Venus

Look at these numbers:

• Orbital period (Venus year): 224.7 Earth days
• Rotation period (Venus day): -243 Earth days (retrograde)

A Venus day (the time its globe takes to rotate 360 degrees around its own axis) is longer than a Venus year (the time it takes to travel a loop the Sun)!

That's pretty damn slow, compared to Earth which manages to spin 365 revolutions before completing one orbit.

But how slowly does the Sun move across the Venus sky? That's usually what we mean when we talk about a "day".

Because of the retrograde rotation, a solar day on Venus (the time from one sunrise to the next sunrise) takes "only" 116.75 Earth days.

I wonder, if the planet's rotation was prograde like most planets, would a solar day be horribly long?

Yes. Normally, if you imagine a non-rotating body orbiting the Sun, it would see the Sun moving across its sky, but a slow prograde rotation cancels out much of that, seemingly slowing down the Sun's path across the sky. So how long would it take from one sunrise to the next?

The difference between its day and its year is about 20 Earth-days, so on every completed orbit, the planet's rotation would only have progressed 20 days, or about a twelfth, of the full 243 days. During the orbit, it was mostly "rotating along" like a tidally locked satellite. In short, if Venus rotation had been prograde, a solar day would take more than 2500 Earth days, or seven years between sunrises!

Venus is one of the best colonization sites in our solar system.

• While it has temperatures around 500 C on the ground, you can find more Earth-like conditions at an altitude of 55,000 m above the ground: livable temperature and pressure. These beat Mars.
  • The Venerian atmosphere is crazy-thick, so even at the 50 km altitude, there's still enough above to protect against solar radiation and meteorites. This beats Mars.
  • The atmosphere can be used to manufacture oxygen more easily than on Mars.
• After we realize that Earth air acts as a lifting gas on Venus, it's clear that you could build cities that float in the air on Venus, same as when you let go of a helium balloon on Earth and watch it float up into the sky. Isn't that great?!
  • And the cities wouldn't need big balloons above, the way airships on Earth are made of a big balloon and only a small gondola underneath where people board. Because we can breathe that air, we can occupy the balloon itself, so to speak (rather, there'd be no balloon, just a very big gondola). That gives you plenty of space to walk around.
• While these cities need be airproof, it won't spell doom if it springs a leak somewhere. The pressure inside the city is the same as the Venus atmosphere outside, so Venerian air would come in at normal gas-mixing rates, affording many hours for engineers to patch the leak.
• In theory, you can even step outside on the balcony without a spacesuit and crack open a beer – all you need is a breathing apparatus in your nostrils. But stay inside when it rains; the rain is sulfuric acid strong enough to dissolve most matter.
  • Not sure if you'd ever dare expose your mouth. You know, when we hold our breath for too long, what induces us into a panic is not the absence of oxygen (the lungs consume oxygen very slowly, actually) but the rising amounts of carbon dioxide. IIRC, less than a percent CO2 is enough to cause us to hyperventilate and behave like we're drowning. On Venus, the air is closer to 100% CO2; what would it feel like to accidentally suck in some of that through your mouth? I guess you'd lose all control of your body and die shortly thereafter.
• A major drawback is the lack of local raw materials – you'd have to send drones to the ground to gather stuff, and all drones sent by the USA and the Soviet Union in the 20th century broke in less than 2 hours under the intense heat and pressure.
  • Reycling would be a big deal.

• Planet