When there's a new Moon and after Sunset the light from the Sun is reflected from the Earth onto the un-lit portion of the Moon. As we approach full-Moon we loose the effect as the strong reflected Sunlight light stops our eyes from seeing the Earthlight.
" For so the Lord has commanded us, saying,
“‘I have made you a light for the Gentiles,
that you may bring salvation to the ends of the earth.’”