There is a device on the streets of Ljubljana that measures how blue the sky is

At some point, any child asks: "Why is the sky blue?" Now we know the answer: the sky is perceived as blue, because molecules in the air scatter the blue light of the sun more than rays with a different wavelength, or other colors. But it took scientists a long time to figure it out.

Source: Amusing Planet

In 1859, the phenomenon was correctly explained by the Irish physicist John Tyndall, but only Lord Rayleigh studied it in detail a few years later and established the dependence of the intensity of scattered light on the wavelength. This concept is called Rayleigh scattering.

70 years before Tyndall and Lord Rayleigh began working on this task, the Geneva meteorologist Horace Benedict de Saussure plunged into it with his head. A big fan of mountain hiking, de Saussure, who is considered by many to be the founder of mountaineering, came up with a simple measuring device called a cyanometer, which allowed to determine the degree of blueness of the sky.

De Saussure assumed—and this was a correct assumption —that the degree of blueness depends on the amount of particles hanging in the air and water vapor in the atmosphere, which he called "opaque vapors."

The de Saussure cyanometer had 53 divisions from white to black through different shades of blue, ordered in a circle. The researcher held his device against the sky to measure its color over the mountains, and used this information to predict the weather.

After 227 years, the Slovenian artist Martin Bricelj Baraga installed an amazing monument in Ljubljana as a tribute to the cyanometer de Saussure. The sculpture in the Slovenian capital is not just a monument, but a functioning scientific instrument with an accuracy exceeding the original de Saussure cyanometer.

A 3.3 meter high glass and steel cyanometer measures the color of the sky and changes its own color to perfectly match the sky blue hue. In addition, there is a built-in computer that collects information about air quality from an online database and shows the degree of air pollution on a color scale from red to green in real time. The cyanometer also periodically photographs the sky and sends the images to a network database.

The cyanometer in Ljubljana is energetically autonomous, as it runs on solar energy.