The Moon has phases because it orbits Earth while sunlight illuminates half of it.
The Moon itself is not growing, shrinking, or changing shape. What changes is how much of its sunlit half we can see from Earth.
That shifting geometry produces the familiar new moon, crescent, quarter, gibbous, and full moon phases.
Half the Moon is almost always lit by the Sun
Just as Earth has a day side and a night side, the Moon has a sunlit side and a dark side.
At nearly any moment, one half of the Moon is illuminated by the Sun.
The important question is how that lit half is oriented relative to us.
As the Moon travels around Earth, our viewing angle changes.
New moon happens when the lit side faces mostly away from us
During a new moon, the Moon is located roughly between Earth and the Sun.
The half facing the Sun is bright, but most of that illuminated side faces away from Earth.
From our viewpoint, the Moon appears dark or nearly invisible.
The Moon did not stop reflecting sunlight.
We are simply looking mostly at the night side.
A full moon is the opposite geometry
About two weeks later, Earth is roughly between the Sun and Moon.
Now the Moon’s sunlit half faces toward us, and we see an almost fully illuminated disk.
That is the full moon.
Between those two positions, we see different fractions of the lit half.
What are crescent, quarter, and gibbous phases?
A crescent moon shows less than half of the visible disk illuminated.
At first quarter and third quarter, we see half of the Moon’s visible face illuminated. The name “quarter” refers to the Moon being about one-quarter or three-quarters of the way through its orbit, not to seeing one quarter of the Moon.
A gibbous moon shows more than half but less than the full disk.
When the illuminated portion is increasing, the Moon is waxing. When it is decreasing, the Moon is waning.
Does Earth’s shadow cause the phases?
No.
Earth’s shadow causes a lunar eclipse, which is a different event.
If Earth’s shadow created the ordinary phases, lunar eclipses would happen every month and the geometry would look very different.
Moon phases are produced simply by our changing view of the Moon’s sunlit half.
This distinction is one of the biggest misconceptions about the Moon.
How long does the phase cycle take?
From one new moon to the next takes about 29.5 days.
During that time, the Moon completes the familiar sequence of phases.
That is why the lunar cycle and the idea of a month have been connected in human calendars for thousands of years.
Why do we always see roughly the same side of the Moon?
The Moon rotates once on its axis in about the same amount of time it takes to orbit Earth.
That synchronized rotation keeps nearly the same hemisphere facing us.
It does not mean the Moon does not rotate.
In fact, if it did not rotate at all, we would eventually see every side as it traveled around Earth.
So the changing Moon in the night sky is not a shape-changing object and not a monthly shadow show.
It is geometry.
Sunlight illuminates half.
The Moon moves.
Our viewpoint changes.
And the sky gives us a different-looking Moon almost every night.
A related part of the natural world appears in Why Does Earth Have Seasons?, where the same kind of physical or biological reasoning answers another familiar question.
And that’s the short explanation.
Related explanations: Why Does Earth Have Seasons? and Why Do We Have Day and Night?