What causes the Moon's illuminated portion to change over the month?

What causes the Moon's illuminated portion to change over the month?

• A. Lunar phases
• B. Eclipses
• C. Tides
• D. Rotation

Answer: (A)

Light from the Sun shines on the Moon, and what we see changes because the Moon's position relative to the Sun and Earth keeps shifting as it orbits Earth. This changing geometry means different fractions of the Moon’s sunlit hemisphere are visible from our perspective, producing the monthly sequence of illuminated shapes we call lunar phases (new, crescent, quarter, gibbous, full, and back to new). It isn’t caused by eclipses, which are specific alignments that momentarily block sunlight, nor by tides, which are gravity effects on Earth. It isn’t driven by the Moon’s rotation, either—the Moon rotates, but it’s tidally locked to Earth, so the visible face doesn’t flip; the changing illumination comes from the changing angle of sunlight as the Moon orbits.

Light from the Sun shines on the Moon, and what we see changes because the Moon's position relative to the Sun and Earth keeps shifting as it orbits Earth. This changing geometry means different fractions of the Moon’s sunlit hemisphere are visible from our perspective, producing the monthly sequence of illuminated shapes we call lunar phases (new, crescent, quarter, gibbous, full, and back to new).

It isn’t caused by eclipses, which are specific alignments that momentarily block sunlight, nor by tides, which are gravity effects on Earth. It isn’t driven by the Moon’s rotation, either—the Moon rotates, but it’s tidally locked to Earth, so the visible face doesn’t flip; the changing illumination comes from the changing angle of sunlight as the Moon orbits.