Someone made the Earth.
They placed man on the surface and gave them plants, animals, the waters, and the mountains to make their life compete.
The Earth exists within a spherical firmament upon which the Moon, planets, stars and the sun are affixed.
The firmament is, of course, an artificial construct. The Moon, stars, and planets are projections upon its surface. The Sun is a radiation source that travels around the sphere, giving light and heat to the inhabitants.
The Earth has an axial tilt, and the Sun's radiation fluctuates with the seasons. The Moon, as well as the planets and stars is a light source, one that waxes and wanes periodically.
Given a civilisation and history that mirrors our own, at what point in history would astronomers prove the actual existence of the firmament and how would they do so?
Assume that the firmament is roughly twice as far away as the Moon is now. The Sun or Moon do not visibly illuminate other items on the inside surface of the firmament.