At which point, as seen from Earth, will a planet appear exactly half-lit (looking like the first- or last-quarter Moon)? (It may help to draw a diagram and show the sunlit side of the planet on it.)
A) greatest elongation
B) opposition
C) inferior conjunction
D) superior conjunction

Question

Quizplus · Accepted Answer

At greatest elongation, a planet (especially an inferior planet like Mercury or Venus, which orbit closer to the Sun than Earth does) appears half-lit from Earth's perspective. This is because, at greatest elongation, the angle between the Earth, the planet, and the Sun allows us to see exactly half of the planet's day side and half of its night side, similar to the first or last quarter Moon phase. Opposition, inferior conjunction, and superior conjunction positions do not provide this half-lit view. At opposition, the planet is fully illuminated from our perspective. At inferior conjunction and superior conjunction, the planet is either nearly or completely obscured by the Sun's light from Earth's perspective.

At Which Point, as Seen from Earth, Will a Planet