In the beginning, there was the Sea before the World, and the Sky before the world, and the Demiurge was riding a blue elk upon the water (if you know Finnic mythology, you’ll probably see where this is going). But an enemy of His shot Him down from the elk, and He fell into the water, wounded.
He lay in the water for nine hundred and twenty seven years, until He saw a bird of the sky flying and seeking a nest. He raised His knee from the water, and the bird lay its nest on it. But the eggs were too hot, and the Demiurge moved his leg, and they fell into the water, and the pieces of the eggs became various bits of creation. (Maybe I’ll have a bird of the water, who dredges up stuff from the bottom of the Sea, too…)
The Demiurge formed the Earth from the stuff that was in the eggs, and rose onto land to continue working, and there he met the Sky Smith. The Demiurge tasked the Sky Smith with making a Dome of the Sky for the new world, and the Sky Smith started on it immediately. He forged the World Pillar that lies at the center of the world, and put up the dark night sky on top of it.
The Trickster was fascinated by his work, and came to the Sky Smith to ask if he could help him with it, and the Sky Smith tasked the Trickster to put up stars in the sky. The Trickster started putting stars in the sky with great enthusiasm, though little skill, but then his interest started to vane, and he didn’t put many stars on one side of the Dome of the Sky.
Because of this, the Dome was off balance, and started to wobble on the World Pillar, and the sky started to tip over; when the Trickster saw this, he became panicked and fearful, and he quickly asked help from the spirits of the Above, but no one could help him; then he asked help from the spirits of Below, but they could not help him either, for they did not know how. Then he heard a voice at the bottom of the barrel where the stars had been. It was the voice of a star that told him to take it and hang it in the sky in the place where the Trickster had lost its interest. The Trickster threw the star in that place, and the star grew in size until it became the greatest of all stars in the sky, and the Dome stopped tipping over, and disaster was averted.
When the Sky Smith came and saw what had happened, he was angered, because now he would have to create a haphazard imprompty mechanism to keep the Dome from topping over in the future. He took the Moon, that he had planned to put in the sky to light the nights when the Sun would not be there, and set it rotating around the sky so that the Dome would not wobble, and he took the rest of the stars in the barrel (six all in all), and put them likewise in motion upon the Dome, but in more complex patterns to compensate for the smallest wobble that the Moon could not fix. The Sky Smith also had to adjust the Sun’s orbit in the sky so that it reached North in one part of the year and South in the second. Finally, he attached the Dome fast to the World Pillar with the Pinion Star that was second in brightness only to the moving stars and the Counterweight Star.
Then the Sky Smith kicked the Trickster in the arse.
And to this day, the Dome of the Sky is tipped to one side, even as it rotates around the Pinion Star, as the World Mill at the bottom of the Pillar turns; one side of the Dome is under the water level of the Cosmic Ocean, and one side is above it, and from this gap there comes light from the outside chaos that lits up the rim of the world, and creates the the months of day and night at the farthest Northern and Southern reaches.
The Ship of the Sun moves over the world from the East to the West each day, rising from the sea and descending into it (which creates clouds). From the ship, the Sun God casts out light that falls down to the Earth like rainfall, but it is easier to throw further in alignment with the keel, so the light doesn’t reach as far at the sides. In the Northern Summer, the Ship travels above the North, and in the Northern Winter it is South. Coupled with the gap in the horizon, this means that the farthest north and south (the “poles”) have a day(summer) and night(winter) months long. The sun rises and sets in 12 hours, but the light of the gap remains. Conversely, in the winter, the sun is so far away that the falling light doesn’t reach, and it resembles a bright, moving star.
In the East, the Sun rises, so the mornings are always warmer than the evenings. In the Eastern summer, in the farthest east (the “East pole”), there is no night, and the mornings are hot and the evenings are warm. In the West (at the West Pole), the sun sets, so the converse is true: it is the evening that is hot, and the morning that is cool. Likewise, in the summer, there is no night; and, as with the East Pole, the “Winter” is cooler, but the day/night cycle is 12 hours/12 hours. At the East and West Pole, in the Autumn the time of no-sun (12 hours) will get darker and darker, until there is true deep night for a few months in the winter. In the spring, the dark of the night will start to become lighter, until you can’t tell the difference between day and night.
In the center of the world the day is always twelve and twelve hours, and there isn’t much of a change in seasons; it gets a bit hotter twice a year, when the East and West are in their Summer/Winter period, and the Sun moves to its absolute zenith on top of the center.
If you move north or south from the center, you get seasons a bit more familiar to our own experiences: between the center and the North and South poles, there is a seasonal difference due to the distance of the Sun combined with a lesser effect from the gap in the horizon, which lengthens the day a bit due to some magic I still haven’t fudged into here. Let’s say the combined effect of sun and chaos light is tremendously effective, and that as the Sun moves closer to the horizon when it rises from the depths of the Cosmic Ocean, the light from the gap also becomes stronger, which lengthens the day marginally in areas that aren’t that close to the Rim… Something like that.
EDIT: Or maybe it takes longer for the sunlight to evaporate, because there’s more of it than usual… Yes, this makes the best sense. :EDIT
Note that Sunlight and the light that comes from the outerworld is different from our light. It’s much heavier, for one, and will have a kindasorta ballistic trajectory. Can’t help it, really, and it’s all in good fun so no harm done.
Yes, I have mixed up the compass points. I noticed that I have them in the wrong order in my charts (I mean, obviously the sun rises on the LEFT, right, and north is UP, right? So obviously East is left and north is up…), and, eh, I can’t be arsed to change it. Take it as another sign of otherworldliness.
Naturally, the Moon rotates around the dome of the sky; which means only the dome of the sky, and not beneath the Earth, etc. so unlike our very own Newtonian companion. I’ll have to come up with a good myth for its phases, etc. The planets are likewise attached to the dome; maybe their “orbits” will be completely unpredictable?
This is just fantasy. It only has to make Common Sense, which is completely different from Actually Making Real Sense. Verissimilitude is the key here, not whether it makes sense in our physical framework, etc. Gravity, for example, will be completely unexplained: I do not know a single ancient myth that attempts to explain its origin. It’s one of those “so obvious in hindsight why haven’t I even thought about why it exists” things that only philosophers at level 12 and above can even think about.
further disclaimer the thing represented here is the physical truth: this is not really a globular Earth with a mythical explanation for seasons, no, this is a project for making a flatworld that makes at least some sense while resembling Earth as much as possible with only superficial differences (like, who cares about the orbits of sky objects? and some sort of seasons and stuff to create interesting geography should be enough, etc..)