When studying a model of moon phases, you should consider the Sun's location.

• A. Sun's location
• B. Moon's distance
• C. Earth's rotation
• D. Star positions

Answer: (A)

Moon phases come from the Sun’s lighting of the Moon and how we see that light from Earth. The Sun’s direction relative to the Earth–Moon system determines which part of the Moon is illuminated and visible to us. As the Moon orbits Earth, the angle between the Sun, Moon, and Earth changes, so the illuminated portion we observe changes too.

When the Sun and Moon are aligned with Earth in between, the near side appears fully lit (full Moon). When the Sun and Moon are on the same side of Earth, the Moon is between the Sun and Earth and we see a new Moon. At roughly a quarter of the way through the cycle, the angle is about 90 degrees, and we see half of the Moon lit (first and third quarter). Distance to the Moon can affect how bright or large it appears, but it doesn’t determine the fraction of the Moon that’s illuminated. Earth’s rotation affects our viewing perspective (day vs. night) but not the lighting geometry of the Moon itself. Star positions don’t influence the Moon’s illumination either.

So to model moon phases, you focus on where the Sun is in relation to Earth and the Moon and how that lighting angle changes over the month.