To start we will make some assumptions:
1. The Sun is much farther away than the Moon. You may take that as a good story for itself, or check the evidence that the ancients used. There is so much background evidence that it might be better to just take this idea as the best story that has been found yet and not spend much time trying to prove it.
2. The Sun, Moon, and all visible planets are roughly spherical.
3. That calculations based on a curved surface Earth show drops in level at greater distances:
Earth
Circumference = 40000 Km = 24800 miles
Diameter = 12732 Km = 7894 miles