The Cyanometer: The Instrument That Measured Sky Color

Imagine inventing a scientific instrument with one simple, poetic purpose: to measure the blueness of the sky.

Not temperature. Not humidity. Not air pressure or wind speed. Just blueness. That's exactly what Swiss physicist Horace-Bénédict de Saussure did in 1789. He created the cyanometer (from the Greek kyanos, meaning "dark blue"), a circular chart with 53 paper swatches dyed in graduated shades of blue, ranging from white through increasingly deep blues to nearly black.

The tool was elegantly simple. You held it up to the sky and found which shade best matched the sky's color. That number became your measurement. The 10th shade? Pale, hazy sky. The 30th shade? Clear day. The 46th shade? The deep, almost purple-blue of high-altitude air measured by explorer Alexander von Humboldt atop the Andean volcano Chimborazo in 1802, the darkest sky ever recorded by cyanometer.

It sounds almost whimsical now, like something from a fairy tale. A device that does nothing but measure blueness. But in the 18th century, before sophisticated atmospheric science, this simple tool helped scientists understand how the atmosphere works, why the sky appears blue, and how altitude affects atmospheric transparency.

This is the story of the cyanometer: the young scientist who dreamed of climbing the highest peak in the Alps and wanted to objectively measure its sky, the explorer who carried the device across three continents setting blueness records, the atmospheric science it helped reveal, and why modern artists are still creating contemporary versions of this 235-year-old instrument.

The Young Scientist and the Mountain: Horace-Bénédict de Saussure

In 1760, a 20-year-old student named Horace-Bénédict de Saussure traveled from Geneva to the base of Mont Blanc, the highest mountain in the Alps at 4,808 meters (15,774 feet). Saussure came from a wealthy family and had just finished his studies at the Academy of Geneva. He was brilliant. Within two years, at just 22 years old, he would become a professor at the same academy. But at this moment, standing at the base of Mont Blanc, Saussure was free to explore. And he was captivated.

Nobody had ever climbed Mont Blanc. Not all the way to the summit. The mountain was considered impossible, perhaps even cursed. But Saussure dreamed of standing at the top. He wanted it so badly that he offered a generous financial reward (amount unspecified, but substantial) to the first person who could reach the summit.

The Observation That Started Everything

During his mountain travels, Saussure noticed something fascinating: the higher you climbed, the deeper blue the sky became.

At sea level, the sky was pale, almost whitish-blue on hazy days. Higher up in the mountains, the blue intensified. At the highest peaks, the sky was a deep, rich blue unlike anything seen from lower elevations.

This wasn't just Saussure's imagination. Every mountaineer noticed it. But nobody had systematically measured it.Saussure, with his scientific mind, wanted data. He wanted to objectively record the color of the sky from Mont Blanc's summit. Not just describe it as "very blue" or "extremely blue," but measure it numerically. But how do you measure blueness?

27 Years of Waiting

It took 27 years for someone to claim Saussure's reward. In 1786, two men from Chamonix, Jacques Balmat (a crystal hunter and mountain guide) and Michel Gabriel Paccard (a physician), finally reached Mont Blanc's summit. They were driven by enthusiasm, ambition, and Saussure's lavish reward. The following year, 1787, Saussure himself made the ascent. At 47 years old, he finally achieved his dream. And he brought his measurement tools with him. Among his scientific instruments: pieces of paper colored in different shades of blue, which he held up against the sky to match its color. This practical test helped him refine his idea into a proper scientific instrument: the cyanometer.

The Cyanometer: Design and Function

In 1789, two years after climbing Mont Blanc, Saussure formalized his blue-measuring tool.

The Design

The cyanometer was remarkably simple:

A circular chart printed on paper (some versions were painted on disks of other materials)

53 sections (some versions had 52 or 16, depending on the iteration) arranged in a circle like a color wheel

Graduated shades of blue: The sections ranged from white (or very pale blue) through increasingly saturated blues to deep indigo or near-black. Each section was numbered.

The dye: Saussure used Prussian blue, a synthetic blue pigment discovered in 1706 that was stable, affordable, and could be mixed to create a wide range of blue shades.

The sections were painted or dyed with carefully graduated intensities of blue. The transitions were smooth, allowing for nuanced comparisons.

How to Use It

Using a cyanometer was straightforward:

Step 1: Go outside on a clear day.

Step 2: Hold the cyanometer up against the sky, typically looking at a point about 90 degrees from the sun (directly overhead if the sun is on the horizon, or at the zenith if measuring midday sky).

Step 3: Compare the color of the sky to the sections on the cyanometer.

Step 4: Find the section that most closely matches the sky's hue.

Step 5: Record the number of that section as your measurement.

That's it. Your result might be "the sky today is 28 degrees on the cyanometer" or "the summit of Mont Blanc measures 39 degrees."

Standardizing Observations

Saussure understood that for measurements to be comparable, they needed to be standardized. He gave specific instructions:

• Observe at the same time of day when comparing different locations (ideally at solar noon)

• Look at the same part of the sky (90 degrees from the sun)

• Avoid looking toward the horizon (which appears paler due to atmospheric thickness)

• Make observations in clear weather, not cloudy or hazy conditions

These guidelines helped ensure that different observers using different cyanometers could compare results meaningfully.

Saussure's Theory: Particles in the Atmosphere

Why did Saussure care about measuring sky blueness? He had a theory. Saussure believed that the sky's color was determined by the concentration of particles suspended in the atmosphere. He theorized that these atmospheric particles had an inherent "opaque blue" color (which he thought corresponded to about the 34th degree on his scale). The more particles in the air, the more blue the sky appeared, but also the hazier and less transparent the atmosphere. At higher altitudes, there were fewer particles (less atmosphere above you), so the sky appeared darker and more transparent, allowing you to see the "true" black of space approaching.

This theory was partially correct and partially wrong.

What Saussure got right: The sky's color is indeed related to particles in the atmosphere. The transparency of the atmosphere does affect how blue or hazy the sky appears.

What Saussure got wrong: The particles don't have an inherent blue color. The blueness of the sky is actually caused by Rayleigh scattering, a phenomenon where molecules of nitrogen and oxygen (and water vapor) scatter shorter wavelengths of light (blue and violet) more than longer wavelengths (red and orange).

When sunlight enters the atmosphere, blue light scatters in all directions. That's why the sky looks blue when you look up at it. Red and orange light mostly pass straight through, which is why the sun itself looks yellowish-white and why sunsets are red (when light travels through more atmosphere, even more blue is scattered away, leaving red). This correct explanation wasn't established until the late 1800s, when physicists John Tyndall and Lord Rayleigh (John William Strutt) confirmed the scattering mechanism. By then, the cyanometer had been largely forgotten as a scientific tool.

Saussure's Measurements

Saussure used his cyanometer to measure the sky's blueness at several locations:

Geneva (373 meters elevation): His home base, where he made baseline observations

Chamonix (1,035 meters elevation): A mountain valley in the Alps, noticeably bluer than Geneva

Mont Blanc, Col du Géant (3,365 meters elevation): A high mountain pass

Mont Blanc summit (4,808 meters elevation): 39 degrees on the cyanometer, the deepest blue Saussure personally measured

Saussure's data confirmed what mountaineers already knew intuitively: higher altitude equals deeper blue sky. But he had done something new: he had quantified it with a standardized measurement scale.

Alexander von Humboldt: The Record Breaker

Enter Alexander von Humboldt (1769-1859), one of history's greatest explorers and naturalists. Humboldt was fascinated by everything, from botany to geology to meteorology to indigenous cultures. He measured, cataloged, and documented the natural world with obsessive thoroughness. When Humboldt learned about Saussure's cyanometer, he became an enthusiastic user. He took the device on his voyages across three continents, measuring the blueness of the sky wherever he went.

Humboldt's Measurements

Atlantic Ocean crossing (at sea level, open ocean): 23.5 degrees at noon. Much paler than mountain skies due to humidity and sea spray particles in the air.

Canary Islands, Summit of Teide volcano (3,718 meters): 41 degrees, surpassing Saussure's Mont Blanc measurement. A new record for the darkest sky ever measured.

Chimborazo volcano, Ecuador, South America (June 23, 1802): At approximately 5,600-5,900 meters elevation, Humboldt reached what was then the highest altitude ever attained by humans. And he took his cyanometer.

The sky measured 46 degrees on the cyanometer.

This was the darkest blue sky ever recorded by a scientific instrument in the 18th or 19th centuries. It stood as the record for cyanometer measurements.

Humboldt's Contributions

Humboldt's extensive use of the cyanometer across different latitudes, climates, and elevations provided valuable data about atmospheric conditions worldwide.

He demonstrated that:

• Tropical skies at sea level could be quite pale (due to high humidity)

• Mountain skies were consistently darker blue

• The highest elevations produced the darkest blues

• Atmospheric transparency varied greatly by location and weather

Though Humboldt didn't fully understand the physics of why the sky was blue (Rayleigh scattering wasn't known yet), his measurements helped build the dataset that later scientists would use to understand atmospheric optics.

A Literary Footnote: Lord Byron's Satirical Use

The cyanometer even made it into English literature. In his satirical epic poem Don Juan (1818-1824), Lord Byron referenced the cyanometer in a mocking way: "Canto IV, 112" alludes to using the device to measure "the blue of bluestocking ladies," poking fun at intellectual women (called "bluestockings" in that era).

Interestingly, Byron credited Humboldt as the inventor of the cyanometer, not Saussure. This historical inaccuracy likely occurred because Humboldt was far more famous than Saussure by the 1820s, and Humboldt's extensive use of the cyanometer had made him associated with it.

Why Did the Cyanometer Fade Away?

By the late 1800s, the cyanometer was obsolete as a scientific instrument.

Reasons for Obsolescence

Better understanding of atmospheric optics: Once Rayleigh scattering was understood, scientists knew that measuring sky color didn't directly tell you about particle concentration. More sophisticated measurements of atmospheric composition, humidity, and transparency were needed.

Subjectivity: Despite Saussure's attempts at standardization, cyanometer readings were inherently subjective. Different observers might pick different shades. Lighting conditions affected perception. Individual color vision varied.

Better instruments: Spectroscopes, photometers, and other optical instruments provided more precise, objective measurements of light and atmospheric properties.

Limited usefulness: Knowing the sky is "39 degrees blue" doesn't give you specific information about atmospheric composition, pressure, temperature, or other meteorologically useful data.

The cyanometer was a beautiful idea, but science moved on.

Modern Revival: The Cyanometer as Art

Though obsolete as science, the cyanometer has been revived as art and cultural commentary.

The Ljubljana Cyanometer (2009)

In 2009, German artist Martin Bricelj Baraga created a contemporary version of the cyanometer in Ljubljana, Slovenia.

A large monolith stands on Slovenska Cesta, one of Ljubljana's main streets. At the top is a circular hole through which viewers can look at the sky. Around the hole, a ring of 52 blue sections (just like Saussure's original) allows viewers to compare the sky's blueness to graduated shades. The installation is permanent, inviting passersby to stop, look up, and measure the sky's color, just as Saussure did 220 years earlier.

Why Revive It?

Modern artists and scientists who revisit the cyanometer do so for several reasons:

Poetic simplicity: In an age of complex instruments and digital sensors, the cyanometer's analog simplicity is charming. It connects us to direct sensory observation.

Mindfulness: The act of stopping, looking at the sky, and carefully comparing its color to a chart forces you to slow down and pay attention to something beautiful that we usually ignore.

Environmental awareness: Some contemporary cyanometer projects use sky blueness as a proxy for air quality. Hazy, polluted air makes the sky paler. Clean air makes it bluer. A daily cyanometer reading could track environmental changes over time.

Historical connection: Using a cyanometer connects you to the history of science, to Saussure climbing Mont Blanc with his paper swatches, to Humboldt crossing oceans and mountains measuring the sky everywhere he went.

The Science We Understand Now: Why the Sky Is Blue

We now know that the sky's blueness is caused by Rayleigh scattering. When sunlight (which contains all colors of the visible spectrum) enters Earth's atmosphere, it encounters molecules of nitrogen (N₂) and oxygen (O₂), the two main components of air. These molecules are much smaller than the wavelength of visible light. When light waves hit these tiny molecules, they scatter in all directions.

The key: shorter wavelengths (blue and violet light) scatter much more strongly than longer wavelengths (red and orange light). Specifically, scattering intensity is proportional to 1/λ⁴, where λ is wavelength. Blue light has a wavelength about half that of red light, so it scatters about 16 times more strongly (2⁴ = 16). When you look up at the sky (not directly at the sun), you see all the scattered blue light coming from air molecules in every direction. The sky appears blue.

When you look at the sun itself, you see the light that traveled straight through the atmosphere without much scattering, which is why the sun appears yellowish-white (blue has been scattered out). At higher altitudes, there's less atmosphere above you, so less scattering occurs. The sky appears darker blue, approaching the black of space.

This is exactly what Saussure observed with his cyanometer, even though he didn't understand the mechanism.

The Bottom Line

The cyanometer is an 18th-century scientific instrument invented by Swiss physicist Horace-Bénédict de Saussure in 1789 for the sole purpose of measuring the blueness of the sky. It consists of a circular chart with 52 or 53 sections of graduated blue shades ranging from white through pale blue, medium blue, deep blue, to nearly black. Each section is numbered. Users hold it up to the sky and find which shade best matches the sky's color.

Saussure invented it after noticing that the sky became deeper blue at higher altitudes during mountain climbs. At age 20 (in 1760), he dreamed of climbing Mont Blanc, the highest peak in the Alps. He offered a reward for the first person to reach the summit. In 1786, Jacques Balmat and Michel Gabriel Paccard claimed it. In 1787, Saussure himself climbed Mont Blanc at age 47, carrying colored paper swatches to measure the sky.

Saussure measured Mont Blanc's summit at 39 degrees on his cyanometer. He theorized that atmospheric particles caused the sky's blue color and that higher altitudes had fewer particles, making the sky darker. Explorer Alexander von Humboldt enthusiastically used the cyanometer across three continents. In 1802, atop Ecuador's Chimborazo volcano, he measured 46 degrees, the darkest sky ever recorded by cyanometer.

The device fell out of scientific use by the late 1800s once Rayleigh scattering (the true explanation for sky blueness, involving scattering of short-wavelength blue light by nitrogen and oxygen molecules) was understood. Modern instruments provided more precise, objective atmospheric measurements. The cyanometer has been revived as art. In 2009, artist Martin Bricelj Baraga created a public cyanometer monument in Ljubljana, Slovenia, allowing people to measure sky blueness just as Saussure did 220 years earlier.

The next time you look up at a brilliantly blue sky, remember: a Swiss scientist once climbed the highest mountain in Europe carrying paper swatches just to measure that exact shade of blue. And an explorer carried that invention across oceans and up volcanoes, setting blueness records that stood for centuries.

Science doesn't always need complex instruments. Sometimes all you need is 53 shades of blue and the curiosity to measure something beautiful.

Sources

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