Imagine that it is a full moon as seen from Earth and there are no eclipses. If a person living on the Moon is looking at the Earth how much of the Earth will she see lit?
a) The entire Earth facing her will seem lit.
b) She will see half of the Earth facing her lit.
c) None of the Earth facing her will seem lit.
d) Trick question --- the Earth will NEVER seem lit from the moon no matter what phase of the Moon it is.

During a full moon, the side of the Moon facing Earth is fully lit by the Sun and likewise, the Earth also appears fully lit from the Moon's perspective. The correct answer is (a) The entire Earth facing her will seem lit.

Explanation:

When it is a full moon as seen from Earth, the side of the Moon facing Earth is fully illuminated by the Sun. Similarly, when observing from the Moon, the Earth would be fully illuminated as it is opposite the Sun in the sky. Therefore, if a person on the Moon is looking at the Earth during a full moon, the entire Earth facing her will seem lit, which corresponds to option (a) The entire Earth facing her will seem lit. This is because both the Earth and the Moon experience phases as seen from each other; when one is full, the other is also full from the opposite perspective. This phenomenon is further explained by the concept of earthshine, which is sunlight reflected from Earth illuminating the Moon, especially noticeable during the crescent phase when only a thin portion of the Moon is directly sunlit.