Earthshine: why the dark part of a crescent moon still shows

Look at a thin crescent low in the west a few days after new moon and the rest of the disc is often there too, a pale grey circle completing the shape. Old almanacs called it the old moon in the new moon's arms. Leonardo da Vinci worked out the cause in one of his notebooks around five centuries ago.

When the moon looks thin to us, Earth seen from the moon is close to full. Clouds, ice and ocean send roughly a third of the sunlight back into space, and part of that lands on the moon's night side. The glow you see has made two reflections, Earth then moon, before it reaches the eye, which is why it is so dim and why it fades as the crescent grows.

The difference in brightness between the lit sliver and the earthshine is several thousand times, more than one exposure can hold comfortably. Set the camera for the grey disc and the crescent turns pure white, set it for the crescent and the disc vanishes. Many photographers accept the burned edge, a small bright arc on a soft round shadow, which keeps the twilight sky behind it dark enough to rest an icon row on.