Are the Moon's True Nodes really True?
This short article appeared in the NCGR ELECTRONIC MEMBERLETTER Issue 11 November 29, 2004: LUNAR NODES: MEAN & TRUE
Since the Moon's Node is a mathematical construct rather than a physical body, its position is a matter of how you define it. The Moon's nodes are formed by the line where the plane of the Moon's orbit intersects the plane of the Earth's orbit. Projected onto the zodiac, this line creates the Moon's North Node, and exactly 180 degrees from this, its South Node.
Because of a slow twisting motion of the Moon's orbital plane, the nodes move completely around the zodiac in retrograde motion every 18.6 years, or approximatley 3 minutes of arc every day. The lunar node position calculated according to this long-term trend is known as the Mean Node. Within this large cycle there is a short-term wobbling of the Moon such that if you projected an orbital plane from the Moon's position at any given moment, the nodes created by the orbital plane would wobble backward and forward in the zodiac over a period of a few days. The lunar node calculated with this short-term wobble added in is known as the True Node.
The two types of node calculation are equally accurate, because over the long run the Mean Node and the True Node cover exactly the same distance in the same time. According to Robert Hand in Horoscope Symbols, "The wobbling nodes are no more "true" than the mean nodes, as they are in their own way just as abstract a concept."