The idea of a regularly scheduled “Green Moon” event, where Earth’s satellite glows with a vibrant green hue, is not a reality in astronomy. The Moon’s intrinsic appearance is a dull brownish-gray, reflecting sunlight. While the Moon can appear to change color from our perspective on Earth, a universally visible, pre-scheduled green spectacle does not exist. The perceived color shifts we observe are temporary, localized, and entirely result from atmospheric conditions.
The Origin of the “Green Moon” Myth
The “Green Moon” myth stems from viral internet hoaxes that circulate on social media. These hoaxes are often tied to specific, predetermined dates, claiming the event is a once-in-a-lifetime occurrence. The original iteration, dating back to 2016, claimed the Moon would turn green on May 29th for the first time since 1847.
This narrative was later adapted to claim the event would occur on April 20th, a date often associated with the number 420. The hoax cleverly incorporated this number by claiming the event hadn’t happened in 420 years, suggesting the entire phenomenon was a joke related to cannabis culture. Another version of the myth attempted a pseudo-scientific explanation, suggesting the Moon’s proximity to the planet Uranus would cause the color change. No planetary alignment can physically alter the Moon’s reflected color.
How Earth’s Atmosphere Affects Lunar Color
The Earth’s atmosphere is responsible for the legitimate, temporary color changes we observe in the sky. Lunar light must pass through our planet’s layers of air, which acts as a vast filter. This scattering process, known as Rayleigh scattering, affects shorter wavelengths of light, like blue and violet, more significantly than longer wavelengths, such as red and orange.
When the Moon is low on the horizon, its light travels through a greater column of atmosphere, causing nearly all the blue and green light to scatter away before reaching our eyes. This leaves the remaining light appearing yellow, orange, or even deep red, similar to the colors of a sunset. Conversely, the Moon appears bright white or silvery when it is high overhead because its light takes a much shorter path through the thinnest part of the atmosphere.
Aerosols, fine solid or liquid particles suspended in the air, can dramatically alter the scattering effect. Large particles from volcanic eruptions, wildfire smoke, or industrial pollution scatter light differently than air molecules alone. After a major volcanic event, the Moon has appeared genuinely blue or, in rare cases, a faint greenish-blue due to the unique size and composition of the ejected particles. This temporary tint caused by specific particles is the only scientific context for a Moon appearing green, and it is not a scheduled event.
Why the Moon’s Surface is Not Green
The Moon’s true color is determined by the composition of its surface material, known as regolith. This layer of dust and fragmented rock is primarily made up of silicate minerals, including oxygen, silicon, iron, magnesium, and calcium. These elements, especially the iron and titanium oxides found in the darker regions called maria, absorb and reflect light to produce shades of gray and brown.
The lunar surface lacks the chemical compounds, such as chlorophyll or copper-rich minerals, required to reflect light in the green spectrum. Since the Moon has no atmosphere or geological processes like weathering and biology, its surface color remains stable. Therefore, the Moon is intrinsically a dull, dusty gray color, making it physically impossible for it to turn a true, vibrant green on its own.