Phases are caused by the relative positions of the Moon, Earth, and Sun; does the Moon’s phase depend on its position relative to the Sun and Earth?

• A. Yes; the Moon’s phase depends on the relative positions of the Moon, Earth, and Sun.
• B. No; phases are determined solely by Earth's shadow.
• C. Yes; but the Moon's phase is fixed.
• D. No; phases depend only on the Sun's position.

Answer: (A)

Phases depend on how sunlight falls on the Moon and what portion of the illuminated side we can see from Earth, and this changes as the Moon orbits around our planet. As the Moon travels around Earth over about a month, the angle between the Sun, Moon, and Earth shifts, so the lit part facing us grows or shrinks. When the Moon is between Earth and the Sun, we see mostly a dark Moon (new moon). When the Moon is opposite the Sun, we see a fully lit Moon (full moon). Half-lit phases occur when the Moon is at right angles to the Sun from our viewpoint (first and last quarters). The Moon’s phase isn’t fixed and isn’t determined solely by the Sun’s position; it requires the Moon’s position relative to Earth as well. Earth’s shadow can cause lunar eclipses, but that’s a separate event from the regular monthly phases.