Earthshine is a phenomenon that allows us to see the dark, night side of the Moon. But how?
Earth is more reflective than the Moon by a considerable margin. Although you might not think it - because the Moon can appear remarkably bright in h night sky - our lunar neighbour's reflectivity is actually about the same as an asphalt road.
We’re all familiar with the phases of the Moon caused by the varying Moon-Earth-Sun angle as our neighbour travels around its orbit, and the thin crescents we see just before and after new Moon represent the times when Earth’s light shining onto the Moon is at its strongest. Earth's light illuminates the dark, night portion of Moon and reflects back to Earth, allowing us to see the lunar night hemisphere glowing gently. This is earthshine.
Another, more poetic term for earthshine is ‘the old Moon in the young Moon’s arms’, describing the appearance we get with the early evening crescent Moon when it reappears just after the new phase.