I imagine a planet similar to ours, except for two details:
- The presence of everlasting clouds
- No yearly seasons
Explanation: In this world clouds are very high and thick, so that sunlight (or starlight, for that matter) only comes through like on a cloudy day here on Earth. There are clouds over deserts (but no rain there) and above the highest mountains. Despite these conditions, enough light and energy reach the planet's crust and allows for life to exist.
The planet's rotation axis is not tilted, so that there are no seasons.
When humanoid civilizations start to appear, they develop myths and religions - usually based on animal figures whom they considered to be gods. Later, after the development of writing and accounting, philosophers start to ponder about the origin of their planet.
My question is threefold:
Without ever being able to see stars or planets, how would such civilizations figure out anything about the universe?
How would the state of their planetary weather shape their development of physics?
What would it feel like when they managed to send their first rocket beyond the clouds?