Jul 13th 2018

What Is a Total Lunar Eclipse?

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This July, much of the world will be treated to a total lunar eclipse — the second of 2018 — according to Sky & Telescope. North America will be left out of July’s upcoming eclipse, but never fear: North America will have a ringside seat next January, per Space.com.

Perhaps it’s time to step back and answer a few questions like, what is a total lunar eclipse? How does it relate to a solar eclipse? What do you see, and are there precautions to take in order to watch it safely?

Cast a Giant Shadow

Eclipses, both lunar and solar, are all about shadows. Both the Earth and moon cast shadows that extend far into space, in a direction away from the sun. The sun causes shadows when its light is blocked but does not cast a shadow itself — it produces light rather than blocking it.

We pass into Earth’s shadow every evening at sunset, but we don’t really see it as a shadow — we just see the sun go down, hidden by Earth. Most of the time, the long shadows cast into space are not noticeable, because they fall only on empty space.

But the long shadows cast by the Earth and moon become view-worthy when the sun, Earth, and moon are perfectly lined up. (See the Sky & Telescope‘s handy diagram.) If the moon is directly between the Earth and the sun, the moon’s shadow (usually) extends just far across space enough to completely block the view of the sun from a small area on Earth.

The result of this sun-moon-Earth shadow play is a total solar eclipse, according to NASA. This is one of nature’s most spectacular sky shows. For a few minutes the blazing disk of the sun is entirely blocked, covered over by the moon. Deep twilight falls in the middle of day, and some animals will be tricked into thinking it is evening, said TIME.

What is a Total Lunar Eclipse?

But what happens if the lineup is sun-Earth-moon? Instead of Earth passing through the moon’s shadow, the moon passes through Earth’s shadow.

For viewers on Earth, the moon is not blocked from view — unless we are on Earth’s day side at the time, in which case, the moon is below the horizon and we wouldn’t see it anyway. If the eclipse is total, the moon is blocked from direct sunlight. What would otherwise be a bright full moon goes dark, for up to more than an hour, according to NASA.

Ghost Moon, Blood Moon

But even in a total lunar eclipse, the moon does not go entirely dark. The reason is that Earth’s atmosphere bends (or refracts) some of the sunlight passing through it, so that a little sunlight still reaches the moon even within Earth’s shadow. Exactly how this sunlight reaches the moon depends on weather conditions on Earth at the time of the eclipse. Sometimes the moon will be so dim it will be barely visible, and sometimes the eclipsed moon will be easier to see, per Space.com.

On a few occasions during a total lunar eclipse, the moon will appear deep red — the so-called blood moon — because red light is more easily refracted by Earth’s atmosphere than other colors. This is the same effect that often makes sunsets appear red, Space.com noted.

Some conditions on Earth, as described by Space.com, including a major volcanic eruption that throws vast amounts of dust into the upper atmosphere, are especially favorable to a blood moon during a total lunar eclipse. There is no way to know in advance what effects refraction will have on an upcoming eclipse. You’ll just have to go outside and find out for yourself.

One bit of good news is that it is perfectly safe to look directly at a total lunar eclipse, unlike a solar eclipse, which requires special precautions for safe viewing. Step outside and enjoy!

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