- The phase known as a full moon occurs when Earth, the sun and the moon are directly aligned. Earth is positioned between the sun and the moon, so the entire half of the moon that is illuminated by the sun is visible to Earth.
- A new moon, also referred to as a dark moon, looks as if there is no moon in the sky. A new moon phase occurs when the moon is positioned directly between Earth and the sun, and the lighted portion of the moon is not visible from Earth.
- The half moon phase occurs twice during the lunar cycle, both before and after the new moon phase. One half of the sunlit side of the moon is visible to Earth during both half moon phases, but the side that is illuminated alternates. The right half appears to Earth before the new moon and the left half appears to Earth after the new moon.
- A crescent moon refers to the lunar phase where less than half of the moon is visible. The gibbous moon refers to the lunar phase where greater than 50 percent of the moon is visible. There are two terms associated with the crescent and the gibbous moons, waxing and waning, which refer to the increasing and decreasing sunlit portions of the moon, respectively.
- Occasionally, a full moon will occur twice in one month, because the lunar cycle is shorter than the calendar month. This occurs once every 33 months and is known as a blue moon. The phenomenon inspired the phrase, "Once in a blue moon."
