Pre-Modern Astronomy vs Habitable Venus

[Cray]

S.M. Stirling has two novels, In the Courts of the Crimson Kings, and The Sky People, in which Mars and Venus resembled the way they were portrayed in pulp fiction. He has a good rationale.

I just re-read that series a couple of months ago and enjoy it enormously. However, Stirling's reasons that Venus are habitable are "Lords of Creation" and "lots of clouds." While "lots of clouds" are exactly what's suggested in the link in my prior post to keep Venus-like planets habitable, those clouds are generated by extremely long rotations (days of 64+ Earth days long) on ocean-covered planets. Stirling's Venus has a 30-hour rotation.

Harry Turtledove involves Mars in several things, and has an alternate Mars in "A World of Difference".

Yep, got that one, too.

I saw the article about the moon not stabilizing Earth; what would be the situations where a large moon would be needed to stabilize a planet? Would a much larger planet ever experience chaotic changes in its rotation or tilt?

It's thought that Jupiter flips Mars on its ear every so many tens of millions of years. Venus, Mars, and Jupiter can influence Earth's tilt and eccentricity: https://www.researchgate.net/publication..._evolution

Having a big moon might stabilize that, though The Astronomer's link indicates that Luna is just a bit too big to do that job.

[sandcastles]

In another solar system, if two Jovian planets are within a few AU's, or a hot Jupiter is close to its sun, would the gravitational forces be enough to make the rotation and tilt vary widely?   If a planet has an entirely liquid surface, maybe oceans many kilometers deep, would that make any difference?

[Cray]

In another solar system, if two Jovian planets are within a few AU's, or a hot Jupiter is close to its sun, would the gravitational forces be enough to make the rotation and tilt vary widely?

I think so.

If a planet has an entirely liquid surface, maybe oceans many kilometers deep, would that make any difference?

Yes. Part of the cause of instability in a planet is uneven mass distribution: uneven continents, equatorial bulges that other planets/moons can get a handle on, etc. An ocean planet would be relatively even and less susceptible.

I think.