What Does an Orange Moon Mean?

The Moon can appear orange or red when it is near the horizon because of the longer path that its light must take through the air before reaching the observer. Oxygen in the Earth’s atmosphere scatters optical light with short wavelengths, and the effect, known as Rayleigh scattering, is more pronounced as light travels through more air. Particles in the air from smoke or dust accentuate the scattering of light.

The Moon emits no light of its own, and it simply reflects the sunlight striking its highly reflective surface. This light must then pass through Earth’s atmosphere before it can be seen on the ground. Earth’s atmosphere tends to scatter light from the blue end of the visible spectrum. This scattering effectively strips moonlight of its bluer wavelengths and makes it appear artificially reddened. The effect is difficult to notice when the Moon is high overhead, because its light travels through comparatively little air before reaching the observer and therefore scatters less than it does on the horizon.

If the Moon appears unusually red or orange or the effect persists while the Moon is overhead, it is possible that fine-grained particles of dust or smoke have saturated the atmosphere and are scattering the light more than normal.