Precession and Apsidal Motion

Unfortunately the term "precession" often is used unadvisedly not only in the binary star literature but also in other areas. For example, many textbooks and public documentaries speak of the "precession" of planet Mercury's orbit in regard to a well-known prediction of General Relativity Theory (GRT). However, the described GRT phenomenon is orbit rotation, not precession. Orbit rotation (apsidal motion) is rotation within the orbit's own plane, so only one plane is involved, whereas precession involves two planes and may be described in terms of (conical) motion of one plane's normal around the other's. So although precession and orbit rotation may arise in a common context and perhaps have some common physics, they are geometrically distinct. Sometimes the term nodal precession is used for true precession and precession (unqualified) for orbit rotation. That does make a distinction if the adopted meaning is made clear, but why use confusing terminology? With such dual terminology, one might refer to precession as true precession, but then there would be an extra unnecessary and conflicting terminology. Precession need not refer to an orbit but can be precession of a gyroscope, including precession of the Earth's equatorial plane (equivalently precession of its rotation axis). We recommend taking a page from a physics text, where precession refers to phenomena such as gyroscopic precession that necessarily involve two planes.

As outlined in Sect. 5.2.1, a third body in a binary system causes apsidal motion accompanied by precession of the orbital rotation axis. Both effects result from the rotation of the binary's orbital frame around the barycenter and are effective on the same timescale.

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