You may have noticed, with Saturday's total lunar eclipse, a dark patch that became visible around the moon. This is caused by the Earth's shadow. The shadow, which includes the atmosphere, suppresses the Rayleigh and Mie scattering components in the atmosphere and causes a local darkening. (The Rayleigh effect is the same effect that causes the sky to turn red at sunset.) While this is actually happening all the time, it is only so pronounced when seen against a low light background.