The Sun always illuminates roughly half of the Moon. So why don’t we always see a half Moon? The answer is that how much of that illuminated half we can see changes as the Moon orbits Earth. Sometimes we see nearly all of it, sometimes just a sliver, and sometimes almost none at all. That changing view is what creates the Moon’s phases: the thin crescents, the half-lit quarters, the bright Full Moon, and the near-invisible New Moon.
Once you understand how that changing view works, the whole lunar cycle starts to make sense. A crescent or half Moon can tell you where the Moon is in its cycle, whether it is heading toward or away from Full Moon, and even something about the positions of the Sun, Earth and Moon in space.
What Causes the Phases of the Moon?
The easiest way to understand Moon phases is to picture just the Sun, Earth and Moon.
At any point in the cycle, sunlight illuminates roughly half of the Moon. As the Moon travels around Earth, what changes from our perspective is how much of that illuminated half we can see.
Imagine four points in the Moon’s orbit. At New Moon, the illuminated half faces mostly away from us, so we see almost none of it. At First Quarter, our view is from the side and we see about half of the illuminated half. At Full Moon, we are looking toward nearly all of the illuminated half. At Last Quarter, we see about half again.
That’s the basic mechanism behind every phase of the Moon.
The 8 Phases of the Moon
The Moon moves continuously around Earth, so its appearance changes gradually rather than jumping from one phase to another. The eight phases of the Moon are simply names for recognizable points along that cycle:
New Moon → Waxing Crescent → First Quarter → Waxing Gibbous → Full Moon → Waning Gibbous → Last Quarter → Waning Crescent → New Moon
Four of these phases give us useful landmarks. New Moon begins the cycle, First Quarter comes roughly one-quarter of the way through, Full Moon about halfway through, and Last Quarter about three-quarters of the way through. The crescent and gibbous phases fill the stages between these four landmarks.
The cycle begins at New Moon, when almost none of the illuminated half is visible from Earth. Soon afterward, a Waxing Crescent emerges and grows night by night. Sometimes you can even make out the rest of the Moon as a faint ghostly disk, illuminated by sunlight reflected from Earth—a phenomenon called Earthshine.
As the illuminated portion grows, the Moon reaches First Quarter, when half of the disk we see appears bright. The name can be confusing: it doesn’t mean that a quarter of the Moon is illuminated. The Moon is roughly one-quarter of the way around its cycle. From our earlier model, we’re seeing about half of the illuminated half.
The bright portion then grows beyond half, becoming a Waxing Gibbous Moon, until we reach Full Moon and can see nearly the entire illuminated half. We’ve now reached roughly the halfway point in the cycle.
From here the sequence reverses. The Moon becomes Waning Gibbous as the illuminated portion we see starts shrinking, reaches Last Quarter when half of the visible disk is bright again, and finally becomes a narrowing Waning Crescent. Eventually that crescent disappears into the Sun’s glare as the Moon returns to New Moon and the cycle begins again.
What Do Waxing, Waning, Crescent and Gibbous Mean?
The names of the intermediate Moon phases become much easier to remember once you know what the words mean. Waxing means the illuminated portion we see is growing, while waning means it is shrinking. So the Moon waxes from New Moon toward Full Moon and wanes from Full Moon back toward New Moon.
Crescent describes a Moon with less than half of its visible disk illuminated, while gibbous describes a Moon with more than half of its visible disk illuminated, but less than the full disk. Put the words together and the names tell you both what the Moon looks like and where it is heading: a Waxing Crescent is less than half illuminated and growing; a Waning Gibbous is more than half illuminated and shrinking.
You may sometimes hear the simple rule that a Moon bright on the right is waxing, while one bright on the left is waning. That isn’t a reliable rule everywhere on Earth, because the Moon’s apparent orientation varies depending on where you are. People around the world still see essentially the same Moon phase at the same time—it is the Moon’s orientation in the sky that can look different.
How Long Does the Lunar Cycle Take?
The Moon takes about 29.5 days to complete one full cycle of phases, from one New Moon to the next. This is called a synodic month, and it’s the reason the lunar phase cycle is often described as lasting about a month.
But the Moon actually takes only about 27.3 days to orbit Earth once. Why the difference? While the Moon is circling Earth, Earth itself has been moving along its orbit around the Sun. So after 27.3 days, the Moon may be back where it started relative to Earth, but Earth and Moon are no longer in the same position relative to the Sun. The Moon therefore has to travel a little farther around Earth before it reaches the same position relative to the Sun again. That takes about another two days, bringing the full phase cycle to roughly 29.5 days.
One orbit around Earth takes the Moon about 27.3 days; returning to the same phase takes about 29.5 days.
This is also why Moon phases don’t occur on the same dates every calendar month. Our calendar months last between 28 and 31 days and aren’t synchronized with the 29.5-day lunar cycle, so the phases gradually shift through the calendar.
Explore More About the Moon
Once you understand the phases, the Moon becomes much easier to make sense of in the night sky. Its appearance can tell you where it is in its cycle, roughly when it will be visible and even give you clues about the position of the Sun when it’s below the horizon.
Continue exploring with our guide to the Moon, discover more facts about the Moon, or find the best features on the Moon for stargazers.
