When a Supermoon appears on the horizon, it often presents a stunning visual spectacle, looking dramatically large and glowing with a deep orange or reddish hue. This vibrant color is not an inherent property of the moon, which is a gray celestial body reflecting white sunlight. Instead, the temporary transformation into a fiery orb is an optical phenomenon caused by the interaction of the moon’s reflected light with Earth’s atmosphere. This effect is common for any low-hanging full moon, but it becomes particularly noticeable when paired with the moon’s slightly increased size.
What Makes a Moon “Super”
The designation “Supermoon” is a widely used term for a full moon that occurs near perigee, the point in the moon’s elliptical orbit when it is closest to Earth. The more precise astronomical term for this alignment is perigee syzygy, referring to the straight-line configuration of the Earth, Moon, and Sun with the moon at its nearest point. Because its orbit is not a perfect circle, the moon’s distance from Earth varies significantly. When a full moon coincides with this closest approach, it appears larger and brighter than an average full moon.
Compared to a typical full moon, a Supermoon can appear larger in diameter. The size difference is more pronounced when comparing it to a micromoon, which is a full moon at apogee (its farthest point), where the Supermoon can be up to 14% larger and 30% brighter. While these are real physical differences, the visual impact of the size increase is often subtle to the casual observer without a side-by-side comparison.
How Earth’s Atmosphere Filters Moonlight
The mechanism responsible for painting the moon orange is atmospheric scattering, which is the same principle that makes the sky appear blue during the day. Sunlight, which the moon reflects, is composed of all colors, each corresponding to a different wavelength. Earth’s atmosphere contains tiny gas molecules, primarily nitrogen and oxygen, which interact with these wavelengths of light. Shorter wavelengths, such as blue and violet, are scattered in all directions more efficiently than the longer wavelengths, which are red and orange.
This process of preferential scattering acts as a natural filter for the moon’s light as it travels toward an observer. The blue light is effectively removed from the direct path, scattered away by the air molecules. What remains to travel straight through to our eyes are the longer, less-scattered wavelengths of red and orange light. This filtering gives the moon its warm color, much like how the sun takes on a reddish hue at sunrise or sunset.
Intensifying the Color on the Horizon
The orange effect is most pronounced when the moon is low in the sky, specifically near the horizon, because of the geometry of the viewing angle. When the moon is directly overhead, its light travels through the thinnest, most direct slice of Earth’s atmosphere. As the moon descends toward the horizon, its reflected light must pass through a dramatically increased column of air.
This longer atmospheric path means the light encounters a much greater density and number of scattering particles. The extended distance intensifies the filtering process, removing virtually all the shorter-wavelength blue light and leaving a saturated, deep orange or red color. Furthermore, the atmosphere often contains particulate matter, such as dust, smoke from wildfires, or pollution, which can further exaggerate this scattering effect. The presence of these aerosols deepens the hue, resulting in the most vivid orange or sometimes a dark red moonrise or moonset.