Why Does the Moon Turn Red During a Lunar Eclipse?

If you look up at the sky during the total lunar eclipse, you might notice something: the Moon doesn't disappear into darkness. Instead, you will observe a “blood moon”. Let’s understand the science behind it.

What Is a Lunar Eclipse?

A lunar eclipse occurs when Earth passes directly between the Sun and the Moon, casting its shadow onto the Moon's surface. This can only happen during a full moon, when the Sun, Earth, and Moon are aligned in a straight line. 

Why Does the Moon Turn Red?

The red color comes from Earth's atmosphere acting like a giant lens. Even though Earth blocks direct sunlight from reaching the Moon, some sunlight still bends around the edges of our planet. As this light passes through Earth's atmosphere, it gets filtered and refracted. Now, Earth's atmosphere scatters different wavelengths of light differently called Rayleigh scattering. Shorter wavelengths like blue and green light scatter widely and get dispersed in all directions, while longer wavelengths like red and orange can pass through more easily. During a lunar eclipse, the sunlight traveling through Earth's atmosphere has all its blue light scattered away, leaving only the red and orange wavelengths to continue their journey. This filtered, reddish light bends around Earth and illuminates the Moon, giving it that distinctive copper or blood-red glow.

How Is This Different From a Solar Eclipse?

It's worth noting the key difference. During a solar eclipse, the Moon passes between Earth and the Sun, blocking sunlight from reaching us. Solar eclipses are dangerous to view directly because you're looking at the Sun. But lunar eclipses are completely safe to watch with the naked eye because you're simply looking at the Moon—there's no risk of eye damage. In fact, lunar eclipses are one of the most accessible astronomical events for everyone to enjoy.