OK, I have had a lot of ideas for exact nature of my world. Currently I am still closest to the "Earth in the far future and several tectonic upheavals later"... but I also considered several other ideas. One of these ideas is to have basically "flat Earth" scenario - but one where worlds are not planets as such, but rather worlds floating in an atmosphere of a gas giant. Similar to islands from Granblue Fantasy, but on a planetary scale (average island would have surface area similar to Earth, so would be much larger looking top-down). Don't worry about how and why they are floating - it is magic, essentially. What does worry me is a) how to ensure that these worlds still have oceans (seriously, this looks like it should have lost seas long time ago, and my continents / worlds would be similar), b) how do people handle navigation on the worlds which would likely be carried around by air currents, c) how much stuff am I not even considering here?
Have each floating world be sort of bowl-shaped. Maybe the outer perimeter is a thick rocky crust, or needs to be porous stone with billions of micro-cells holding lighter-than-air gas or whatever.
On second thought, if the worlds are as big as the surface area of the earth, you don't have anything to worry about. Even if bodies of water near the edge would drain, there's plenty of room for oceans away from the edges.
Seriously, once you're getting into a setting like this, conventional planetology just isn't going to work. I mean, Discworld was on the back of four elephants on the back of a turtle. No one cared how it worked. It just did.
Thanks. I am actually more interested in about how day-to-day things would work on such a world: calendar, navigation and so on.
The same way as in our world. Or maybe a different way. It's up to you. In a world that does conform to realistic physics, it works how you want it to work. You mean how they get from world to world? Maybe they have a magic map. Or maybe each world projects a pillar of coloured light that they sail towards. Be as creative as you want,
Actually, it could work for real, without magic. Imagine a hollow world, a shell with holes in it. This could be thick enough and heavy enough to have significant gravity and therefore produce atmospheric pressure. Inside the shell, there would be no gravity as the gravitational field of the matter in the shell would cancel itself out. then you could have rocks floating in this atmosphere which would be large enough to have some gravity of their own (but not much).
Okay, you'll definitely need to look into how the gas giant would affect things. Any ships taking off from your planetoids would have to escape the gravity of its own rock as well as the gravity of the giant. There's also implications of using the gas giant in long distance space travel as a kind of gravity slingshot to fling the ship further. A calendar could be based more on the cycle around the gas giant rather than on whatever star happens to be present -- although you'll have to make the star it revolves around very distant. Because of solar radiation, there's a limit to how close a gas planet can be to a star. That's why all of the closer planets to Sol are rocky ones, and we don't get a gas giant until Jupiter. Bear in mind also that the color of the gas giant will continually be in the sky. Since your worlds are actually in the atmosphere, it might be that they can't see any stars from the surface, or only at the points where the gas giant's clouds are thinnest. As far as water goes, you may be able to have some sort of weird gravity thing where the water flows from one planet back to another as the two orbit together. Like a water loop. I don't know how physically possible that is in the real world, but in yours it sounds like it could work.
The gravity would depend on the gas giant's mass. Saturn's surface gravity (taking the surface to be the cloudtops) is only slightly more than Earth's. However, it a depth great enough to have an atmosphere of 1 bar, the situation would be quite different. And remembering that the lower the atmospheric pressure, the lower the boiling point of water, which is why liquid water can't persist on the surface of Mars. Gas giants have been discovered in very close proximity to their parent star - the so-called "hot Jupiters". Gas giants also have violent storms and very active atmospheres. Winds on Saturn can reach nearly 2000 kph. All in all - they're not hospitable places for life. You'd essentially have to explain everything with magic.
Gravity for gas giants is actually given at the 1 bar pressure level of the atmosphere. For Saturn, surface gravity is 1,07 g, while for Jupiter it is 2,53 g (that Saturn tidbit is one of things which made me consider this idea). So depending on a gas giant, gravity at livable pressure depth is not a problem. Problem are the winds, as you mention; I imagine same magic which keeps worlds "in the air" would have to shield them from the environment. And I imagine there might be something else I haven't considered. What I meant is actually navigation not just between the worlds (that would be solved with magic - and yeah, those things you mention are pretty much what I considered), but on the worlds themselves. Some of these floating continents have surface area of a small-ish planet (say, Mars, or even Earth for a particularly massive examples), and many are the size of actual continents. This includes their own seas and stuff. I tried searching for navigation on a flat Earth and then I remembered this. But I am not certain it would work in this "islands in the sky" scenario, as a lot depends on the Sun moving across the sky on a relatively small distance. This however is just a normal gass giant but with floating continents. Thanks. Thanks. Yeah, that might be the other option - I haven't really considered it due to the problem of getting sunlight inside, but well, there could be magical sun at the center, or maybe shell itself would produce light.
