- [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.