What primarily determines Moon phase visibility from Earth?

• A. Sunlight facing the Moon from Earth
• B. Distance to the Moon
• C. Earth's rotation
• D. Moon's orbital speed

Answer: (A)

The main idea here is that Moon phases come from how the Sun lights the Moon and how much of that sunlit surface we can see from Earth. The Sun always shines on half of the Moon, but as the Moon orbits, the angle between the Sun, Moon, and Earth changes. When the Moon is between the Sun and Earth, we see little or none of the lit side (new Moon). When the Sun and Moon are on opposite sides of Earth, the entire sunlit hemisphere faces us (full Moon). Intermediate positions give crescent and quarter phases. So the visibility of a given phase is determined by how the Sunlit portion is oriented toward Earth, not by the Moon’s distance, Earth's rotation, or the Moon’s orbital speed. Distance might affect brightness a little, and rotation just affects when you happen to see it, but the phase itself hinges on the Sun–Moon–Earth alignment.