The Moon can appear a striking orange color, especially when it’s low on the horizon. This phenomenon isn’t due to rust, a recent discovery on the lunar surface, but rather the Earth’s atmosphere.
When moonlight travels through a thicker layer of atmosphere, as it does when the Moon is near the horizon, shorter wavelengths of light like blue and violet are scattered away. This leaves the longer wavelengths, such as orange and red, to dominate, resulting in the Moon appearing orange. This is the same reason why sunrises and sunsets often appear reddish-orange.
The effect is more pronounced during summer months when the full Moon is lower in the sky for observers in the Northern Hemisphere. Its position below the celestial equator prevents it from rising high enough to escape the thicker atmospheric layer. This results in the orange hue persisting throughout the night.
While the scattering of light is the primary reason for an orange Moon, there are other contributing factors. The amount of dust and pollution in the atmosphere can also enhance the orange color. Volcanic eruptions, for instance, can spew massive amounts of particles into the atmosphere, leading to intensely orange moonrises and sunsets.
Specific areas on the Moon itself possess a genuine orange tint. Using binoculars, one can observe a distinct orange patch close to the prominent Aristarchus crater, known as Wood’s Spot. This coloration is due to the presence of specific minerals on the lunar surface.
Another instance where the Moon takes on a reddish-orange color is during a total lunar eclipse. In this scenario, Earth casts its shadow on the Moon. However, some sunlight still reaches the Moon indirectly, bending through Earth’s atmosphere. Again, longer wavelengths like red and orange are preferentially refracted, leading to the Moon appearing a deep red or orange.
Despite occasional claims, there’s no scientific basis for a “green Moon.” There’s no known natural phenomenon that could cause the Moon to appear green. The orange appearance, however, is a well-understood and frequently observed phenomenon related to atmospheric optics and the scattering of light.
