Science Facts

Why Moon Turns Red? – Blood Moon & Lunar Eclipse

Red Moon

Why does the moon turn red? Moon does not have a light of its own. Why? Didn’t he pay the electricity bill? Sunlight or white light is a mixture of visible colors. Now, when the sunlight falls on the moon, it reflects all these visible colors equally. Making the moon mostly appear white. However, when the earth is precisely between the sun and the moon during a lunar eclipse, the moon appears red. It happens because of the earth’s atmosphere.

When the sunlight coming from the sun passes through the earth’s atmosphere. The earth’s atmosphere scatters the colors of the sunlight, having shorter wavelengths. However, the red color, which has the longest wavelength, is the least scattered. Hence, it makes its way to the moon, making it look red.

Why moon turns red?

When the Earth’s moon looks orangy-brown or red, also sometimes referred to as a “Blood Moon.” It’s because your moon is in a total lunar eclipse. This happens when the Earth moves between the sun and the moon, causing the moon to be in Earth’s shadow fully. The moon gets that red color because a bit of light from the Earth’s sunrises and sunsets make their way over.

A full moon happens once a month. The Moon moves in a circle-shaped pathway around the Earth. The Sun’s light causes the earth to have a shadow that goes out this way into space. Outer space is dark, so you usually can’t see the shadow. But sometimes, when the moon is over in this part of its pathway, it goes into the Earth’s shadow.

Lunar eclipse
Lunar eclipse

The shadow of the Earth blocks out the light from the Sun, so the Moon starts to go dark. It starts to look like it’s disappearing. What happens after the moon begins to go into the earth’s shadow? How the Moon turns reddish-orange or red color? It is because of the lunar eclipse, and the blood moon happens.

During a lunar eclipse, the moon travels directly behind the earth, into the Earth’s shadow. The Moon’s surface, which generally looks white, would have reddish-orange light shining on it.

  • A new moon visible when its orbit is the closest point to the sun.
  • A full moon is visible when its orbit is farthest from the sun.

If the Earth is sitting here in the middle, why doesn’t Earth cast a shadow during every full moon?

The moon’s orbit around the Earth is tilted by a few degrees relative to our path around the sun. So during most full moons, the Earth isn’t directly in the way. But every so often, that full moon happens right in the middle of Earth’s shadow. Lunar eclipses are pretty standard as astronomical phenomena go.

On average, there are about one and a half lunar eclipses per solar year, but that means anywhere from 0 to 3 because I’m not sure how you get half of an eclipse. Not every eclipse is the same. Like every orbit, the moon follows an elliptical path. When a full moon lines up with its closest approach to Earth, a supermoon will become visible. And the lunar face can appear up to 14% bigger and 30% brighter.

The opposite phenomenon where the full moon happens at the farthest point is called an apogee-syzygy. The sun is so much bigger than the Earth. We cast a two-part shadow on the moon. The wider outer shadow, where the Earth is only partially blocking the sun’s light, is called the penumbra.

The moon barely dims as it enters this part. The narrow shadow in the middle, where Earth is blocking the sun’s light, is called the umbra. Temperatures on the moon can quickly fall from over 100˚C in the sun to -150˚C in Earth’s shadow.

A lunar eclipse from the moon looks a lot like a solar eclipse from Earth. During totality, where the moon is entirely inside Earth’s umbra. A tiny bit of the sun’s light is bent through our atmosphere and comes out the other side. But along that journey, the shorter wavelengths have been filtered out by the air around us. It’s the same reason that sunsets and sunrises are red here on Earth.

When we stare at the red color of a lunar eclipse, we’re seeing every sunrise and sunset on Earth reflected at us at the same time!


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Sources:

McClure, Bruce. “Century’s Longest Lunar Eclipse.” EarthSky.
Karttunen, Hannu. Fundamental Astronomy. Springer.

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