Definition
A total lunar eclipse happens when the Sun, Earth, and Moon line up almost perfectly in space. As the Moon slips entirely into Earth's darkest shadow, its usual silver glow transforms into a mysterious, rusty red.
When the Moon Hides in Earth's Shadow
Stand in front of a flashlight, and your body casts a sharp shadow on the wall.
The exact same thing happens in space. The blazing Sun acts like a giant flashlight, and when Earth stands in front of it, it casts a long, deep cone called Earth's umbra. Most of the time, the Moon circles Earth at a slight tilt, dodging this shadow entirely.
Every so often, though, the Sun, Earth, and Moon line up in a nearly straight row. The Moon slowly wanders into the dark silhouette trailing behind Earth. If only part of the Moon clips the shadow, it is a partial lunar eclipse. When the entire Moon is swallowed by the shadow, it is called a total lunar eclipse.
You can watch a total lunar eclipse with the naked eye from anywhere on the night side of Earth. Unlike a solar eclipse, which demands special protective eyewear, a lunar eclipse is completely safe and comfortable to watch with just your eyes or a pair of binoculars.
Why Does It Glow Red in the Dark?
Once the Moon is buried in Earth's shadow, you might expect it to vanish into pitch darkness. But when you look up, the Moon is still there, glowing with a soft, reddish hue. People often call this a 'Blood Moon.'
The secret behind this eerie color lies in Earth's atmosphere. Sunlight is a mixture of all the colors of the rainbow. When sunlight slices through the blanket of air wrapping our planet, short wavelengths like blue light scatter in every direction.
Longer wavelengths like red light pass through with far less scattering. Earth's atmosphere acts like a lens, bending this surviving red light inward toward the dark shadow zone. This refracted light travels all the way to the lunar surface. In other words, the red glow during a total lunar eclipse is the combined light of every sunrise and sunset on Earth projected onto the Moon at the same time.
Why Doesn't It Happen Every Month?
A full moon rises once every month, so why isn't there an eclipse every time? The answer comes down to the tilt of their orbital paths.
The Moon's path around Earth is tilted by about 5 degrees compared to Earth's orbit around the Sun. That 5-degree tilt might sound small, but across the vast distance of space, it is more than enough for the Moon to pass safely above or below Earth's shadow.
The two points where the Moon's tilted orbit crosses Earth's orbital plane are called nodes. An eclipse can only happen when a full moon coincides closely with one of these nodes. Earth's shadow is also divided into a lighter outer ring (the penumbra) and a dark inner core (the umbra). Only when the Moon dives completely into that dark umbra does the grand show of a total lunar eclipse begin.
Common misconceptions
During a total lunar eclipse, the Moon gets completely blocked by Earth's shadow and disappears from the sky.
Because red sunlight bends through Earth's atmosphere toward the Moon, it almost never vanishes completely; instead, it glows a dim, coppery red (a Blood Moon). Depending on dust and clouds in Earth's atmosphere, it may look quite dark, but rarely invisible.
Where you meet it
Sunlight filters through Earth's atmosphere, scattering blue light away and leaving only red light to reach and tint the Moon.
It works on the same principle as a sunset on Earth: the reddish glow seen at dusk is projected onto the face of the Moon.
In one sentence
When the Sun, Earth, and Moon line up and the Moon slips entirely into Earth's dark inner shadow, filtered and bent red sunlight turns it an eerie copper red.