Air scatters sunlight toward your eyes from across the daytime sky, with a strong contribution from shorter wavelengths. Without a nearby atmosphere, sunlight does not create the same bright sky around an astronaut.
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Sunlight meets air
Sunlight contains a mixture of visible colours. Each colour corresponds to a range of wavelengths, the distance between repeating parts of a light wave.
Gas molecules in the atmosphere redirect some of that light. This process has the name scattering. Light that originally travels from the Sun can then reach your eyes from another direction.
The atmosphere is not a blue ceiling. Its appearance depends on the sunlight that its molecules redirect.
Why shorter wavelengths scatter more
For air molecules, Rayleigh scattering increases strongly as wavelength decreases. In the simple relationship, scattering strength varies inversely with the fourth power of wavelength.
The video compares blue light near 450 nanometres with red light near 700 nanometres. A nanometre is one billionth of a metre.
Dividing 700 by 450 and taking the fourth power gives about 5.9. This explains the video's statement that blue scatters about six times more strongly than red.
The ratio depends on which blue and red wavelengths you choose. It is not one fixed number for every colour within those broad ranges.
Blue light from across the sky
Scattered light reaches your eyes from directions away from the Sun. Shorter wavelengths contribute strongly to this light, so the daytime sky appears blue.
The video's arrows show possible directions of scattering. They do not mean that every photon repeatedly bounces from one molecule to the next before reaching your eye.
Why the sky is not violet
Violet has an even shorter wavelength than blue and scatters strongly. However, scattering strength alone does not determine the colour you perceive.
The Sun supplies different amounts at different wavelengths. The atmosphere also absorbs some light, and the human eye responds less strongly to violet.
Your visual system combines signals from different colour-sensitive cells. The resulting mixture appears blue. It does not simply select the single wavelength with the greatest scattering strength.
The longer path at sunset
Near sunset, direct sunlight travels through a longer path in the atmosphere. More blue light leaves that direct path through scattering.
The remaining direct light contains a larger proportion of longer wavelengths. This helps produce yellow, orange, and red sunset colours. Particles and local atmospheric conditions also affect the exact appearance.
The same scattering process therefore helps explain both a blue sky and a red sunset. The difference is the light path that you observe.
A black sky in space
An astronaut can receive bright sunlight while seeing a dark surrounding sky. Space lacks the dense surrounding atmosphere that scatters daylight across the view on Earth.
This does not mean that space contains absolutely no matter or light. It means that the local scattering glow is too weak to produce an Earth-like blue sky.
What this means
To explain the colour, follow the light from the Sun through the air to the eye. The source spectrum, scattering, path length, and visual response all matter.
FAQ
Is sunlight itself blue?
No. It contains a mixture of visible wavelengths.
Is the six-times comparison exact for all blue and red light?
No. It uses the example wavelengths of 450 and 700 nanometres.
Does a black sky mean the Sun is absent?
No. Direct sunlight can remain bright without an atmosphere to spread it across the sky.