Why Does the Moon Turn Red During a Total Lunar Eclipse?

Total lunar eclipse of July 16, 2000. CREDIT: Fred Espenak/NASA's Goddard Space Flight Center View full size image

Early tomorrow morning (Dec. 10), a ruby-colored light show will appear in the sky as a total lunar eclipse turns the normally pallid moon scarlet.

Skywatchers in western Canada and the western United States should have a great view of the eclipse, which will start around 7:45 a.m. EST (4:45 a.m. PST, 1245 GMT), when the Earth's shadow begins to creep across the lunar disk. By 9:05 a.m. EST (6:05 a.m. PST, 1405 GMT), the moon will be fully engulfed in a glow that could range from light orange to blood red.

Our planet is big enough by a factor of three to block the entire sun, so you'd expect Earth's shadow to completely black out the sun. Instead, it makes the moon glow an eerie, rusty hue. Why?

Even as Earth blocks the sun's rays, the sunlight bends around the edge of the Earth, and this light is reflected onto the moon. The moon's reddish tint comes from the indirect rays of light being filtered through our atmosphere – the same visual effect that makes sunsets that striking coral color . Our atmosphere acts like a filter, removing most of the blue light, while leaving the red and orange light to grace the moon's surface. [How to Get to the Moon in 5 'Small' Steps ]

The moon will change various shades during different stages of the eclipse, going from an initial gray to orange and amber. The brightness of the colors can also be affected by atmospheric conditions. Extra particles in the atmosphere, such as ash from a large wildfire or a recent volcanic eruption, may cause the moon to appear a darker shade of red, according to NASA.

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