The term “Purple Moon” is not a standard astronomical classification but describes an exceptionally rare and transient visual effect caused by unique atmospheric conditions on Earth. This phenomenon occurs when moonlight is filtered through the dense layers of the atmosphere, making the Moon appear violet or lilac. Since the Moon’s surface is actually a consistent brownish-gray, the purple appearance is entirely an optical illusion created by our planet’s air. Due to the precise and temporary nature of the required atmospheric mix, an authentic purple moon is one of the most elusive color phenomena an observer can witness.
How Atmospheric Scattering Colors the Moon
The appearance of any color in the Moon other than its natural gray results from the way Earth’s atmosphere interacts with reflected sunlight, a process known as scattering. The atmosphere acts as a filter, preferentially scattering shorter wavelengths, such as blue and violet light, while allowing longer wavelengths, like red and orange, to pass through more directly.
This effect is governed by Rayleigh scattering, which explains why the daytime sky appears blue; tiny nitrogen and oxygen molecules scatter blue light across the sky. When the Moon is high overhead, its light passes through a minimal amount of atmosphere, resulting in its normal bright, whitish-yellow appearance. Conversely, when the Moon is low on the horizon, the light must travel through a significantly thicker column of air, scattering away the blue light and leaving only the warmer, longer-wavelength colors like orange and red to reach the observer’s eye.
Specific Conditions for a Purple Appearance
A genuine purple or violet hue requires a highly unusual and precise combination of atmospheric filtering. Since purple is visually a blend of red and blue light, the atmosphere must simultaneously allow the longest (red) and shortest (blue) wavelengths to pass through, while efficiently blocking the middle wavelengths, particularly green and yellow. This mandates a complex, layered atmosphere containing a mix of particulate matter of specific sizes.
Smaller air molecules scatter blue light, but larger particles are necessary to scatter other colors. Sources like ash from volcanic eruptions, smoke from large wildfires, or unusual dust events introduce particles of varying sizes into the air column. For a purple appearance, a layer of intermediate-sized particles must filter out the green and yellow light. This layer must also be thin enough to allow the longer red light to pass, while blue light is scattered toward the observer by smaller molecules, resulting in the combined purple coloration. The precise conditions for this filtering are ephemeral and localized, making the sight unpredictable and rare.
Comparing Purple to Blue and Red Moons
The purple moon differs significantly from more commonly referenced colored moon phenomena, which are either predictable astronomical events or calendrical terms. The “Blue Moon,” for instance, is a calendrical term referring to the second full moon occurring within a single calendar month, not a color description. An actual blue-colored moon is an extremely rare atmospheric event, typically caused by specific, large particles, such as ash from a volcanic eruption, which selectively scatter red light, allowing only blue light to pass through.
The “Red Moon,” or “Blood Moon,” occurs during a total lunar eclipse. The red color is predictable because Earth’s atmosphere bends sunlight onto the Moon, filtering out other colors and leaving only red light to reach the lunar surface. The purple moon is not caused by an eclipse or a calendar cycle, but by localized, unpredictable atmospheric pollution or dust that creates a momentary, unique light filter. This purple effect is much more fleeting and geographically restricted than the large-scale, predictable red color of an eclipse.
Non-Scientific Meanings
Because of its rarity, the concept of a purple moon has been embraced outside of science as a metaphor for uniqueness or mystical occurrence. It frequently appears in fiction, poetry, and song as a symbol of an event that is nearly impossible or a time of significant change. The phrase evokes a sense of the extraordinary, similar to the saying “once in a blue moon,” often associated with royalty and magic.
In some spiritual and astrological circles, the term has been adopted to describe a specific alignment of the lunar cycle with a person’s menstrual cycle. A “Purple Moon cycle” refers to the pattern where menstruation begins during the waning phase of the moon, after the full moon. This alignment is interpreted as a time for personal reflection, realization, and the release of old patterns.