It is claimed that the moon stabilizes the Earth's axial tilt, but Mars and Venus (and Mercury, too) are rocky planets without large moons. Why are their axes stable?
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1$\begingroup$ Their axes are not stable. What makes you think that they are? $\endgroup$Dale– Dale2025-09-29 15:50:51 +00:00Commented Sep 29 at 15:50
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5$\begingroup$ "stable" needs the context of timescale. In a human lifetime something can appear stable, but on a longer timescale (and this is typically what astronomers use) you may see deviations. $\endgroup$StephenG - Help Ukraine– StephenG - Help Ukraine2025-09-29 15:54:53 +00:00Commented Sep 29 at 15:54
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$\begingroup$ Related question: How exactly does the Moon stabilize Earth axial tilt? In my answer to that question I discuss the findings that are present in the article by Laskar & Robutel titled: 'The chaotic obliquity of the planets' $\endgroup$Cleonis– Cleonis2025-09-29 16:30:32 +00:00Commented Sep 29 at 16:30
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The paper The chaotic obliquity of the planets by Laskar and Robutel¹ discusses the stability of the obliquity for all of the plants in the Solar System, though sadly it is behind a paywall. The summary is:
Numerical study of the global stability of the spin-axis orientation (obliquity) of the planets against secular orbital perturbations shows that all of the terrestrial planets could have experienced large, chaotic variations in obliquity at some time in the past. The obliquity of Mars is still in a large chaotic region, ranging from 0° to 60°. Mercury and Venus have been stabilized by tidal dissipation, and the Earth may have been stabilized by capture of the Moon. None of the obliquities of the terrestrial planets can therefore be considered as primordial.
Anyhow, the obliquity of Mars is not stable and it is calculated to vary widely with time. Curiously the orbit of Venus is calculated to be stable, and this is due to the moment of inertia of its thick and dense atmosphere. For the gory details see The four final rotation states of Venus² by Correia and Laskar, though again this is behind a paywall.
¹ Nature, Volume 361, Issue 6413, pp. 608-612 (1993)
² Nature volume 411, pages 767–770 (2001)
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1$\begingroup$ About the 1993 article 'The chaotic obliquity of the planets'. A file with scanned pages is available on Jacques Laskar's personal repository on the IMCCE website (Institut de Mécanique Céleste et de Calcul des éphémérides) The chaotic obliquity of the planets No paywall. The pages of that PDF-with-scanned-pages were scanned with gray level to dark. Back when I downloaded that article I extracted the scans, adjusted the gray level, and reassembled to PDF-with-scanned-pages. If you would like to have that version contact me. $\endgroup$Cleonis– Cleonis2025-09-29 16:33:50 +00:00Commented Sep 29 at 16:33
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$\begingroup$ I notified in the comment section of this question: there is a related question: Jan. 2023: How exactly does the Moon stabilize Earth axial tilt? $\endgroup$Cleonis– Cleonis2025-09-29 16:44:10 +00:00Commented Sep 29 at 16:44