The first person to have explained, with reasonable accuracy, why the sky is blue was Leonardo da Vinci in 16th century Italy. Da Vinci was a man who embodied childhood curiosity throughout his life. In his notebooks it was not uncommon to find entries on his to-do list such as “Describe the tongue of a woodpecker and a crocodile’s jaw.”. Put on there, it seems, out of pure curiosity. The crocodile’s jaw being of note because it has two hinges, not one, allowing for a greater clamping force and the woodpeckers tongue being truly bizarre but allowing it to repeatedly smash its beak into a tree with 10 times the equivalent force that would kill a human doing so.

Leonardo is one of the greatest geniuses to have ever lived, having a life dedicated solely to following his curiosity. He accrued expertise on a vast range of topics and, along with being one of the greatest artists to have ever lived, was the forefather of several scientific disciplines including anatomy, palaeontology, optics, civil engineering and astronomy.

Aligned with this curiosity, around his mid-50s Leonardo questioned why the sky was blue, dutifully recording his investigations in a notebook now owned by Bill Gates.

The overwhelming majority of the light from the sun travels straight through the atmosphere and hits the earths surface. This is the typical white light we associate with sunlight; and this is not blue at all. What is blue is the sky. How does this happen?

Leonardo observed, whilst on a hike, that the sky appeared to get a richer more beautiful blue as he climbed higher. While many of us would simply admire the view on a mountain hike, for Leonardo like everything it was an exercise in observation. Leonardo was aware of moisture in the air from his studies on rivers and the water cycle; where he had correctly concluded (for the first time in history) that all water falling as rain came from evaporating water on the ground. He suggested water vapour consisted of tiny droplets, too small for the eye to see. And that there were far more of these particles at lower altitudes, nearer the puddles and lakes they originated from; than at higher altitudes.

He suggested that the blue tint of the sky came from the scattering of light off these tiny droplets. The sky was darker at higher altitudes as there was less scattering due to less particles. The closer to sea level, the thicker the atmosphere and hence the brighter the blue tint was.

To test this scattering idea, Leonardo set up a dark room, with a beam of sunlight going from one side to the other. He set up a wood fire in the path of the beam, that billowed smoke. A beam of white light entered the room; yet Leonardo observed a light blue tint to the light whilst looking at the scattered light perpendicular to the path of the beam.

Smoke has an extremely high density of particles in it. Therefore, it will scatter light far more often than regular air. This means that we can achieve the same amount of scattering in hundreds of metres of air, in just a small volume of smoke. By observing this blue tint, Leonardo effectively confirmed, that it is the small particles in air that scatter light and somehow this leads to a blue tint. He had no clue why the scattering led to a blue tint as a pose to a tint of any other colour.  ....[read more]