How the light bends
A ray at distance r from the centre bends toward it with curvature
k · 2 sin θ / r, where θ is the angle between the ray and the direction to the centre.
k = 0: ordinary straight light. The sun lights the whole cell forever and the far side of the
world hangs overhead. That's why concave-earth models need bent light or a special sun.
k = 1: every ray is an arc of a circle through the centre. This is exactly
sphere inversion (x → R²x/|x|²) applied to ordinary optics. Everything you see matches a
normal globe with the sun at R²/d, with the same day/night line, sunsets and sky. The model makes
no different predictions, which is why looking around can't settle it. It also means light,
rulers and time all have to warp in exactly compensating ways.
Two-faced sun: Cyrus Teed's idea. A sun at the centre that shines from one face and
rotates, giving night.
The ball of stars at the centre is the celestial sphere. After inversion, "infinitely far away" ends
up at the centre, so the whole night sky is there. Its true size is far smaller than a pixel, so it's
drawn enlarged. The moon sits between the stars and the ground, at its real apparent size, with the
correct phase for the date.