The moon, a familiar sight in our night sky, occasionally appears in hues beyond its typical pale white. This raises questions about its true color and whether a purple moon is a natural possibility. Understanding how Earth’s atmosphere interacts with moonlight helps explain these visual transformations.
The Moon’s Natural Hue
The moon does not emit light; instead, it reflects sunlight. Its inherent color is primarily shades of gray, brown, and white. This appearance stems from minerals composing its surface, such as magnesium, iron, feldspar, and basalt, which form a dusty regolith.
Any perceived colors beyond these inherent grays and browns are not due to changes on the lunar surface. The moon’s true color remains consistent, meaning shifts in its appearance are entirely a result of external factors affecting how we see it from Earth.
Atmosphere’s Colorful Influence
Earth’s atmosphere alters our perception of the moon’s color through processes like light scattering. Rayleigh scattering explains why shorter wavelengths of light, such as blue and violet, are scattered more efficiently by atmospheric molecules than longer wavelengths like red and orange. This phenomenon is the same reason the sky appears blue during the day and sunsets are often red or orange.
When the moon is low on the horizon, its light must travel through a greater thickness of Earth’s atmosphere. This increased atmospheric path scatters away more blue light, allowing longer red and orange wavelengths to reach our eyes, making the moon appear reddish or orange. During a total lunar eclipse, Earth’s atmosphere filters sunlight, bending red light onto the moon’s surface, which creates the “blood moon” effect.
Fine particles like dust, smoke, or volcanic ash can also influence light scattering. For example, a “blue moon” can occur when specific-sized particles, such as those from volcanic eruptions or large forest fires, scatter red light, allowing blue light to pass through. Conversely, a clear atmosphere with minimal pollutants results in a brighter, more whitish moon.
The Unlikelihood of a Purple Moon
Given the principles of light scattering, a naturally occurring purple moon is improbable. While atmospheric conditions can favor the transmission of red or blue light, the precise combination required to create a purple hue from natural scattering is not present. Purple is a mix of red and blue, and the atmospheric conditions that allow red light to pass through often scatter blue light, and vice-versa.
Although some rare and localized atmospheric events, such as the Chinchaga forest fire in 1950, were reported to have caused a moon to appear purple during a lunar eclipse, these instances are uncommon. Such observations are tied to unusual atmospheric compositions. Therefore, any widespread perception of a purple moon is more likely due to photographic effects, digital manipulation, or specific and fleeting atmospheric conditions.