Look up at the super blue blood full moon Jan. 31 — here’s what you’ll see and why

To see the phenomenon, you need to be on the night side of the Earth; this eclipse will be visible mostly in Asia, Australia, the Pacific and North America. But don’t worry if you miss it, lunar eclipses happen on average a couple times a year. The next one visible in North America will be on Jan. 21, 2019.

A blue moon that looks red

When a lunar eclipse happens, the moon appears to darken as it moves into the Earth’s shadow called the umbra. When the moon is all the way in shadow it doesn’t go completely dark; instead, it looks red due to a process called Rayleigh scattering. The gas molecules of Earth’s atmosphere scatter bluer wavelengths of light from the sun, while redder wavelengths pass straight through.

This is why we have blue skies and red sunrises and sunsets. When the sun is high in the sky, red light passes straight through to the ground while blue light is scattered in every direction, making it more likely to hit your eye when you look around. During a sunset, the angle of the sun is lower in the sky and that red light instead passes directly into your eyes while the blue light is scattered away from your line of sight.

In the case of a lunar eclipse, the sunlight that makes it around Earth passes through our atmosphere and is refracted toward the moon. Blue light is filtered out, leaving the moon looking reddish during an eclipse.

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