Marcus wrote from Billings, Montana, to ask, “Why does the sky look blue?”

Marcus, if you saw Earth from outer space you’d see a world with blue oceans – surrounded by black outer space. It’d only be when you came close enough to Earth to be inside our planet’s atmosphere that the sky would begin to look blue. So space is black, but Earth’s sky looks blue. But, it wouldn’t be blue if the Earth didn’t have an atmosphere – or, if the light in ordinary sunshine didn’t have some blue color in it. You can see the blue in sunlight in a rainbow – or if you shine light through a prism.

And you know, Marcus, the air surrounding Earth is made of different kinds of gases. And these gases take the form of molecules – or collections of atoms. And it just so happens that the molecules of gas in Earth’s atmosphere are just the right size to send sunlight flying in all directions, as the light strikes the air. Scientists call this scattering.

It also just so happens that the blue color in sunlight is scattered more than the other colors. And that’s why the sky looks blue – because, during the daytime, the blue in sunlight scatters across the sky.