The moon often appears orange due to an interaction between moonlight and Earth’s atmosphere. The moon reflects sunlight, and as this light travels through our planet’s atmospheric layers, it undergoes transformations that influence the color we perceive.
How Our Atmosphere Filters Light
Earth’s atmosphere filters light through Rayleigh scattering. Light, including moonlight, consists of a spectrum of colors, each with a different wavelength. Blue and violet light have shorter wavelengths, while red and orange light have longer wavelengths.
As light enters the atmosphere, it encounters atmospheric molecules. These molecules scatter shorter wavelengths, like blue and violet, in all directions. This scattering is why the sky appears blue during the day. Longer wavelengths, like red and orange, are scattered much less efficiently and pass through the atmosphere more directly. When the moon is low in the sky, its light travels through a greater amount of atmosphere, causing more blue light to be scattered away and allowing red and orange hues to dominate what reaches our eyes.
Conditions That Create an Orange Moon
An orange moon is most commonly observed when it is low on the horizon, either during moonrise or moonset. At these times, moonlight travels through a greater thickness of Earth’s atmosphere. This intensifies the scattering of blue light, allowing red and orange wavelengths to pass through more readily. This same principle explains why sunrises and sunsets often appear red or orange.
Beyond the moon’s position, atmospheric particles also contribute to an orange or red appearance. Natural occurrences like smoke from wildfires, volcanic ash, or dust storms introduce additional particles. These particles enhance the scattering of blue and green light, allowing more red and orange light to reach our eyes. Human-made pollution can also add to this atmospheric haze.
The Moon’s Apparent Size
When the moon appears orange and low on the horizon, it often seems unusually large, a phenomenon known as the “moon illusion.” This perceived increase is an optical illusion, not a physical change in the moon’s size or distance. The moon’s angular diameter remains constant whether it is near the horizon or high in the sky.
This illusion stems from how our brains interpret visual information, especially in relation to surrounding objects. Near the horizon, the moon is viewed with foreground objects like trees and buildings, providing a sense of scale that makes it appear larger. When high in the sky with no immediate reference points, our brains may perceive it as smaller.