Disclaimer: the overall idea of the world as presented is basically lifted from Strugatsky Brothers "Inhabited Island". The details of the question are for a derivative work in that universe.
Imagine a planet whose atmosphere has an unusually large refraction, resulting in the observer getting the impression that the surface of the planet is concave. The upper atmosphere is very dense and opaque, constantly, phosphorescent, resulting in no observable sun or stars from the planet's surface.
If a sentient species with civilization developed on such a planet (without - to date - access to high level atmospheric flight, but otherwise, a 1900-1940 level of overall scientific/technological development - what would be the likely cosmology be for such a species?
Their direct visual experience is that they live on an inner circle of a large bowl or a sphere.
Would they likely be able to detect that they indeed live on an outer surface of a spheroid, by gravitational observation/experiments? Geographical travels?
Would they be able to somehow deduce the existence of extra-planetary space, their solar system/sun, or other astronomical bodies/structures?
Assume that all the physical laws of the universe are 100% identical to Earth, aside from whatever needs to be minimally bent to produce such an unusual atmosphere.
Desired answers will be based on physics/astronomy/cosmology, as well as history of science.