This system has its Z axis perpendicular to the plane of the Earth's orbit around the Sun (positive North) and its X axis towards the First Point of Aries (the direction in space defined by the inters
ection between the Earth's equatorial plane and the plane of its orbit around the Sun (the plane of the ecliptic). This system is (to first order) fixed with respect to the distant stars. It is subject to slow change owing to the various slow motions of the Earth's rotation axis with respect to the fixed stars.
This system has its X axis towards the Earth and its Z axis perpendicular to the plane of the Earth's orbit around the Sun (positive North). This system is fixed with respect to the Earth-Sun line.
This coordinate system has its Z axis parallel to the Sun's rotation axis (positive to the North) and its X axis towards the intersection of the solar equator and the solar central meridian as seen fr
om the Earth. This system is sometimes known as heliocentric solar (HS).
This system has its Z axis parallel to the Sun's rotation axis (positive to the North) and its X axis towards the intersection of the solar equator and the solar central meridian as seen from the Eart
h. This system is sometimes known as heliocentric solar (HS).