Wednesday, April 26, 2017

Why Is the Sky Blue?

The Cyanometer of Ljubljana

Amusing Planet  
At some point in life, almost every child on Earth asks, “Why is the sky blue?” Today we know the answer—the sky appears blue because molecules in the air scatter blue light from the sun more than they scatter the other wavelengths, or colors. But it took a long time for scientists to figure that out. 

It wasn’t until 1859 when the phenomenon was first correctly explained by Irish physicist John Tyndall, although it was Lord Rayleigh, who studied it in more detail a few years later, after which the phenomenon of Rayleigh scattering is named. 

A modern cyanometer in Ljubljana, Slovenia. Photo credit:

Seventy years before John Tyndall and Lord Rayleigh began working on the problem, a Genevan meteorologist named Horace-Bénédict de Saussure was consumed by it. De Saussure, who was an avid mountaineer and is often attributed to the founding of this challenging sport, invented a simple measuring instrument called a cyanometer that enabled him to measure the blueness of the sky.

De Saussure reasoned, and correctly, that the azure must be dependent on the amount of suspended particles and water vapor in the atmosphere, which he called “opaque vapors”.

De Saussure's cyanometer had 53 sections, ranging from white to varying shades of blue and then to black, arranged in a circle. De Saussure would held his device against the sky and use it measure the color of the sky over the alps, and then use that information to forecast the weather.

Two hundred and twenty-seven years later, Slovenian artist Martin Bricelj Baraga erected a striking monument in Slovenia's capital city Ljubljana, as a homage to de Saussure’s cyanometer.

Baraga’s Cyanometer sculpture is not just a monument, but a functional scientific instrument with an accuracy that surpasses De Saussure’s original cyanometer.

The 3.3-meter-tall glass and steel cyanometer measures the color of the sky and then changes its color to match it perfectly. The cyanometer also has an inbuilt computer that gathers air quality measurement data from an online archive and displays the air pollution level on a color scale from red to green on the instrument in real time. The cyanometer also periodically photographs the sky and sends the photos back to the online archive.

The cyanometer is completely energy self-sufficient, since it runs on solar energy.

De Saussure’s original cyanometer. Photo credit: Kirsten Carlson

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