Why does the total eclipse moon look red? | The essentials of astronomy

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Great moon of red-orange color eclipsed.

Photo via Fred Espenak.

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During a lunar eclipse, you will see the shadow of the Earth crawling across the face of the moon. The shadow will appear dark, like a snack taken from a cookie, until the shadow completely covers the moon. Then, during the awesome moment of totality, the shadow on the face of the moon often changes suddenly. Instead of dark, it appears red. Why?

The reason comes from the air we breathe. During a total lunar eclipse, the Earth lies directly between the Sun and the Moon, which causes the Earth to cast its shadow on the Moon. If the Earth did not have an atmosphere, then, when the moon was completely within the shadow of the Earth, the moon would appear black, perhaps even invisible.

Thanks to the atmosphere of the Earth, what really happens is much more subtle and beautiful.

Earth's atmosphere extends about 50 miles (80 km) above the surface of the Earth. During a total lunar eclipse, when the moon is submerged in the shadow of the Earth, there is a circular ring around the Earth, the ring of our atmosphere, through which the sun's rays pbad.

Sunlight is composed of a range of frequencies. As sunlight pbades through our atmosphere, the green to violet part of the spectrum of light It is, essentially, filtered. This same effect, by the way, is what makes our sky blue during the day. Meanwhile, the reddish part of the spectrum is the least affected.

In addition, when this reddish light first entered the atmosphere, it inclined (refracted) to the surface of the Earth. It bends again when it comes to the other side of the Earth. This double curvature sends reddish light to the moon during a total lunar eclipse.

Dark planet of the Earth with a pink border on the left. Orange moon on the right.

The surprising photo montage on the left combines an image of Earth, taken by the Apollo astronauts, and another of the Japanese lunar probe Kaguya during a lunar eclipse in 2009. A person standing on the moon during a total lunar eclipse would witness the eclipse of the Earth. Sun. The pink ring around the Earth is our radiant atmosphere with the combined light of all sunrises and sunsets on the planet. On the right, the totally eclipsed moon acquires its color. Photos via JAXA (left), Jim Fakatselis. Image and legend through AstroBob.

Depending on the conditions of our atmosphere at the time of the eclipse (dust, humidity, temperature, etc., can make a difference), the surviving light will illuminate the moon with a color ranging from copper to intense red .

In December 1992, shortly after the eruption of Mount Pinatubo in the Philippines, there was so much dust in the 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 occurs, you will often listen to people who speculate about it. That uncertainty is part of the fun of eclipses, so enjoy! And watch the red moon during a lunar eclipse.

Earth diagram aligned with the sun and the moon.

A lunar eclipse takes place when the sun, the Earth and the moon align in space. The moon pbades through the shadow of the Earth. Image via Wikimedia Commons.

Conclusion: The moon can look red during a total lunar eclipse due to sunlight that is filtered and refracted by the Earth's atmosphere.

Deborah Byrd

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