We see different phases of the Moon because, as it orbits Earth, we see different amounts of its sunlit half from our point of view on Earth.
Sunlight and the Moon
The Moon does not make its own light; it only reflects sunlight. At any moment, sunlight always illuminates half of the Moon’s surface, creating a “day side” and a “night side,” just like on Earth.
The Moon’s orbit around Earth
The Moon goes around Earth about once every 29.5 days. As the Moon moves along this orbit, the angle between the Sun, Earth, and Moon changes, so we see different fractions of the lit half.
How this creates phases
- New Moon: The Moon is between Earth and the Sun, and the sunlit side faces away from us, so the Moon is hard to see.
- Full Moon: Earth is between the Sun and Moon, and we see the entire sunlit half.
- In between: We see only part of the lit half, giving crescent, quarter (half), and gibbous shapes that grow (wax) from new to full and shrink (wane) from full back to new.
Not Earth’s shadow
A common misconception is that phases come from Earth’s shadow on the Moon. In reality, Earth’s shadow only causes lunar eclipses, which are rare, while phases happen every month due to the Moon’s own day-night side and its changing position in the sky.
