what if Earth and Mars switched places?

How often do eclipses happen?

That's a great question.

The moon is one of my favorite things to look at.

It has so many interesting things that you can notice.

Well, there's lots of things you might have noticed

about the moon before, like maybe you've noticed

how the moon seems to look bigger whenever it's low

in the sky, or you've probably also noticed

how the moon appears to change shape.

We call these the phases of the moon,

and of course there's the full moon.

That's when you can see the whole thing.

Now, here's something that's very strange.

People have noticed that sometimes, on special occasions,

when it's a full moon out, the moon starts to disappear.

Now, you might think, "Oh, maybe that's how

"it changes phases, like maybe that's how

"it goes form being a full moon to a crescent moon.

But that takes days to happen.

is something that happens in just a few hours.

What is going on here?

Is the moon disappearing?

It turns out what we're seeing when this happens

is an eclipse of the moon, a lunar eclipse.

A lunar eclipse is different from a solar eclipse,

or an eclipse of the sun.

That's when the moon appears to go in front of the sun.

So a solar eclipse always happens during the daytime.

And a solar eclipse is pretty rare.

The Great American Solar Eclipse of 2017

was the first time in nearly a hundred years

that a total eclipse of the sun was visible

across the United States.

But in a lunar eclipse, the moon is the main character.

A lunar eclipse is something

that can only happen at nighttime.

You're not looking at the sun.

You don't need special glasses to watch it.

And lunar eclipses happen a little more often

than solar eclipses do.

You can usually see one about every two years or so.

For a long time, people had no idea why the moon

would seem to disappear during a lunar eclipse.

In fact, if you didn't know that the moon

was going to come back, you might really worry

that it was actually disappearing and leaving us forever.

But eventually we were able to figure out

that as the sun shines on the Earth,

the Earth casts a shadow outwards into space.

It's really just like how we cast shadows

here on the ground.

A lunar eclipse happens when the moon

passes through the shadow of the Earth.

That's why the moon seems to disappear.

In fact, you can notice that the shape of the shadow

is curved, just like the Earth.

Think about it, the Earth is round.

Once the moon moves out of our shadow,

the full moon reappears again.

But I've saved the best part of a lunar eclipse for last.

I haven't mentioned the strangest thing of all

about a lunar eclipse.

Not only does the full moon seem to start disappearing

as it goes into the Earth's shadow, but watch what happens

as the moon gets into the very center part

of the Earth's shadow.

Now, you'd think that, in the center of the Earth's shadow,

the moon would look totally black, but it doesn't.

It turns reddish orange.

Isn't that weird?

Some people say it's the color of blood.

For this reason, people sometimes call

a lunar eclipse a blood moon.

Now, why does it do this?

It's a great questions.

To be honest with you, the answer is a little complicated.

From having watched a lot of lunar eclipses

since I was a kid, I've learned that

no two lunar eclipses are exactly alike.

Sometimes it looks orangy-red,

but sometimes it looks really deep red,

and other times more brownish orange.

For me, that's part of the thrill

of watching a lunar eclipse.

You never know exactly what color it's going to be

until you watch it happen.

And you'll have a chance to find out soon.

Now, it won't be visible from everywhere on Earth.

For most of North America it will be seen

during the very early hours before sunrise.

But don't worry, if you're not able to see it,

there'll be another one in Mars 2020

that's visible during the evening.

So, in summary, a lunar eclipse happens

when the moon moves through the shadow of the Earth.

It makes a full moon seem to disappear

and then reappear again.

Surprisingly, once the moon gets into the center part

of the shadow, it turns a reddish orange color,

what some people call the "blood moon".

Lunar eclipses happen about every two years or so,

so you've got a lot more chances to see them

than you do a solar eclipse.

That's all for this week's question.