The longest total lunar eclipse of the 21st century will turn the moon blood red in the night sky this week.

The July 27 eclipse will be visible for 1 hour and 43 minutes when the moon is fully engulfed in Earth’s shadow — a period known as totality. This celestial show will not be visible from North America, but skywatchers in parts of South America, eastern Africa, the Middle East and central Asia will be in for an impressive event.

“This is a really cool eclipse,” said Noah Petro, a planetary geologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, adding that the event will be extra special because, “[t]his is going to be the longest eclipse of this century.”

What is a lunar eclipse?

Lunar eclipses occur when the moon, Earth and the sun are aligned and the moon passes into Earth’s shadow. This typically happens up to three times a year. The last lunar eclipse occurred on Jan. 31, an event that was dubbed a “super blue blood moon.”

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Unlike with solar eclipses, there’s no need to wear protective eyewear when witnessing a lunar eclipse. In other words, these events can be viewed safely with the naked eye.

What happens during a lunar eclipse?

As the moon passes into Earth’s shadow, it will dim and darken. But then, something special happens.

“When it’s fully engulfed, [the moon] will turn this copper, blood-red color,” Petro said. “It will turn a deep, deep red when it’s at the greatest point in the eclipse.”

This dramatic coloring is actually caused by Earth’s atmosphere, which scatters light from the sun and casts it onto the face of the moon. “It’s like the projection of sunrises and sunsets onto the lunar surface,” Petro said.

What’s the best time to view the lunar eclipse?

The best time to see the eclipse depends on where you are in the Eastern Hemisphere. The best places to view the celestial event from start to finish are in eastern Africa, the Middle East, India and central Asia, according to NASA. In southern Africa and the Middle East, for example, totality will occur at or around midnight local time. In India and central Asia, the moon will start passing into Earth’s shadow at 10:44 p.m. local time, and the eclipse will peak at around midnight.

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In parts of South America, western Africa and Europe, the eclipse will be partially visible just after sunset, as the moon rises. And in eastern Asia, Australia and parts of the western Pacific, the eclipse will be visible as the moon sets, in the early morning hours before sunrise on July 28.

Can I watch the lunar eclipse online?

If you live in a region of the world where the lunar eclipse isn’t visible, you can still experience the spectacular sky show. NBC News will be hosting a livestream — part of a digital special hosted by Simone Boyce called "Space Is Awesome" — starting at 4 p.m. EDT (20:00 UTC).

Why is this lunar eclipse so long?

Unlike solar eclipses, which have short-lived periods of totality, the peaks of lunar eclipses can be quite lengthy. This month’s lunar eclipse is particularly long because of the moon’s position as it slips into Earth’s shadow.

When the sun, Earth and the moon are aligned, Earth casts a shadow that projects out into the shape of an imaginary cone. The longest lunar eclipses occur when the moon moves right down the center of this cone.

“In shorter [eclipses], the moon won’t pass through most of that shadow,” Petro said.

Coincidentally, the moon will also be at its most distant point from Earth in its monthly orbit in late July, meaning the moon will appear smaller in the night sky and will take longer to completely pass through Earth’s shadow.

When is the next lunar eclipse?

The next total lunar eclipse will occur on Jan. 21, 2019, and will be visible from North America, South America and parts of Europe, Africa and the central Pacific. The period of totality during this eclipse will last for 1 hour and 2 minutes.

There will also be a partial lunar eclipse on July 16, 2019, visible from South America, Europe, Africa, Asia and Australia.

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