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Orbital resonances are a powerful phenomenon over a long enough time. This article doesn't mention it, but Mars's rotational axis is also chaotic. The whole p

Orbital resonances are a powerful phenomenon over a long enough time. This article doesn't mention it, but Mars's rotational axis is also chaotic. The whole planet can change its axis of rotation over the course of a few million years. It is likely that something similar happened to Mercury, though tidal friction has slowed its rotation enough that it's unlikely to occur anymore. The only reason this doesn't happen to the Earth is that the presence of the Moon stabilizes our rotation. Also, there is a few percent chance that Mercury will be ejected from the Solar System due to orbital resonances before the Sun dies. A general principle of planetary dynamics is that systems are only barely stable --- they are stable only over timescales of approximately their lifetime. Then they will suddenly go through a period of chaos, only to settle into a slightly more stable state and start the whole process over again, with a longer timescale.

pavel_lishin 10y ago
> A general principle of planetary dynamics is that systems are only barely stable --- they are stable only over timescales of approximately their lifetime I think I might be misunderstanding what you wrote, because it sounds tautological to me - "Systems are only stable as long as they remain stable systems."
- antognini 10y ago
  I find it a bit counterintuitive. After all, the natural timescale for planetary dynamics should be the orbital period --- i.e., a few years. So one might think that if the system is unstable, it will dissociate within a few years. If it doesn't do that, then it should be stable for all time. But instead the next system takes 10 years to fall apart. And the next one takes 100 years, and then 1000 years, and so on. And eventually these systems (whose natural timescales are still years) take billions of years to fall apart. You would think that if it is unstable, then it should reorganize itself into a system that is stable for all time. It's a little bit odd that every time the system reorganizes itself, it is only a little more stable than it was when it started. Somehow, planetary systems are always just sort of end up teetering on the edge of stability.
  - mannykannot 10y ago
    That is interesting - it seems counter-intuitive to me that the periods of apparent stability would tend to increase in duration over time.

pavel_lishin 10y ago

> A general principle of planetary dynamics is that systems are only barely stable --- they are stable only over timescales of approximately their lifetime I think I might be misunderstanding what you wrote, because it sounds tautological to me - "Systems are only stable as long as they remain stable systems."

antognini 10y ago
I find it a bit counterintuitive. After all, the natural timescale for planetary dynamics should be the orbital period --- i.e., a few years. So one might think that if the system is unstable, it will dissociate within a few years. If it doesn't do that, then it should be stable for all time. But instead the next system takes 10 years to fall apart. And the next one takes 100 years, and then 1000 years, and so on. And eventually these systems (whose natural timescales are still years) take billions of years to fall apart. You would think that if it is unstable, then it should reorganize itself into a system that is stable for all time. It's a little bit odd that every time the system reorganizes itself, it is only a little more stable than it was when it started. Somehow, planetary systems are always just sort of end up teetering on the edge of stability.
- mannykannot 10y ago
  That is interesting - it seems counter-intuitive to me that the periods of apparent stability would tend to increase in duration over time.