Hello. Can anyone please provide me with an authoritative definition of planetary precession? Any link to an authoritative source would be best. I did do my googling first. I tried to provide my search results but an ostensibly spam-blocking feature does not allow me to post the links because I have not made 15 posts on these forums yet. Stupid in that it does not allow even legitimate results. The Wolfram Labs page on this mentions merely the change of the orientation of the ecliptic plane in space. Wikipedia mentions two things: the 47" /cy shift of the ecliptic plane (same as above) and something else "whose center lies on a circle about 6° away from the poles". Another website gives a shift of 0.13" /yr caused by the movement of the ecliptic. One website says "the planetary precession (which makes not the ecliptic pole the centre, but a circle about 6° away from it)". So which is the correct definition? I know that the ecliptic rotates around an axis at the rate of 47" /cy, and that this causes a change in the ecliptic longitude of bodies, but is this the 0.13" /yr mentioned in one site? And what is this "6° away from the poles" point? Thanks in advance.