The moon sometimes appears to glow with a vivid orange or reddish hue, especially when it is just rising or setting on the horizon. This striking color change is not a property of the Moon itself, but rather an atmospheric trick played on our eyes by the planet Earth. The coloration is entirely dependent on the conditions of the air between the observer and the lunar surface.
What the Moon Really Looks Like
If you were observing the Moon from space, outside the influence of Earth’s atmosphere, its true coloration is a dull, dark gray. The lunar surface is primarily composed of silicate materials rich in oxygen, silicon, magnesium, iron, and aluminum. The darker areas, known as maria, are ancient plains of solidified basaltic lava that contain slightly more iron and titanium. The lighter highlands are composed of rock types such as anorthosite, which are rich in calcium and aluminum. The Moon possesses a very low reflectivity, or albedo, reflecting only about 7 to 12% of the sunlight that hits it, making its surface comparable in brightness to worn asphalt.
How Earth’s Atmosphere Filters Light
The light from the Moon that reaches our eyes must first travel through the Earth’s atmosphere, which is a blanket of gases like nitrogen and oxygen. These tiny gas molecules and other small particles in the air act as scattering agents for light waves. The amount of light scattered is highly dependent on the light’s wavelength, a phenomenon known as atmospheric scattering.
Shorter wavelengths of visible light, such as blue and violet, are scattered much more effectively by these small molecules than the longer wavelengths, like red and orange. This is the same mechanism that causes the sky to appear blue during the day. The atmosphere essentially removes the blue and violet components from the light beam, leaving the remaining light enriched with longer wavelengths.
Why the Moon Looks Orange Near the Horizon
The Moon appears orange near the horizon because of the sheer volume of atmosphere its light must penetrate to reach an observer. When the Moon is directly overhead, its light travels through the minimum possible thickness of the atmosphere. When the Moon is low in the sky, however, its light must take a much longer, more oblique path through the densest layers of the air. This significantly extended path length maximizes the scattering process.
As the light travels farther, virtually all the shorter-wavelength blue and green light is scattered away from the line of sight. Only the least-scattered, longest-wavelength light—the reds and oranges—remain to illuminate the Moon’s disk for the observer.
Furthermore, the presence of larger particles in the lower atmosphere, such as dust, smog, smoke from wildfires, or high humidity, can dramatically enhance this effect. These larger particles scatter even more of the light, sometimes resulting in a deep, fiery red appearance.