Why does the moon look red during a total lunar eclipse?

During a total lunar eclipse – like the lunar eclipse on the night of June 15, 2011 – Earth’s shadow on the moon often looks red. Why?

During a lunar eclipse, you’ll see the Earth’s shadow creeping across the moon’s face. The shadow will appear dark, like a bite taken out of the moon’s face, until the shadow completely covers the moon. Then, during the breathtaking moments of totality, the shadow on the moon’s face often often suddenly changes. Instead of dark, it appears red. Why?

A very long total lunar eclipse on night of June 15

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Lunar eclipse of March 3, 2007. Image by Joshua Valcarcel. (Wikimedia Commons)

The reason stems from the very air we breathe. If the Earth had no air, then Earth’s shadow on the moon during a lunar eclipse would appear black. The presence of Earth’s air means that, during a lunar eclipse, sunlight can filter through Earth’s atmosphere onto the shadow on the moon. This filtered sunlight makes the moon appear red during a total eclipse.

But red isn’t the only color of a totally eclipsed moon. An eclipsed moon can appear brown, red, orange or yellow. The color depends on the presence of dust and clouds in Earth’s atmosphere. If there has been a major volcanic eruption, for example, the shadow on the moon will appear dark throughout an eclipse. In December 1992, not long after the eruption of Mount Pinatubo in the Philippines, there was so much dust in Earth’s atmosphere that the totally eclipsed moon could barely be seen.

Can anyone know in advance how red the moon will appear during a total lunar eclipse? Not precisely. Before an eclipse takes place, you’ll often hear people speculating about it. Still, no one knows for certain how red the moon will appear when eclipse time comes. That uncertainty is part of the fun of eclipses, so enjoy! And watch for the red moon during a lunar eclipse.