A planet appears exactly half-lit (looking like the first- or last-quarter moon)when it is
A) in retrograde motion.
B) at inferior conjunction.
C) at greatest elongation.
D) at opposition.

Question

Quizplus · Accepted Answer

A planet appears exactly half-lit when it is at its greatest elongation. This is because, at greatest elongation, the planet's position relative to Earth and the Sun forms a right angle, allowing us to see exactly half of the planet's day side and half of its night side, similar to the first or last quarter phase of the Moon.

A Planet Appears Exactly Half-Lit (Looking Like the First- or Last-Quarter