shadow, like many imaged on the Moon's surface, is surrounded
by a bright aureole. It is an example of the "Opposition Effect".
Lunar soil has an open structure with many areas of deep shadow.
But, when looking in a direction directly away from the sun, shadows
are hidden by the object casting them. The antisolar
point and the adjacent areas therefore appear brighter than elsewhere
because they have more sunlit surfaces and less shadow. There are
other factors that contribute to the glow, retroreflection by crystalline
minerals and a phenomenon called coherent backscattering. The heiligenschein,
also at the antisolar point, is a separate effect.
The opposition effect was so named because it is substantially responsible
for the brightness of the Moon and Mars at
opposition, i.e. when they are near the antisolar point in our sky. The Moon's brightness at full is greater than can
be accounted for by the increase in its illuminated area compared
with its partial phases.