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Hmm, but if the perihelion changes position and the orbit doesn't change shape, the only other option is that the entire orbit shifts—is that it? My understand

Hmm, but if the perihelion changes position and the orbit doesn't change shape, the only other option is that the entire orbit shifts—is that it? My understanding of precession is that there are two levels of rotation: an object rotating on some axis A, with axis A rotating around another axis B. So in the case of the perihelion precessing, what would be the corresponding parts? I was assuming the only precession taking place was of Mercury itself because the axis it rotates on (axis A) rotates about some other B axis. Edit: to clarify, if the above is a correct description of precession, then in order for the perihelion itself to precess it would need to be rotating, and the axis it's rotating about would need to be rotating about another axis. My understanding is that neither the perihelion nor the orbit are rotating, so they can't be precessing.

lutorm 9y ago
"Precession" is just another word for rotation. It sounds like you're thinking of a spinning top, whose rotation axis will precess (rotate) around another axis (usually the local vertical). In the context use here, the precession in question is the rotation of the perihelion (or, if you want to think of it more akin to the above case, the semimajor axis of the ellipse) that rotates in the orbital plane (around the normal vector to the orbital plane). https://en.wikipedia.org/wiki/Apsidal_precession https://en.wikipedia.org/wiki/Apsidal_precession
- westoncb 9y ago
  Thanks, that article does clear it up. So the orbit doesn't change shape nor does it shift, instead it rotates (from the article: "More precisely, it is the gradual rotation of the line joining the apsides of an orbit"—so I guess the ellipse major axis rotates). From what I can tell using precession as a direct substitute for rotation is less common, even in astronomy—but maybe I've just had poor samples so far. I was thinking of it as: "Precession is a change in the orientation of the rotational axis of a rotating body." (https://en.wikipedia.org/wiki/Precession https://en.wikipedia.org/wiki/Precession)

lutorm 9y ago

"Precession" is just another word for rotation. It sounds like you're thinking of a spinning top, whose rotation axis will precess (rotate) around another axis (usually the local vertical). In the context use here, the precession in question is the rotation of the perihelion (or, if you want to think of it more akin to the above case, the semimajor axis of the ellipse) that rotates in the orbital plane (around the normal vector to the orbital plane). https://en.wikipedia.org/wiki/Apsidal_precession https://en.wikipedia.org/wiki/Apsidal_precession

westoncb 9y ago
Thanks, that article does clear it up. So the orbit doesn't change shape nor does it shift, instead it rotates (from the article: "More precisely, it is the gradual rotation of the line joining the apsides of an orbit"—so I guess the ellipse major axis rotates). From what I can tell using precession as a direct substitute for rotation is less common, even in astronomy—but maybe I've just had poor samples so far. I was thinking of it as: "Precession is a change in the orientation of the rotational axis of a rotating body." (https://en.wikipedia.org/wiki/Precession https://en.wikipedia.org/wiki/Precession)