What explains the Moon phases?

What explains the Moon phases?

• A. The Sun's light output changes monthly.
• B. The Moon's distance to Earth changes the phase directly.
• C. The Earth's shadow periodically changes the phase.
• D. The Moon's position relative to Earth changes which portion of its sunlit face is visible.

Answer: (D)

The Moon’s phases come from the changing geometry of sunlight on the Moon as it orbits Earth. The Sun always lights half of the Moon, but we see different fractions of that sunlit half depending on where the Moon is in its orbit relative to Earth. When the Moon is between the Sun and Earth, the sunlit side is mostly away from us, so we see a new Moon. As the Moon moves, we start to see more of the lit side, leading to crescent shapes, then half of the lit side at quarter phases, then more than half at gibbous phases, and finally a full Moon when the entire sunlit face is visible. This pattern isn’t caused by changes in the Sun’s output, nor by large changes in the Moon’s distance from Earth, and Earth’s shadow only affects the Moon during occasional eclipses, not the regular monthly cycle. The key is the angle between the Sun, Moon, and Earth determining how much of the illuminated face we can see.

The Moon’s phases come from the changing geometry of sunlight on the Moon as it orbits Earth. The Sun always lights half of the Moon, but we see different fractions of that sunlit half depending on where the Moon is in its orbit relative to Earth. When the Moon is between the Sun and Earth, the sunlit side is mostly away from us, so we see a new Moon. As the Moon moves, we start to see more of the lit side, leading to crescent shapes, then half of the lit side at quarter phases, then more than half at gibbous phases, and finally a full Moon when the entire sunlit face is visible.

This pattern isn’t caused by changes in the Sun’s output, nor by large changes in the Moon’s distance from Earth, and Earth’s shadow only affects the Moon during occasional eclipses, not the regular monthly cycle. The key is the angle between the Sun, Moon, and Earth determining how much of the illuminated face we can see.