Why is the sky blue?

Why is the sky blue - and those sunsets so red!!

Inspired by some crazily gorgeous skies recently, cycling back from my morning with my accountability partner - talking social media and how to use it - the blue skies in the wintry cold Dulwich Park inspired me to answer the classic question, why is the sky blue?

1. The View from the Moon

Imagine standing on the Moon. You look up, and despite the Sun blazing overhead, the sky is completely black. Stars are visible against the dark background. The Sun itself appears as an intense yellow-white disc, then all around is the dark night sky.

Why? Because the Moon has no atmosphere. The sunlight travels directly from the Sun to your eyes, which is why on the Moon the Sun is bright and the remaining sky is dark.

2. The View from the Earth

Now back to Earth. When you look up on a clear day, the entire sky is blue, not black. What's the difference? Our atmosphere.

Sunlight appears white, but it's made of a mixture of wavelengths - colours - red, orange, yellow, green, blue, and violet. Each colour is a different wavelength of light. Red light has longer waves; blue has shorter wavelengths.

As sunlight shines on the Earth, here is where the difference lies. All those oxygen, nitrogen and other molecules in the atmosphere scatter light, more at the shorter blue wavelengths rather than the longer red wavelengths. If you fancy more info then this phenomenon is called Rayleigh scattering first explained it mathematically in the 1870s.

Blue light scatters in random directions – up, down, sideways. Then it hits another molecule and scatters again and again. This happens everywhere in the atmosphere, constantly. So when you look at any patch of sky, you're seeing blue light that has been scattered toward you from air molecules in that direction.

The red and green light? It mostly travels straight through the atmosphere without being scattered. So the Sun appears whitish-yellow, as it has all wavelengths of light, and the rest of the sky has scattered blue wavelengths of light, so looks blue.

3. And Those Red Sunsets

Last week it looked like the sky above Turney Road was on fire. Why was that sunset so red?

When the Sun is low on the horizon, its light must travel through much more atmosphere to reach you – perhaps 30 or 40 times more air than when the Sun is directly overhead.

As sunlight takes this long journey through the atmosphere, almost all the blue light gets scattered away – creating the lovely blue sky for people in other parts of the world who are seeing some of that scattered blue light which is missing at sunset.

What remains for you watching the sunset? Primarily the red, orange, and yellow wavelengths that make it through without being scattered. These longer wavelengths mixed together make the sky redder.