Why do we observe different Moon phases?

Why do we observe different Moon phases?

• A. The Sun's illumination changes over time
• B. The Moon orbits Earth and we see varying amounts of its sunlit side over time
• C. The Moon's distance to Earth changes dramatically
• D. The Earth's shadow causes the phases

Answer: (B)

Different Moon phases happen because the Moon orbits Earth and we see different portions of its lit side as it moves around us. The Sun provides light consistently, but the angle between the Sun, the Moon, and Earth changes over the month. As the Moon travels in its orbit, more or less of the sunlit hemisphere faces us, creating the sequence from near side not visible (new Moon) through crescent, first quarter, gibbous, to fully lit (full Moon), then back again. The Moon’s distance from Earth varies, but that doesn’t drive the phases. Earth’s shadow can dim the Moon during a lunar eclipse, but regular Moon phases are due to the changing geometry of Sun–Moon–Earth.

Different Moon phases happen because the Moon orbits Earth and we see different portions of its lit side as it moves around us. The Sun provides light consistently, but the angle between the Sun, the Moon, and Earth changes over the month. As the Moon travels in its orbit, more or less of the sunlit hemisphere faces us, creating the sequence from near side not visible (new Moon) through crescent, first quarter, gibbous, to fully lit (full Moon), then back again. The Moon’s distance from Earth varies, but that doesn’t drive the phases. Earth’s shadow can dim the Moon during a lunar eclipse, but regular Moon phases are due to the changing geometry of Sun–Moon–Earth.