Guess what? The sky is not blue. At least, there’s no blue stuff, no blue pigment, in the sky. It’s a trick of the eye. Up there, and all around us, are gases of different kinds, such as oxygen, nitrogen and carbon dioxide. There’s also dust, water vapour, spores and even tiny airborne animals.
When sunlight hits something, it gets reflected. Big objects, such as the Moon, reflect the light very well. Moondust is dark, but so reflective that the Moon shines brightly in the night sky. But a tiny gas molecule is too small to act as a mirror. Instead, it absorbs light, and then sends that light bouncing back out again in a random direction. In other words, every molecule in the air is a tiny, flickering light source.
Imagine for a moment that light was sound. Sunlight is not just one note of a certain pitch played on one instrument; it is a vast orchestra playing every imaginable pitch at every imaginable volume! We see just some of this music. Our eyes perceive different pitches of light as colours: violet, blue, green, yellow, orange, red and purple.
Air molecules absorb blue light very easily, and they send it bouncing away just as easily. This is why blue light is scattered all around the sky, and why it reaches our eyes from all directions. Everywhere we look, we are bombarded with blue light. That is why the whole sky looks blue.
The other colours aren’t nearly so easily scattered by the Earth’s atmosphere, and they come to us in a more or less straight line. DON’T look directly at the Sun, because if you do, every colour there (aside from a little sky blue) will be hitting the back of your eye at the same time. That much light really can damage your eyes.
If Mars had more gas in its atmosphere, it too would have a blue sky. As it is, there isn’t enough gas for this scattering effect to work. If you could stand on the surface of Mars and look up, you’d see the sky there is the white of raw sunlight, tinted beige by dust.
Towards the Earth’s poles, the Sun sits low in the sky and sunlight has more atmosphere to pass through before it reaches the ground. Here the sky is especially blue.