Apsidal precessionIn celestial mechanics, apsidal precession (or apsidal advance) is the precession (gradual rotation) of the line connecting the apsides (line of apsides) of an astronomical body's orbit. The apsides are the orbital points farthest (apoapsis) and closest (periapsis) from its primary body (therefore it can be also called after any of the apsides). The apsidal precession is the first time derivative of the argument of periapsis, one of the six main orbital elements of an orbit.
Orbital nodeAn orbital node is either of the two points where an orbit intersects a plane of reference to which it is inclined. A non-inclined orbit, which is contained in the reference plane, has no nodes. Common planes of reference include the following: For a geocentric orbit, Earth's equatorial plane. In this case, non-inclined orbits are called equatorial. For a heliocentric orbit, the ecliptic or invariable plane. In this case, non-inclined orbits are called ecliptic.
