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Looking at the History of Mars observation, if Venus had remained habitable per discussion in this thread, I can make some guesses about when Venus's albedo features (ice caps, seas) become detectable. However, a couple of questions remain:

1. Assuming Venus has plot-convenient inhabitants with 17th-19th Century technology, they'll have some cities that are lit up at night. When would human astronomers be able to reliably detect pre-electric city lighting? To put it another way, would 19th Century human astronomers be able to detect Venus's equivalent of 19th Century London and its gas- and oil lamps? Would 18th Century human astronomers be able to spot London?

2. If Venus had an inclination-stabilizing moon (call it "Cupid") that was half Luna's diameter but closer to Venus (deeper inside its Hill Sphere so the sun doesn't cause problems for Cupid), would it be naked-eye visible from Earth in ancient times? Or would a telescope be required to spot it?
The so-called Ashen Light was still a feature of Venus observations when I was a youngster, but it has not been reliably observed in recent years. I think that means that lights cannot be reliably observed on Venus from Earth even today.
I found an answer about a fiction moon of Venus: since Earth's moon would be quite visible to the unaided eye from Mars, presumably a Luna-scale moon of Venus would be visible from Earth.
Oh, yes. That is definite. It would be a bit dimmer than Mercury, but still easily visible.
Translocating Luna to Venus in Celestia shows that the two objects would be close together in the sky, about as far apart as various stars in the Pleiades. This separation would be very variable, since Venus varies in distance significantly over its orbit. Neith would be easily visible, about at bright as Arcturus. Of course Neith would look brighter than Luna at similar distances and angles, even if it were otherwise identical, because sunlight is brighter in Venus orbit.

Curiously, the brightness of Neith (and of Venus) would not vary much over time as seen from Earth, even though the distance varies so much, because the angle of the crescents also change according to the position with respect to the Sun. Venus ranges in brightness from -3.8 to -4.8, and Neith would vary to a similar extent (about a magnitude).
dsmike, thank you for the historical reference. Steve, thank you for the simulation.

So, my habitable Venus will have its inclination stabilized by the sun's tides, not Cupid/Neith. I don't want to mess up human prehistory with an extra astronomical object near Venus.
Interesting article. Has there been follow-up research verifying the idea?

Quote:Slow rotation, in
turn, reduces the temperature contrast between the
poles and equator which may play a role in making
the Earth’s Ice-Ages rare and relatively mild

Eenteresting. So if rotation speeds up a bit, perhaps to 18-20 hours, that might make the poles more habitable.

On the other hand, apparently slow rotating planets can get closer to stars and handle heat fluxes higher than Venus's:

Gee, 256-day rotation keeps a cloudy Venus positively frosty according to that paper.
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.
Harry Turtledove involves Mars in several things, and has an alternate Mars in "A World of Difference".

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?
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