The Moon’s appearance in our night sky often shifts through various shades, from white to yellow or red. This perceived variability is largely influenced by factors external to the Moon itself. Its true color is not always apparent, requiring an understanding beyond its surface.
The Moon’s True Hue
The Moon’s inherent color, when viewed without interference from Earth’s atmosphere or specific lighting conditions, is primarily various shades of gray, brown, and tan. This is due to the composition of its surface material, known as regolith, a dusty, rocky substance covering the entire lunar surface. Regolith is made up of lunar rocks, including basaltic rock, which tends to be gray. Observations from Apollo missions and close-up images confirm the Moon’s surface consists largely of dark gray and brownish-gray material, similar to asphalt in brightness. These subtle hues are derived from minerals like magnesium, iron, feldspar, pyroxene, olivine, and titanium-bearing minerals found in the lunar crust.
How Earth’s Atmosphere Influences Perception
Earth’s atmosphere significantly alters our perception of the Moon’s color. When the Moon is high in the sky, its light travels through a relatively thin layer of atmosphere, making it appear bright and almost white. As the Moon approaches the horizon during moonrise or moonset, its light must traverse a much greater thickness of Earth’s atmosphere to reach our eyes.
This extended path causes Rayleigh scattering, the same process responsible for colorful sunrises and sunsets. Rayleigh scattering preferentially scatters shorter wavelengths of light, such as blue and violet, more effectively than longer wavelengths.
As moonlight passes through more air, blue light is dispersed, leaving the longer red, orange, and yellow wavelengths to pass through. Dust, pollution, and water vapor particles in the atmosphere can further enhance this filtering effect, intensifying the warm colors we observe. This explains why the Moon often appears yellow, orange, or reddish when low in the sky.
The Striking Colors of a Lunar Eclipse
During a total lunar eclipse, the Moon often appears in shades of red or coppery brown, commonly referred to as a “blood moon.” This occurs when Earth positions itself directly between the Sun and the Moon, casting its shadow upon the lunar surface. While direct sunlight is blocked, some sunlight still reaches the Moon by passing through Earth’s atmosphere.
As this sunlight travels through our planet’s atmosphere, shorter blue and violet wavelengths are scattered away. The longer red and orange wavelengths are less scattered and are instead bent or refracted around Earth’s edges, projecting onto the Moon.
The amount of dust, clouds, or volcanic ash in Earth’s atmosphere can influence the exact shade of red or orange the Moon exhibits during an eclipse. This filtered and bent red light is then reflected by the Moon’s surface back to Earth, creating the crimson hue.