Phases of the moon | Middle school Earth and space science | Khan Academy

- [Narrator] Imagine that one day,

all of the clocks and
computers on Earth broke,

and all the calendars disappeared.

How would you keep track of
how much time had passed?

Well, you could look to the Moon.

Humans have used the Moon
to keep track of time

for thousands of years.

It isn't a coincidence that the word moon

is related to the word
month in old English.

The Moon is Earth's
only natural satellite.

A natural satellite is a
naturally occurring body

that orbits a planet.

Some of the planets in our solar system

have more than 50 satellites or moons,

but the Earth just has one.

The Moon does not generate its own light.

We can see the Moon from the Earth,

because it is partially lit
by the light from the sun.

In fact, moonlight is just sunlight

reflected from the Moon onto Earth.

The Moon takes about 27 days

to make a full orbit around Earth.

And as it does, the lit part of the Moon

appears to change shape
to us here on Earth.

These shapes are called the
moon phases, or lunar phases.

So why do we see different lunar phases?

Well, the 27 days it takes the
Moon to orbit the Earth once

is the same amount of time

it takes the Moon to
rotate on its axis once.

This means that the same side of the Moon

is always facing us.

At the same time, the sun always lights up

half of the sphere of the Moon.

However, the side of the
Moon that's facing us

isn't always the same part of the Moon

that the sun lights up.

This causes the Moon to appear
to have different shapes,

or faces in the sky,

depending on the time of the month.

Let's take a closer look
at the Moon's phases.

First, we have the new moon,

which happens when the
Moon is closest to the sun

in its orbit.

During this phase, the
lit side of the Moon

is completely facing away from Earth.

So it looks like the Moon has disappeared.

In a day or two, we'd be able
to see a little sliver of moon

in the sky.

Over the next few days,

the crescent moon will appear
to get bigger and bigger.

When the Moon appears to get bigger

from one day to the next, we
say that the Moon is waxing.

So this phase of the Moon

is called the waxing crescent moon.

You might notice that sometimes

we can still see the rest
of the Moon in the shadow.

This is because the Earth
reflects sunlight onto the Moon,

just like the Moon
reflects light onto Earth.

Eventually, the Moon appears
to change shape so much

that it isn't a crescent anymore,

but a half circle in the sky.

This is called a first quarter moon.

There are two ways to think about

why this phase is called a quarter moon.

Even though it looks like
the Moon is half illuminated,

the Moon is a sphere.

So we can only ever see
half of the Moon from Earth.

During a quarter moon,

the Moon forms a right angle
with Earth and the sun.

This means that we a half lit portion

of the half of the Moon
that's always facing us.

It's half of a half, so it's a quarter.

Also, a first quarter moon

occurs when the Moon is a
quarter of the way through

its new cycle.

Next, there's the waxing gibbous moon.

The word gibbous comes from the
Latin word meaning humpback.

Once the Moon is farthest
from the sun in its orbit,

the full sun lit side
of the Moon faces Earth.

We call this phase the full moon,

but the Moon is not done yet.

It's only finished half of the cycle.

Next, we have the same
phases, but in reverse.

As the Moon appears to
get smaller and smaller,

we say that it's waning.

The full Moon appears to shrink,

and then we see a waning gibbous.

Then there's the third quarter moon.

This happens when the
Moon is three quarters

the way done with its cycle,

and the Moon forms another right angle

with the sun and the Earth.

The Moon appears to shrink even more,

and it becomes a waning crescent,

and the cycle starts anew
with another new moon.

Even though the Moon completes
an orbit every 27 days,

the lunar phases actually
repeat about every 29.5 days.

This is because the Earth
is revolving around the sun

while the Moon completes its orbit.

So the Moon has to travel
a little extra to catch up.

The Moon isn't just something beautiful

to look at in the sky,

or just an easy way to keep track of time.

The Moon's gravity controls the tides,

which are the rise and
fall of water in oceans,

lakes, and rivers.

Tides allow for unique ecosystems,
like tide pools to exist.

And we can use the tides
to create electricity,

and tidal power plants.

The Moon also helps keep
Earth's access stable.

Without it, our planet would
wobble more dramatically

on its axis over long periods of time,

which would change up our
weather and our seasons.

So even though your calendar

is pretty unlikely to
spontaneously combust tomorrow,

you could still thank the Moon

for being such a stabilizing influence,

and good companion to our planet.