The phase of the moon depends on the relative positions of the Sun, Earth, and Moon. As the Moon orbits Earth, different parts of it are illuminated by the Sun, and we see different amounts of the lit portion from our perspective on Earth.

When the Moon is directly between the Sun and Earth, the side of the Moon facing Earth is not illuminated, and we see a New Moon. As the Moon moves in its orbit, more of the side facing Earth becomes illuminated, and we see a Waxing Crescent Moon. When the Moon is at a right angle to the Sun and Earth, half of the side facing Earth is illuminated, and we see a First Quarter Moon.

As the Moon continues to move in its orbit, more of the side facing Earth becomes illuminated, and we see a Waxing Gibbous Moon. When the Moon is opposite the Sun in the sky, the entire side facing Earth is illuminated, and we see a Full Moon. After the Full Moon, the amount of illuminated side facing Earth begins to decrease, and we see a Waning Gibbous Moon, Third Quarter Moon, and Waning Crescent Moon, until the cycle repeats with a New Moon.