File:Illuminated-arimass.png

Summary
Amount of atmosphere that is directly illuminated by the sun after sunset, at the horizon, directly above the spot where the sun is below the horizon.

Data was calculated as follows
 * $$\int_\gamma \rho(h \circ \gamma)\,ds$$

where γ is the path from where the sun hits the atmosphere along the line of sight, and continues in the direction of the oberserver's horizon. ρ is taken to be the usual isothermal atmospheric density. The integral is normalized so that the path from ground to zenith is exactly 1 airmass. The earth's curvature was taken into account.