In the middle of a bright, sunny day, the sky is usually a shade of blue.

Why blue—and not purple, green, or orange?

Well, it's all because of how the Sun's light reaches Earth!

If you see light from the Sun hitting the ground, it probably looks like plain white

light.

However, that white light is made up of all the colors of the rainbow.

You can actually separate and see all these colors if you shine sunlight through a specially

shaped crystal called a prism.

Light energy travels in waves.

And different colors of light are created by different types of waves.

For example, when our eyes recognize the color red, what we're actually seeing is long,

stretched-out light waves with peaks that are far apart.

And when our eyes recognize the color blue, we're really seeing shorter, choppier light

waves with peaks that are close together.

The different colors of light are all determined by how stretched out the light waves are.

Together, all the colors make up the spectrum of visible light—the light we can see.

So, why do we only see blue light when we look up in the sky on a sunny day?

The blue color we see comes from sunlight hitting Earth's atmosphere—a layer of

gases that gives us air to breathe and keeps the planet warm enough to live on.

When sunlight reaches Earth's atmosphere, it is scattered in all directions by the gases

in the air.

The types of gases in Earth's atmosphere mostly scatter the shorter, choppier waves

of blue light.

So, when we see a blue sky, we're really just seeing all of these blue light waves

scattering in our atmosphere.

But wait a minute: why is the sky a different color at sunset?

Well, as the Sun gets lower in the sky, its light is passing through more of the atmosphere

to reach you.

So, even more of the blue light is scattered away before it gets to you.

This allows more of the orange and yellow light to pass straight through the atmosphere

and directly to your eyes.

However if the whole sky is glowing red, it could be due to particles of dust, pollution,

or smoke in the air.

These particles also scatter lots of blue light, which can lead to a very red sky.

NASA's Earth-observing satellites monitor how many of these particles—called aerosols—are

in our air.

The information from these satellites help forecasters keep an eye on particles in the

air and make sure the air is safe to breathe.

In general, a blue sky is good news.

And now you know why!

Find out more about our home planet at NASA Space Place.