It may have been remiss not to mention this earlier in our tour but now I must tell you this world is, in fact, A SPACE SHIP!
Originally named Wanderer IV (for the original meaning of the word 'planet' - wandering astral body), it was constructed in orbit of Earth by using nano-terraforming technology on tethered meteors; it took all of the nations of the world pooling their technology and knowledge to pull it off, and then off it was sent into deep space, a shining and lonely hope for humanity's continued existence far away by terraforming or at least populating a distant Earth-like planet. It wasn't the first ship of this purpose, hence the IV or 4, but it was certainly the first of its kind - and ambitious attempt to create a home-away-from-home of sorts for the generations of humans who would live on it.
It is a doughnut shape, terraformed on the inside surface to resemble Earth, and with technology to simulate its environment. For example: the Sun.
The sun of this world keeps a circadian 24 hour day and night cycle - not by travelling across the sky (or seeming to) but by rotating (or seeming to).
In fact, the doughnut planet rotates around the sun. One half of the spherical sun is shaded and opaque, the other bright and warm - creating night and day, depending on which part of the planet's surface is passing in front. The consequence of this is that the sun remains stationary in the sky and has a crescent cycle - first a thin smile of light at the bottom, which extends until there's a half-sun, and then a full midday circle, then wanes away until there's a thin n of sunset light at the top, which then disappears to give way to dark night.
The fact that the face of the planet directly above you at night is fully lit means it can be seen in the night sky, as so:
(apologies for my drawings, just.... apologies. -_-)
The Sun, as well as sustaining life on the ship, propels it through space; it is an ion engine, the kind that accelerates very slowly but continuously until great speeds are reached. The light trail of this engine - the Sun's 'Tail', as it's called, can be seen at all times but most vividly at night, as above.
The technology that allows the planet to rotate around the sun is the particle accelerator buries in the crust, looping all the way around the planet - along its 'equator', where you'd slice a bagel. This accelerator artificially increases gravity on board as well as rotating the planet, keeping the atmosphere at the correct pressure and concentrated around the inner area of the planet.
It's been hundreds of years since this ship left home, and things have changed; for one, the ecosystems have settled into new balances, then resettled, and again - this was to be expected of course, as the foremost ecologists of the world told government officials and project forecasters with a collective shrug as they designed the environments and food webs of their 'ark'; there's always an element of chaos when it comes to living things.
But not all the changes have for the better; there've been tragedies in the past, decimations of the population, though people now wouldn't be able to give you specifics - those have passed away with older generations. They've left their scars on the planet, and on the collective conscious too.
Because the collective knowledge of this population is overall rather patchy, where those who would have taught it on have been lost - no-one on board actually knows their world is a spaceship, for one. (Apart from some suspicions on the part of the Captain and her confidantes, who've stumbled on some things they don't yet understand). The theory behind quite a bit of the technology salvaged and used isn't fully understood, though there have been advances in the workshops in the Coral City in recent years. Astronomy is less than rudimentary - knowledge of genetics and biology is very specific, of practical application in farming - the religion of the world can be a little confused, with general themes and morals agreed on but a lot of disagreement between any two groups about specifics.
Perhaps making a traditionally habitible ship was a mistake, and the unspoken-of catastrophes of the past were caused by the onboard-born generations' inability to fully live on their own world and leave the future for their children - and as a result, they eventually thought it better for their children to never know the truth. Or perhaps something that could not be predicted, an outside factor, descended on the ship...
Whatever the case, the past has certainly left its traces.