How do we know?
↻ One of the pages with a moving picture
John Tyndall made a little blue sky in a glass tube.
- Sunlight zooms into the air around Earth. Air is made of tiny bits called molecules.
- Light bounces off the molecules in new directions. This is scattering.
- Blue light gets scattered much more than red or yellow light.
- So blue light reaches your eyes from every part of the sky, and the whole sky looks blue.
In 1869, John Tyndall shone a white light through a glass tube and slowly filled it with smoke. From the side, the beam looked blue. From the far end, it looked red. His blue-sky tube is kept at the Royal Institution in London.
The scattering of blue light is named after Lord Rayleigh, who had his home lab at Terling, England.
It also explains two other puzzles. Space looks black because it has too few molecules to scatter light, and the sky looks paler near the horizon.
Wow number
78% of the air is a gas called nitrogen. Its tiny molecules are a big part of the air that scatters blue light.
Myth or fact?
High up in space, the sky is still blue. Myth. Space is black. It has too few molecules to scatter light.
On Mars, the sky near the setting Sun looks blue-gray. Fact. Mars dust scatters light differently.
Think about it
Look low, near the horizon. Why might the sky look paler there than overhead?
Where in the world?
Royal Institution, London, England where John Tyndall made a blue “sky” in a glass tube in 1869. Lord Rayleigh had his home lab at Terling, England — 51.5089° N, 0.1423° W.
Where these facts come from
Every number on page 30, and who measured it
This page explains how air scatters blue light, and shows the blue-tinted sunset sky on Mars.
Why Is the Sky Blue? — NASA Space Place, NASA's science site for kids
spaceplace.nasa.gov/blue-sky/en
We checked it on 31 October 2026
This page tells us why the sky is paler near the horizon and why space looks black.
Why Is The Sky Blue? — The U.S. National Weather Service
weather.gov/fgz/SkyBlue
We checked it on 31 October 2026
This page tells us that 78% of the air is nitrogen.
What's in the Air? — UCAR, a group of U.S. universities that study the air and weather
scied.ucar.edu/learning-zone/air-quality/whats-in-the-air
We checked it on 31 October 2026
This page shows John Tyndall's blue-sky glass tube from 1869.
John Tyndall's blue sky apparatus — The Royal Institution in London, where John Tyndall worked
rigb.org/explore-science/explore/collection/john-tyndalls-blue-sky-apparatus
We checked it on 31 October 2026
This page tells us about Lord Rayleigh and his home laboratory at Terling.
Lord Rayleigh – Biographical - NobelPrize.org — The Nobel Prize, the world's most famous science award
nobelprize.org/prizes/physics/1904/strutt/biographical
We checked it on 31 October 2026
Has this changed?
Why the sky is blue is not going to change.
What changes is the sky itself: dust, smoke and clouds make it look different from day to day.
Words from this page
- molecule
- A tiny bit of something. Air is made of molecules.
- scattering
- When light bounces off tiny bits and goes in new directions.
- nitrogen
- The gas that makes up most of the air.
Try it!
Make a little sky in a glass.
1. In a dark room, fill a clear glass with water. 2. Shine a flashlight through the side of the glass onto white paper behind it. 3. Stir in milk, a few drops at a time. Watch the water and the spot on the paper.
What happens: the water starts to look a little blue from the side, and the spot on the paper turns orange-yellow.
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