Axial Precession - Precession Nomenclature

Precession Nomenclature

Etymologically, precession and procession are terms that relate to motion (derived from the Latin processio, “a marching forward, an advance”). Generally the term procession is used to describe a group of objects moving forward whereas the term precession is used to describe a group of objects moving backwards. The stars viewed from earth are seen to proceed in a procession from east to west on a daily basis, due to the earth’s diurnal motion, and on a yearly basis, due to the earth’s revolution around the sun. At the same time the stars can be observed to move slightly retrograde, at the rate of about 50 arc seconds per year, a phenomenon known as the “precession of the equinox".

In describing this motion astronomers have generally shortened the term to simply “precession”. And in describing the cause of the motion physicists have also used the term “precession”, which has led to some confusion between the observable phenomenon and its cause, which matters because in astronomy some precessions are real and others are apparent. This issue is further obfuscated by the fact that many astronomers are physicists or astrophysicists.

It should be noted that the term "precession" used in astronomy generally describes the observable of the precession of the equinox (the stars moving retrograde across the sky), whereas the term "precession" as used in physics generally describes a mechanical process.