Longer Path Through Air

The Clear Sky Blanket

The Earth wears a clear blanket. We call this blanket the atmosphere. It is full of invisible gas molecules. Nitrogen and oxygen make up this air 1.

The atmosphere protects our planet. It keeps us warm. It gives us air to breathe. It also plays tricks with the sunlight.

Sunlight must travel through this air. It must pass through the gases. This journey changes the sky colors.

In plain terms: The air around our planet catches energy from the sun. It spreads this light and heat all over the world.

Empty Space to Thick Air

Space is completely empty. Space has no air. It has no gas molecules.

Sunlight travels through space easily. It does not scatter at all. The light stays perfectly white. Light moves very fast. It zips through empty space in straight lines 3.

Then the light reaches Earth. It hits the top of our atmosphere. The journey suddenly gets much harder. The light slows down just a tiny bit. It bumps into the gas molecules.

Straight Down at Noon

Think about the sun at noon. The sun is high up in the sky. It sits right over your head.

The sunlight points straight down at you. It takes a short path to the ground. It cuts right through the thinnest part of the sky blanket.

Imagine poking a needle straight through a quilt. The needle pops out the other side fast. It only touches a little bit of fabric.

Sunlight at noon does the same thing. It only touches a small amount of air. It hits fewer gas molecules on its way down. This short trip keeps the sky looking bright blue.

The Long Sunset Journey

Later in the day, the sun moves. It sinks low toward the horizon. The horizon is the line far away. It is where the sky meets the ground.

Now the sun is not over your head. It is far away on the side of the Earth. The sunlight cannot point straight down anymore.

The light must travel sideways. It moves across the sky blanket. It does not move straight through it. This sideways path is much longer.

Imagine poking that needle sideways through the quilt. The needle must slide through a lot of fabric. It takes a long time to come out.

Sunlight at sunset takes a long trip. It travels through a very thick slice of air.

The Giant Flashlight

Imagine holding a giant flashlight. You shine it straight down at a glass of water. The light goes right through the thin middle. It takes a very short path.

Now, turn the glass on its side. Shine the flashlight through the long way. The light must travel through much more water.

The Earth's atmosphere works the exact same way. The air is like the water in the glass. A low sun shines through the long way. The light must travel through miles of extra air.

Bumping Into More Air

A longer path means more air. The sunlight must push past many more gas molecules. The light bumps into extra nitrogen. It bumps into extra oxygen. Every bump scatters the light a little bit.

Think about running across a shallow pool. You can run fast. The water barely slows you down. This is like the noon sunlight.

Now think about walking the long way. Imagine walking from the deep end to the shallow end. The water pushes against you the whole time. You get very tired. This is like the sunset light. Hitting so much air makes the light tired.

Let us compare the two paths. Look at this chart.

At noon, the light has an easy trip. At sunset, the light runs an obstacle course. It must dodge millions of extra gas molecules!

What Happens Next

This long path is a big secret. It makes the sunset look so colorful.

White light holds all the rainbow colors. The long trip changes those colors. Some colors cannot survive the long journey.

Blue light waves are short. They scatter very easily. During the long sunset trip, the blue light bounces away. It gets completely lost in the thick air!

Soon, you will learn the rest of the secret. You will watch the blue light get lost. You will see only the red colors reach you.

Key Terms

• Atmosphere 2 — The clear blanket of gas molecules that wraps around the Earth.
• Horizon — The line far away where the sky seems to meet the ground.
• Path Length — The distance that light must travel to reach a certain spot.