The Moon does not actually change shape. What changes is the amount of its sunlit half that we can see from Earth.
The simple idea behind Moon phases
At every moment, sunlight illuminates roughly half of the Moon. As the Moon moves around Earth, our viewing angle changes. Sometimes we see mostly the dark half, sometimes a thin bright crescent, and sometimes almost the entire illuminated half. That repeating pattern is the lunar phase cycle.
A complete cycle from one new moon to the next takes about 29.5 days. The useful thing to remember is that phases are a matter of geometry: the Sun, Earth and Moon are constantly changing position relative to one another.
The eight commonly named phases
1. New Moon
The Moon is near the same direction in the sky as the Sun, so the illuminated side mostly faces away from us. The Moon can be difficult or impossible to see in the bright daytime sky.
2. Waxing Crescent
A slim bright crescent appears and grows a little wider each day. “Waxing” simply means the visible illuminated portion is increasing.
3. First Quarter
About half of the Moon's visible disk looks illuminated. Despite the name, this phase occurs roughly one-quarter of the way through the lunar cycle.
4. Waxing Gibbous
More than half of the visible disk is bright, but the Moon is not yet full. “Gibbous” describes a shape that is more than half illuminated.
5. Full Moon
Earth lies roughly between the Sun and Moon, so we see most of the Moon's illuminated side. A full Moon usually rises around sunset and is visible through much of the night.
6–8. Waning phases
After full Moon, the bright portion begins to shrink. The sequence is waning gibbous, third (or last) quarter, and waning crescent before the cycle returns to new Moon.
Why Moon phases are not caused by Earth's shadow
This is one of the most common beginner misconceptions. Earth's shadow causes a lunar eclipse, not the normal monthly phases. Most months, the Moon's orbit is tilted enough that it passes above or below Earth's shadow.
A good way to observe the cycle
Look at the Moon at roughly the same time on several clear evenings. Notice not only its shape but also where it appears in the sky. A simple observation journal quickly makes the cycle feel intuitive.
For deeper lunar science, explore the NASA Moon overview. You can also continue with our beginner's night-sky guide.
Key takeaway
Moon phases are a predictable result of sunlight and orbital motion. Once you picture half the Moon always illuminated and imagine your viewpoint changing as it orbits Earth, the whole cycle becomes much easier to understand.