Wikipedia:Reference desk/Archives/Science/2019 May 10

Science desk
< May 9	<< Apr | May | Jun >>	May 11 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

May 10

Semi-major axes of planetary orbits

Do the semi-major axes of planetary orbits align with each other? I understand that because of the different orbital planes in reference to the ecliptic, they might not lie in the same "horizontal" plane. But consider Earth's semi-major axis, and a plane passing through it perpendicular to the ecliptic. Would the semi-major axes of the other planets also lie on this perpendicular plane? Another way of putting it is, are the aphelions of the planets all on the same side of the Sun? (And the perihelions too.) If they do not, is there any term for the angle of one planet's semi-major axis in relation to another's? Thank you.    → Michael J Ⓣ Ⓒ Ⓜ 05:09, 10 May 2019 (UTC)

Offhand I wouldn't expect this. It's pretty well established that the motion of the solar system is chaotic. There might be some locking effects between planets in particular configurations though. 67.164.113.165 (talk) 08:13, 10 May 2019 (UTC)

It's not chaotic in every aspects - for example after Newton's laws of motion all periodic orbits are ellipses (although eccentricity is very low for most of the solar system's bodies). For a more precise answer to the OP, we have: Apsis, and Eccentric anomaly. No, semi-major axes do not align. Orbits also although very nearly circular are not centered entirely regularly relatively to the Sun, see the Apsis article, # Inner planets and outer planets. --Askedonty (talk) 09:13, 10 May 2019 (UTC)

The answer is no. All planetary orbits have different longitude of perihelion. Ruslik_Zero 16:10, 10 May 2019 (UTC)

Askedonty, the solar system being chaotic implies that the planetary orbits are approximately periodic but not exactly so. Thus over long enough time periods, they move around unpredictably. 67.164.113.165 (talk) 01:58, 11 May 2019 (UTC)

There is that famous quote attributed to Albert Einstein, well regarding mechanics and playing dice. On the other hand, finding such a regular alignment of the planets certainly would have postponed to later the diagnostic of chaos so near of our own planetary orbit. --Askedonty (talk) 08:46, 12 May 2019 (UTC)

For purposes of this question, chaos is irrelevant: the important thing is not whether the alignment of the major axis of a planet's orbit changes chaotically, but whether it changes at all. It does: see Apsidal precession. So since the axes are always moving slowly, each at its own rate, any of them may align temporarily but none of them can remain aligned. --76.69.46.228 (talk) 02:39, 11 May 2019 (UTC)

Yes, and unless a solar system is young or recently perturbed, say by a passing rogue star (loose definition), it would really have to be stable. That's because a lack of stability would be resolved by the unstable objects eventually hitting others, breaking up, falling into a stable orbit, or being ejected from the solar system. Specifically, planets can't have orbits which take them close to other planets, at least not for long. But, in the early solar system, this probably was the case, as there is evidence of cataclysmic impacts on the remaining bodies back then. One theory for the Moon's formation is that it resulted from such an impact with Earth. See Giant-impact hypothesis. SinisterLefty (talk) 10:54, 11 May 2019 (UTC)

Thanks User:Ruslik0 for giving the relevant term and article. I searched longitude of perihelion table mars neptune venus and got this link which shows what does indeed look like a pretty random set of directions. It gives a change of seconds of arc per century for them (often over a thousand though) The longitude of the ascending node can change at a very different rate not proportional to this; I don't understand it. Note that the advance of perihelion of Mercury (planet) was a major confirmation of relativity. Wnt (talk) 01:28, 12 May 2019 (UTC)

Yes, thank you @Ruslik0:. And @Wnt:, thank you for the link to the table, but I do not understand completely the abbreviations. Which column is for longitude of perihelion?    → Michael J Ⓣ Ⓒ Ⓜ 01:43, 12 May 2019 (UTC)

There's a legend that says that it's the ~omega column. The first table is the actual value and the second is the change in arcseconds per century. Wnt (talk) 11:14, 12 May 2019 (UTC)

Wnt, the Moon's orbit major axis moves forwards at 360°/8.85 years (with numerous oscillations of ~several tens of degrees around the linear trendline) and its line of nodes move backwards at 360°/18.6 years (with numerous oscillations of ~several degrees around the linear trendline). Thus the two lines few frequently pass each other from opposite directions. Orbits don't necessarily have to rotate like solid objects. Sagittarian Milky Way (talk) 12:04, 13 May 2019 (UTC)