Blue Moon, Blood Moon and Honey Moon have all been working on fiction and folklore, but the changing shades of the planet’s biggest satellite remain rooted in science. The moon does not produce its own light, instead reflecting the white light from the sun. So, what colour is the moonlight, and why does it change from an Earth perspective, and why does it sometimes change to a different colour?
The answer concerns how much of this reflected light reaches us on Earth. Samples taken from the moon reveal that it is a light-gray rock called anorsosite, with some dark areas of basalt, said Christine Splat, scientific engagement manager at the Institute of Moon and Planetary Research.
“Grey rocks are equal energy absorbers,” she told Live Science. It takes a bit from every part of the spectrum, but roughly speaking, it reflects white light on the Earth, giving the moon a typical grayish white look.
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However, various atmospheric and astronomical phenomena can limit the wavelengths that reach us. “The process of light scattering in Earth’s atmosphere is key to how we perceive the colour of the moon,” explained Evelyn Hesse, an atmospheric optics researcher at the University of Hertfordshire in the UK.
From high in the sky, the moonlight has a relatively short path through the atmosphere, so this scattering effect can be negligible, with all reflected light reaching the surface. However, as we approach the horizon, the moonlight must pass diagonally through the atmosphere, creating a longer path, allowing molecules of oxygen and nitrogen in the atmosphere to interact with incoming light.
The moonlight initially contains a mixture of visible wavelengths between 380 nanometers (violet to human eye) and 750 nanometers (red), each of which is scattered to a different degree depending on the particles in the atmosphere. For atmospheric gases, blue light is the strongest scattered. This is a process known as Rayleigh scattering. This phenomenon causes the moon to appear orange when the horizon is low. When light takes a longer path through the atmosphere from this position, only these orange wavelengths reach the surface.
Related: Can the Earth lose the Moon?
Shupla said the effect is exaggerated against blood moon or lunar eclipse. “During the lunar eclipse, the only light reflected is the light that is bounced back through our atmosphere and into the moon,” she explained. “All the blue light is absorbed into the atmosphere, and all that remains is orange and red.”
However, it does not only change the composition of the moonlight, but also the composition of the moonlight. High clouds, pollution and dust all affect the appearance of the moon, scattering different parts of the visible spectrum depending on the size of the particles.
“After a volcanic eruption, the air contains a large amount of ash particles (about 1 micron in size), which preferably scatters red light. [so] The moon may look blue, as only this component of the visible spectrum reaches the observer,” Hesse told Live Science via email.
However, the general definition of a blue moon is when there are two full moons within a calendar month. Another definition up to 1528 describes the blue moon as the third full moon in a season with four full moons. However, neither definition is connected to the real color of the moon. Most of these blue moons actually look whitish.
The moonlight is also displayed in other colors. Full moons throughout June often appear honey-colored, creating the name Honey Moon, Spra said. The traditional wedding holidays that many chose to film in June and July probably take their name from this phenomenon, she added.
Even more prominently, when the moonlight path, known as refraction, is decoupled, individual wavelengths begin to separate, creating extraordinary colors. “In very specific conditions, when the moon is close to the horizon, we can observe the green flash of the moon,” Hesse said. “This is when the moon light is refracted atmospheric times, resulting in different wavelengths taking slightly different paths. But this only lasts a few seconds.”
This same dispersion effect is also responsible for the rare rainbow halo on the moon. “If you have high ice crystals in the atmosphere, the light from the moon will bounce off those crystals and refract in the same way you do with raindrops, creating a little rainbow around the moon,” Spra said.
So, what color is the moonlight? The light that reaches the moon is certainly whitish, but optical interactions can change its appearance, making it difficult to assign moonlight to the critical colour. “I’d say it’s temporary!” Sprah said.
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