Earthshine is illumination seen during which phase of the moon?
a. full
b. new
c. last quarter
d. first quarter

d. first quarter

Explanation:

    When the moon appears as a thin crescent in the sky during twilight, we can often observe that the rest of your disc also shines faintly. This phenomenon is called cinereal light and is due to the reflection of sunlight by the Earth, which in turn illuminates the lunar surface. Once reflected by the earth, the colors of the radiation, which appear as a rainbow in the image, change significantly.

    Earthshine can be best seen during the crescent phases (the 1-5 day period before or after a New Moon.

    By observing cinematic light, astronomers can study the properties of Earth's reflected radiation as if it were an exoplanet and look for signs of life. The reflected radiation is also highly polarized and the study of this polarization, as well as the intensity of the different colors, allows testing of the presence of life very sensitively.