Earthshine, Crescent Moon and Venus

Only a day after new Moon, this stunning twilight display graced the skies over southern California. The crescent Moon and Venus were only about two degrees apart – about the length of your thumb when held at arm’s length. Photographer: David Lynch, Dave’s web sites: 1, 2, 3

The dark side of the Moon — the region where lunar sunrise had not yet happened — was plainly visible as “earthshine” or “ash light.” Illumination still comes from the Sun but indirectly, having first reflected from the Earth to the Moon and back again to our eyes.

When we see the crescent phase of the Moon, we’re seeing sunlight reflected directly from the Moon –- see the light paths in the diagram. Note that the Sun has just set and the Earth’s rotation has carried the observer to the dark side of the Earth. Light from the dark side of the Moon has taken a different path. Sunlight first reflects off the sunlit side of the Earth and onto the dark side of the Moon. It is again reflected from the dark side of the Moon where we see it as earthshine, the ghostly illumination of the lunar dark side.