📚 Study helper: This page helps you learn and practice for school. Read it, listen to it, and try to answer this: can you name the Moon's phases in order, starting with the new moon?
The Moon looks like it changes shape in the sky.
Those shapes are called the phases of the Moon.
But the Moon does not really change shape.
The Sun always lights half of the Moon.
As the Moon travels around Earth, we see different amounts of that lit half.
That is why the Moon seems to change.
NASA StarChild says the Moon goes through a cycle of eight phases, and the cycle repeats every 29.5 days.
Pick your grade:
Elementary: The Eight Phases, In Order
The phases follow one behind the other in a strict order, NASA StarChild says.
Here they are, starting with the new moon.
1. New Moon
The lighted side of the Moon faces away from Earth.
So we cannot see the Moon at all.
2. Waxing Crescent
A small part of the Moon appears lighted.
It grows larger each day.
3. First Quarter
The right half of the Moon appears lighted.
The lighted part keeps growing larger.
4. Waxing Gibbous
More than half of the Moon appears lighted.
More and more becomes lighted each day.
5. Full Moon
The lighted side of the Moon faces toward Earth.
The whole face we see is lit up.
6. Waning Gibbous
More than half of the Moon appears lighted.
But less and less becomes lighted each day.
7. Third Quarter
The left half of the Moon appears lighted.
The lighted part keeps growing smaller.
8. Waning Crescent
A small part of the lighted Moon is visible.
It grows smaller each day.
Then the cycle starts over with a new moon.
Waxing and waning
Waxing means the lighted part we see is growing.
Waning means the lighted part we see is shrinking.
First the Moon waxes, from new moon to full moon.
Then the Moon wanes, from full moon back to new moon.
Middle School: Why the Phases Happen, Step by Step
NASA StarChild says the Moon orbits Earth at an average distance of 382,400 kilometers.
The lunar month is the 29.53 days it takes to go from one new moon to the next.
Just like Earth, half of the Moon is lit by the Sun, while the other half is in darkness.
The side lit by the Sun is always the side pointed toward the Sun.
But the lit side does not always face Earth.
As the Moon circles Earth, the amount of the lit side we see changes. Those changes are the phases of the Moon.
We only see the Moon because sunlight reflects back to us from its surface, NASA StarChild says.
New moon: lined up with the Sun
At new moon, the Moon is lined up between Earth and the Sun.
We see the side of the Moon that is not being lit by the Sun.
In other words, we see no Moon at all, because the brightness of the Sun outshines the dim Moon.
When the Moon is exactly lined up with the Sun, as viewed from Earth, we experience an eclipse, NASA StarChild says.
Waxing: the lit part grows
As the Moon moves eastward away from the Sun in the sky, we see a bit more of the sunlit side each night.
A few days after new moon, we see a thin crescent in the western evening sky.
The crescent Moon waxes, or appears to grow fatter, each night.
When half of the Moon's disc is illuminated, we call it the first quarter moon.
The name comes from the fact that the Moon is now one-quarter of the way through the lunar month.
From Earth, we are now looking at the sunlit side of the Moon from off to the side.
The Moon continues to wax. Once more than half of the disc is illuminated, it has a shape we call gibbous.
Full moon: the whole lit face
The gibbous moon appears to grow fatter each night until we see the full sunlit face of the Moon. We call this phase the full moon.
It rises almost exactly as the Sun sets, and sets just as the Sun rises the next day.
The Moon has now completed one half of the lunar month.
Waning: the lit part shrinks
During the second half of the lunar month, the Moon grows thinner each night. We call this waning.
Its shape is still gibbous at this point, but it grows a little thinner each night.
As it reaches the three-quarter point in its month, the Moon once again shows us one side of its disc illuminated and the other side in darkness.
However, the side that we saw dark at the first quarter phase is now the lit side.
As it completes its journey and approaches new moon again, the Moon is a waning crescent.
Try it with a lamp and a baseball
NASA StarChild says you can show the phases yourself with a lamp and a baseball, with a grown-up's help.
Place the lamp with its shade removed in one end of a darkened room.
Sit in the other end of the room, and hold the baseball up in front of you, between your face and the lamp.
Now move the ball around your head at arm's length, slowly, from right to left.
As the baseball orbits your head, you will see it go through the same phases as the Moon.
Your head is like Earth. The ball is like the Moon. The lamp is like the Sun.
Words to Know
Phase: A phase is one of the shapes the Moon seems to have in the sky. The Moon passes through a cycle of eight phases, and the cycle repeats.
Orbit: To orbit is to travel around something. The Moon orbits Earth, and as it travels, we see different amounts of its lit side.
Lunar month: The time it takes to go from one new moon to the next. NASA StarChild's activity says the cycle of phases repeats every 29.5 days, and its question page gives the lunar month as 29.53 days.
Waxing: Waxing means the lighted part of the Moon we see is growing larger. The crescent Moon waxes, or appears to grow fatter, each night.
Waning: Waning means the lighted part of the Moon we see is growing smaller. During the second half of the lunar month, the Moon wanes, or grows thinner, each night.
Crescent: A crescent phase is when only a small part of the Moon appears lighted — a thin sliver of light.
Gibbous: A gibbous phase is when more than half of the Moon appears lighted, but not the whole face.
Quarter: At a quarter phase, half of the Moon's disc is illuminated. The first quarter comes when the Moon is one-quarter of the way through the lunar month.
📚 Study helper: Great job! Now try this. Say the eight phases out loud, in order: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, waning crescent. Then answer: does the Moon really change shape? No — we just see different amounts of its sunlit half.
Sources
NASA StarChild (Goddard Space Flight Center) — What are the phases of the Moon?: http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question3.html
NASA StarChild (Goddard Space Flight Center) — Moonlight Madness (the eight-phases activity): http://starchild.gsfc.nasa.gov/docs/StarChild/solar_system_level2/moonlight.html