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The Moon can look orange or red for two different reasons: near the horizon, its light passes through more of Earth’s atmosphere on the way to your eyes; during a lunar eclipse, sunlight filtered through Earth’s atmosphere reaches the Moon. Both involve atmospheric scattering, but the light takes a different path in each case.
Why does the Moon look orange near the horizon?
When the Moon is low in the sky, its light travels through a longer stretch of Earth’s atmosphere before reaching you than it does when the Moon is high overhead. Air molecules scatter shorter wavelengths more strongly than longer wavelengths. As some of the blue light is scattered out of the direct path, the remaining light can appear warmer—yellow, orange, or reddish. NASA describes this same general effect in the colors of sunsets: the atmosphere’s scattering makes sunsets red.
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This is an ordinary viewing effect, not evidence by itself that a lunar eclipse is taking place. The Moon may appear especially warm-colored when it is near the horizon, but its color alone does not tell you whether it is entering Earth’s shadow.
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A lunar eclipse happens when the Moon passes into Earth’s shadow. Earth blocks direct sunlight from reaching the Moon, but some sunlight travels through Earth’s atmosphere and reaches the lunar surface. The atmosphere scatters shorter wavelengths more strongly, so the light that continues through is weighted toward orange and red. NASA’s Goddard explanation describes the wavelength difference: shorter-wavelength light is scattered more than orange and red light.
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The two situations are related by atmospheric optics, but the light path differs. For a low Moon, moonlight crosses more atmosphere on its way to the observer. In an eclipse, sunlight crosses Earth’s atmosphere before illuminating the Moon. NASA summarizes the connection between the eclipse color and the colors of the sky in Eclipses and the Moon.
Why does an eclipsed Moon vary in color?
An eclipsed Moon can look coppery, orange, deep red, or more muted. The atmosphere’s condition affects how much and what kind of light reaches the lunar surface. Dust and clouds can make the eclipsed Moon appear redder, as NASA explains in its overview of scattering by the atmosphere and its answers about red-orange eclipse illumination.
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How can you tell an eclipse from an ordinary orange Moon?
Look for the event, not just the color. A reddish Moon near the horizon may simply be seen through a longer atmospheric path. A lunar eclipse requires the Moon to pass into Earth’s shadow; during that event, the Moon’s appearance changes as it moves through the shadow. NASA’s eclipse overview explains this geometry.
Do you need special equipment to see a lunar eclipse?
No. A lunar eclipse can be viewed without special equipment. Binoculars or a telescope are optional ways to observe more detail, but they are not necessary to see the event. NASA’s eclipse guide notes that no special equipment is required.
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