the northern autumn equinox and Ls=270 the northern winter solstice.