In the summer of 1831, people across the world witnessed something both beautiful and terrifying. The Sun, instead of shining its usual golden light, appeared blue and even violet in some places. At the same time, unusual weather gripped the Northern Hemisphere harvests failed, temperatures dropped, and skies looked hazy and strange. For nearly 200 years, historians and scientists puzzled over what could have caused this bizarre episode.
Now, researchers believe they have finally solved the mystery. A powerful volcanic eruption in a remote corner of the Pacific Ocean appears to be the culprit.
A Forgotten Eruption Hidden in the Records
For decades, scientists knew that something big happened in 1831 because ice cores from Greenland and Antarctica showed strong traces of sulfur from that period. Sulfur in the atmosphere is often a fingerprint of a major volcanic eruption. But there was a problem: no one could find a documented eruption large enough to explain the evidence.
Some speculated that an eruption in Babuyan Claro in the Philippines might have been responsible, while others guessed it could have been an unknown volcano somewhere in the tropics. The event remained a scientific mystery until recently.
In 2025, a team of researchers published evidence pointing to the Zavaritskii Caldera, a volcano on Simushir Island in the Kuril Islands chain, located between Russia and Japan. This eruption, almost forgotten by history, appears to have been one of the largest of the 19th century.
How the Sun Turned Blue
When a volcano erupts with extreme force, it can blast sulfur dioxide high into the stratosphere. There, the gas reacts with water vapor to form tiny particles called aerosols. These aerosols scatter sunlight in unusual ways. Instead of the familiar golden glow, the Sun can look bluish or even green, especially when viewed through a hazy sky.
Historical records from Asia, Europe, and North America describe these strange optical effects in 1831. Farmers and city dwellers alike wrote about an eerie “blue Sun” and “violet-tinted skies.” While it must have been breathtaking, it also came with a cost.
A Year of Cold and Hardship
The sulfur-rich aerosols from the eruption spread around the Northern Hemisphere, blocking part of the Sun’s warmth. Scientists estimate that temperatures dropped by about 1°C (1.8°F) over the following year. That might sound small, but it was enough to disrupt farming seasons and cause food shortages in some areas.
Reports mention unusually cold summers, early frosts, and reduced harvests in parts of Europe and Asia. For communities dependent on stable weather for survival, the 1831 event was another reminder of how much human life depends on Earth’s delicate climate balance.
Why It Took So Long to Solve the Puzzle
Unlike famous eruptions such as Krakatoa in 1883 or Tambora in 1815, the Zavaritskii Caldera eruption went mostly unnoticed outside the region. Simushir Island was remote, sparsely inhabited, and lacked detailed records at the time. Without eyewitness reports, the eruption was overlooked, even though its effects were felt worldwide.
Only through the combination of ice-core analysis, historical documents, and modern climate modeling have scientists been able to identify the true source.
Lessons for Today
The 1831 “blue Sun” event highlights the power of volcanoes to alter Earth’s climate in dramatic ways. Even a single eruption, if strong enough, can cool the planet, change weather patterns, and affect millions of lives.
Today, scientists keep close watch on volcanic activity around the globe. Modern satellites and atmospheric monitoring allow us to track ash and sulfur plumes in real time. If another event of this scale were to occur, we would at least have the tools to understand it quickly, though the climate effects would still be unavoidable.
What once seemed like a strange legend that the Sun turned blue and the world chilled in 1831 now has a scientific explanation. A massive eruption from the Zavaritskii Caldera spread sulfur into the skies, altering both light and climate for months.
Nearly two centuries later, we can finally say with confidence why the skies looked so eerie back then. The mystery of the blue Sun has been solved, but it also serves as a reminder of the immense forces of nature that can reshape our world in an instant.