A full moon sometimes appears with a distinct orange hue. This phenomenon prompts the question: why? The answer lies not with the moon itself, but with how light interacts with Earth’s atmosphere.
The Moon’s True Appearance
The moon is not inherently orange. Its surface is primarily composed of various shades of gray, white, and brown due to its rocky and dusty composition. When viewed from space, or when high in the sky and less affected by Earth’s atmosphere, the moon appears in these neutral tones. The light we see from the moon is simply sunlight reflected off its surface.
How Our Atmosphere Filters Light
Earth’s atmosphere is a mixture of gases and microscopic particles that interact with light from the sun, including sunlight reflected by the moon. This interaction is known as scattering, where light waves are redirected from their original path. The extent to which light scatters depends on its wavelength and the size of the particles it encounters. Shorter wavelengths of visible light, such as blue and violet, are scattered more efficiently by the tiny nitrogen and oxygen molecules in the atmosphere than longer wavelengths like red and orange. This preferential scattering of blue light is why the daytime sky appears blue; as sunlight travels through the atmosphere, blue and violet light disperses, leaving longer red and yellow wavelengths to pass more directly.
Why the Moon Appears Orange at Horizon
When the moon is near the horizon during moonrise or moonset, its reflected light must travel through a significantly greater amount of Earth’s atmosphere to reach our eyes compared to when it is high overhead. This extended path through the dense lower atmosphere means that even more of the shorter-wavelength blue and violet light is scattered away. Consequently, the longer-wavelength red and orange light penetrates the atmosphere more effectively, making the moon appear distinctly orange or reddish. As the moon rises higher in the sky, its light passes through less atmosphere, resulting in less scattering of blue light, and the moon regains its more typical white or yellowish appearance.
Other Factors Affecting Moon Color
Beyond the general atmospheric scattering, other elements present in the atmosphere can also influence the moon’s color. Larger particles, such as dust from arid regions, smoke from wildfires, or volcanic ash, can further scatter or absorb specific wavelengths of light. For instance, smoke and dust particles, which are larger than air molecules, can scatter blue light more intensely, enhancing the reddish or orange tint of the moon. Volcanic eruptions can inject ash and sulfur dioxide into the atmosphere, leading to red or orange moons, and in some rare cases, even a blue or green moon, depending on the particle size and composition. Air pollution can also contribute to a deeper red appearance of the moon by adding more dust particles that scatter blue light.