What if the Moon turned red?

The full Moon is usually bright and silvery white.

But on some nights, it slowly turns red or orange. What is happening to the Moon?

On most nights the full Moon shines a bright silvery white. But every so often, over a few hours, something remarkable happens: the full Moon dims, and then glows a deep orange or red.

The Moon itself has not changed at all. Something has come between it and the Sun, and something else is painting it with color. In this thought experiment you can follow what happens step by step: where the shadow comes from, why the light turns red, and when and how you can watch it yourself.

NASA describes a lunar eclipse as Earth's shadow falling on the Moon when Earth is positioned precisely between the Moon and the Sun, dimming it and sometimes turning the lunar surface a striking red over the course of a few hours.

Two questions follow. First, geometry: why does this happen only at some full Moons, and what is the difference between the umbra and the penumbra? Second, optics: why is the Moon, deep in Earth's shadow, not simply dark? The answers connect the Moon to the same physics that makes our sky blue and our sunsets red.

The Moon goes around Earth once a month.

Most months, it does not go into Earth's shadow. Sunlight still reaches it. So the full Moon stays bright.

The Moon travels around Earth about once a month. You might expect Earth to block the Sun's light from the Moon every time it is full, but it doesn't. NASA explains that the Moon's path around Earth is tilted, by about 5 degrees, compared with Earth's path around the Sun.

Because of that tilt, the full Moon usually misses Earth's shadow and stays brightly lit, even though it is behind Earth. Only now and then do the Sun, Earth and Moon line up exactly. That is why a lunar eclipse is a special event.

Lunar eclipses occur only at the full Moon phase, NASA notes, but not at every full Moon. The Moon's orbit is tilted about 5° to the plane of Earth's orbit around the Sun, so at most full Moons it misses Earth's shadow.

Counting both kinds, NASA says that anywhere from four to seven times a year, the Sun, Earth and Moon line up closely enough to produce an eclipse, lunar or solar. Solar eclipses happen only at new Moon, when the Moon's shadow falls on Earth instead.

On eclipse night, Earth is right between the Sun and the Moon.

Earth's shadow slowly covers the Moon. But a little reddish light still gets in. So the Moon glows red!

During a total lunar eclipse, the Moon moves into the darkest middle part of Earth's shadow, called the umbra. You might expect it to vanish, but it doesn't. A little sunlight still reaches it by passing through Earth's air, which bends some of the light toward the Moon.

On the way, the air takes away most of the blue light, so the light that arrives is orange and red. NASA puts it this way: it is as if all the world's sunrises and sunsets were shining onto the Moon at once.

NASA distinguishes three types. In a total eclipse the Moon enters the umbra and is lit dimly by sunlight filtered through Earth's atmosphere. In a partial eclipse the alignment is imperfect: the Moon crosses only part of the umbra, and the shadow grows and recedes without ever covering it. In a penumbral eclipse the Moon passes only through the faint outer penumbra and dims so slightly that it can be hard to notice.

In totality, the light reaching the Moon has crossed a thick slice of atmosphere; NASA notes that more dust or clouds in it make the Moon look redder.

Sunlight looks white, but it is made of all the colors of the rainbow.

Earth's air bounces the blue light away. The red light keeps going. That red light reaches the Moon.

Sunlight looks white, but NASA explains that it is really a mix of all the colors of the rainbow. When sunlight passes through Earth's air, blue light gets scattered, bounced around in all directions, much more than red. That is why the daytime sky looks blue.

At sunset, sunlight takes a long, low path through the air before it reaches you. Most of the blue is scattered away along the way, so mostly yellow, orange and red light gets through. The light that reaches the eclipsed Moon has taken that same kind of long trip through the air, edging past Earth, so it is red too.

Shorter wavelengths, the blues and violets, scatter more easily in Earth's atmosphere than longer red and orange wavelengths, NASA explains; reddish light travels more directly through the air. When the Sun is near the horizon, its light takes a longer, low-angle path, the blue is scattered away in the distance, and only the yellow-to-red part reaches observers.

During a lunar eclipse, the light that is not blocked by our planet is filtered through a thick slice of atmosphere around Earth's edge on its way to the Moon. NASA Space Place adds that Earth's atmosphere bends some sunlight toward the Moon while absorbing the other colors.

You can watch a lunar eclipse with just your eyes. It is safe!

Anyone on the night side of Earth can see it. You do not need to travel far.

Lunar eclipses are easy to see. NASA explains that each one is visible from the whole half of Earth where it is night, so lots of people can watch the same eclipse. And unlike a solar eclipse, you never look toward the bright Sun: NASA's Astronomy Picture of the Day notes that you need no special equipment, just your eyes.

Scientists use eclipses too. NASA's Lunar Reconnaissance Orbiter, which runs on sunlight, switches off most of its instruments during a lunar eclipse to save power. It leaves one on, called Diviner, to measure how quickly the Moon's surface cools in the sudden darkness.

NASA notes that each lunar eclipse is visible from half of Earth, and an APOD explanation calls lunar eclipses some of the most commonly witnessed astronomical events: no special equipment is needed, and unlike a solar eclipse the observer's eyes and camera do not point toward the Sun. NASA's table of lunar eclipses for 2021 to 2030 lists the next total one, the kind that turns the Moon red, on December 31, 2028, visible from Europe, Africa, Asia, Australia and the Pacific, with a total phase of about 1 hour 11 minutes. The lunar eclipses before it, starting with February 20, 2027, are penumbral or partial.

During lunar eclipses, controllers shut down most instruments on the solar-powered Lunar Reconnaissance Orbiter but keep Diviner running. Because rocks of different sizes cool at different rates, its temperature data let scientists infer the size and density of rocks on the Moon.