Physics
Light sources
Sunlight, the Earth's shadow, sunlight refracted into it, earthshine — and why moonlight is not one of them.
Sunlight and the Earth’s shadow
Section titled “Sunlight and the Earth’s shadow”The illumination is the fraction of the solar disc visible from the satellite, from the apparent discs of the Sun and of the Earth (the atmosphere counts for 20 km). It goes from 1 (sunlit) through the penumbra to 0 (umbra).
Sunlight refracted into the umbra
Section titled “Sunlight refracted into the umbra”In the umbra a satellite is not completely dark: the Earth’s atmosphere refracts a little sunlight into the shadow, which is why an eclipsed Moon looks reddish. LightCurve adds an empirical term calibrated on lunar eclipses (Danjon scale, average atmosphere): about 10 magnitudes below full sunlight at the edge of the umbra, 14 on its axis. This reddened light is treated as grey in the V band. Switch: Refracted light in the shadow (Geometry tab).
Earthshine
Section titled “Earthshine”Sunlight reflected by the Earth also lights the satellite. The visible cap of a Lambertian Earth (albedo 0.3 by default, adjustable) is integrated numerically; the result is applied as an equivalent directional light (magnitude and direction). It is exact for the cap geometry, valid from LEO to GEO, and vanishes deep in the Earth’s shadow. The results panel shows the earthshine share of the flux.
Moonlight
Section titled “Moonlight”Moonlight is not a light source of the satellite. The full Moon is about 14 magnitudes fainter than the Sun (−12.7 vs −26.7), so its reflected light is negligible on a sunlit satellite. It would matter only deep in the Earth’s shadow, where it is comparable with the refracted sunlight. The Moon is used for the sky background (scattered moonlight) and the Moon–satellite separation.
Earthshine is not shadowed by the satellite itself.