Sunset, seen from the Moon
143 years ago, several observers noticed something they had never seen before: the Moon appeared to have turned blue or purplish and the Sun was described as “bright blue” and even “splendid green”.
The Moon, gray, did not change color. What changed was the way its light, reflected towards the Earth, passed through the Earth’s atmosphere, explains the . Under normal conditions, the Moon tends to appear silvery-white when it is high in the sky.
When it appears closer to the horizon, it can take on yellowish, orange or reddish tones, because the light travels through a larger layer of atmosphere and shorter wavelengths, such as blue ones, are more easily dispersed.
But in 1883 the opposite happened: instead of the red tones dominating, it was the blues and greens that became more visible. This meant that for some reason the red light was being blocked or deflected more than normal.
For a long time, one of the hypotheses pointed to water vapor present in the atmosphere. However, more recent studies and models consider this explanation unlikely.
The most accepted interpretation today is that sulfur dioxide and other particles released by Krakatoa volcano eruption played a decisive role.
According to scientific research cited by IFL, the phenomenon would have resulted from the presence of atmospheric particles with very specific sizes, large enough to anomalously affect the passage of light.
These particles, with dimensions slightly larger than the wavelength of red light, made the transmission of red tones more difficult than that of blue. The result was a blue-green coloration of the Moon, the Sun and even the twilights.
Similar phenomena can occur after large forest fires or other volcanic eruptionswhenever particles with comparable characteristics are released.
Today, the blue Moon has another meaning: “blue Moon” normally designates the occurrence of a extra full moon in a calendar year with 13 full moonssomething that happens approximately every two and a half years.
