We are accustomed to the Moon appearing vividly white, silvery, yellow, or tinged with orange or red. However, on rare occasions, the Moon can manifest as blue or even violet. Such was the case in 1883 when observers reported the Moon turning blue, and the Sun displaying as “bright blue” and “magnificently green.”
The lunar surface itself, of course, did not undergo a color change. Throughout 1883, just as throughout its entire history, it remained overwhelmingly gray, punctuated by patches of lighter and darker areas to add visual interest. So, what exactly happened?
The perceived color of the Moon, from our vantage point here on Earth, is dictated by our atmosphere. On clear, clean nights when the Sun is high, it seems silvery-white because light travels through the atmosphere with relatively little obstruction before reaching your eyes.
Nevertheless, blue spectrum light scatters more readily in the atmosphere than red spectrum light due to its shorter wavelength. If the light reflected from the Moon traverses a sufficient depth of atmosphere, this scattering accumulates to a degree that noticeably affects the Moon’s appearance, although typically not to the extent witnessed in 1883.
“There is one memorable aspect where the Moon’s appearance actually differs when it is low in the sky. It tends to have a more yellow or orange hue compared to when it is higher overhead,” NASA explains. “This occurs because the moonlight travels a greater distance through the atmosphere. As the path lengthens, more of the short, blue wavelengths of light are scattered away, leaving a higher proportion of the long, red wavelengths. (Dust or pollution can also intensify the reddish cast.)”
Yet, in 1883, the Moon and Sun became blue, not red. This necessitates that more blue light—contrary to the usual scattering effect—reached human eyes than red light. The culprit was relatively obvious—the spectacular eruption of Krakatoa had occurred earlier that year—but the exact mechanism remained elusive.
“The suppression of red light in the transmitted spectrum can fundamentally be caused by selective absorption or scattering based on wavelength, where absorption might be due to gaseous constituents or aerosols,” states a study on the subject, essentially suggesting that the presence of gases and other matter in the atmosphere could account for the peculiar scattering.
It was previously suggested that the blue Sun and Moon of 1883 were caused by water vapor in the atmosphere, but later models indicate this is highly improbable. Current evidence leans toward an explanation involving sulfur dioxide, alongside other particulates ejected by the volcano.
“Green volcanic twilight can be accounted for by anomalous scattering occurring for sufficiently large particles (i.e., those with radii around 500–700 nm) and a preferentially narrow size distribution for the particles,” explains another paper on the topic.
Because these particles were slightly wider than the wavelength of red light, that end of the spectrum had a harder time penetrating the atmosphere than blue light, resulting in the bluish-green appearance of the Moon, Sun, sunrises, and sunsets. A similar effect can be seen during wildfires, as well as subsequent volcanic eruptions, when similarly sized particles are released.
To further mystify astronomers and those fond of the phrase “once in a blue moon,” we also use the term “blue moon” to refer to a far more frequent and commonplace occurrence. Since the Moon’s orbit unfortunately does not perfectly align with our 12 calendar months, roughly every two and a half years, we experience 13 full Moons in a year instead of the usual 12. This extra full Moon is termed a blue moon and—contrary to the original implication of the expression—happens quite regularly and predictably.
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