The moon usually appears silvery-white, but sometimes displays a subtle pinkish tint. This captivating transformation is not due to any change on the moon itself, but rather a natural optical effect caused by the interaction of light with Earth’s atmosphere.
Understanding Light and Color
Our perception of color begins with light, a form of electromagnetic radiation. Visible light is composed of various colors, each with a different wavelength. Violet and blue light have shorter wavelengths, while red and orange light have longer ones. When light encounters an object, some wavelengths are absorbed, and others are reflected or transmitted, determining the color we perceive.
The human eye contains specialized cells called cones that detect color. We have three types of cone cells, sensitive to different wavelengths (blue, green, red). The brain interprets these signals, allowing us to distinguish millions of colors.
The Role of Earth’s Atmosphere
Earth’s atmosphere filters light from celestial bodies. A key process is atmospheric scattering, specifically Rayleigh scattering. This occurs when light interacts with particles much smaller than its wavelength, like nitrogen and oxygen molecules.
Rayleigh scattering preferentially scatters shorter wavelengths (blue and violet) more efficiently than longer ones (red and orange). This explains why the sky appears blue. During sunrise and sunset, sunlight travels through a greater thickness of atmosphere. This scatters away most blue and violet light, allowing longer red and orange wavelengths to reach our eyes, creating warm hues.
Why the Moon Takes on a Pink Hue
Atmospheric scattering explains why the moon sometimes appears pink, especially when low on the horizon. At this position, moonlight must traverse a significantly greater amount of Earth’s atmosphere. As it travels through this denser, longer path, shorter wavelengths, like blue and green light, are scattered away. The remaining light consists predominantly of longer red and orange wavelengths, creating a pinkish hue. Additional atmospheric particles, such as dust, aerosols, or increased humidity, can enhance this scattering, leading to a more pronounced pink or reddish appearance.
Beyond Pink: Other Factors and Appearances
While atmospheric scattering is the primary reason for a pink moon, other factors influence its perceived color. Pollutants, wildfire smoke, or volcanic ash can absorb and scatter more blue and green light, making the moon appear intensely orange or red. During a total lunar eclipse, often called a “blood moon,” the moon takes on a deep coppery red. This occurs because sunlight filters through Earth’s atmosphere and bends towards the moon, scattering shorter wavelengths and leaving red and orange light to illuminate the surface. The moon’s color can vary widely depending on the specific composition and thickness of the atmospheric path its reflected light travels through.