FIG. 1.

Geometry of the triple system of a star, a planet, and a moon with illuminations indicated by different shadings (pole view). For ease of visualization, the moon's orbit is coplanar with the planet's orbit about the star, and the planet's orbital position with respect to the star is fixed. Combined stellar and planetary irradiation on the moon is shown for four orbital phases. Projection effects as a function of longitude φ and latitude θ are ignored, and we neglect effects of a penumbra. Radii and distances are not to scale, and starlight is assumed to be plane-parallel. In the right panel, the surface normal on the subplanetary point is indicated by an arrow. For a tidally locked moon this spot is a fixed point on the moon's surface. For ϕ_ps=0 four longitudes are indicated.