If the moon is a narrow crescent, you can observe a strange phenomenon: the actually dark part of the moon glows ashen. Leonardo da Vinci explained in his work "Codex Leicester" of about 1510 that the "ghostly glow" of themoon came from the fact that sunlight was reflected from the oceans of the earth and hit the moon.
Earthshine is best observed in the days immediately before or after the new moon, when the crescent moon is very narrow and does not yet shine strongly.