The Moon, which shines by reflecting sunlight, exhibits phases because

Question

BlackTom AI · Accepted Answer

To understand why the Moon shows phases, we need to consider the geometry of the Sun, Earth, and Moon as a system.
Option 1: 'the rotation of the Moon on its axis' — While the Moon does rotate, it is tidally locked to Earth, meaning the same side always faces us. This rotation does not cause the changing phases; the phases come from how much of the Moon's sunlit side we can see from Earth, not from the Moon’s rotation itself.
Option 2: 'the Earth orbits the Sun' — The Earth’s orbit around the Sun affects the overall year-long path of the Moon and Earth together, but it does not by itself cause the sequential bright/dark portions we call Moon phases. Phases are a result of the relative positions of Sun, Moon, and Earth, more specifically the Moon-Earth-Sun alignment.
Option 3: 'the Sun orbits the Earth' — This is incorrect in modern astronomy; the Sun does not orbit the Earth. The apparent motion of the Sun across the sky is due to Earth's rotation and orbit around the Sun, not a Sun-centered orbital change that would produce Moon phases.
Option 4: 'shadows are cast by nearby planets' — Planets do not cast the shadows that create the Moon’s phases. The phases are caused by sunlight illuminating different portions of the Moon as seen from Earth, not by planetary shadows.
Option 5: 'of the changing angle between Earth, Moon and Sun' — This is the correct mechanism: as the Moon orbits Earth, the angle between the Earth–Moon line and the Sun changes, altering how much of the Moon’s sunlit hemisphere is visible from Earth, producing new, crescent, first quarter, gibbous, and full phases.

The Moon, which shines by reflecting sunlight, exhibits phases becauseSingle choice

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