Earth has a central 'sun'. Every planet has a central 'sun' (this is why Jupiter, Saturn, Uranus and Neptune have auroras - just like Earth does). Leonhard Euler, the greatest mathematician of all time, advocated a Hollow Earth model that included a central 'sun'. A hollow Earth would have gravity in its interior. Earth's centre of gravity is NOT at Earth's centre (contrary to popular belief), but is instead in the centre of Earth's crust. This is why the people who live in the Inner Earth World (AKA Agartha) walk upside-down by our perspective here in the Outer Earth World. Furthermore, gravity is both attractive AND repulsive. The 'sun' at Earth's centre (which is actually a naturally occurring nuclear fission reaction) emits repulsive gravity, so that the people who live on Earth's inner surface don't get pulled into it. Celestial bodies are kept in stable orbits due to electromagnetic forces, not gravity. Furthermore, gravity is vastly weaker than electromagnetism. Also, Earth's inner surface has an upward-pointing curvature. This means that if you stood on the inner surface of Earth, you would actually see land in the sky. Pic related.