- The longest total lunar eclipse or blood moon in a century will happen overnight on July 27.
- From the moon, Earth will look like it's surrounded by a ring of fire — with its sunset and sunrise connected in a loop.
- NASA has an animation showing what Earth's glowing red ring might look like during a total lunar eclipse.
- North America won't see the eclipse, but anyone can watch via a live video webcast.
A total lunar eclipse happens when Earth slips in front of the sun to cast a ruddy-orange to deep-red shadow on the moon.
This is why the astronomical event is often called a blood moon. People in Earth's Eastern Hemisphere can see the longest lunar eclipse of the 21st century starting at 19:30 Universal Time (UT) on Friday, July 27.
However, imagine you're an astronaut who happens to be on the surface of the moon during a total lunar eclipse, and you look back home. What would you see?
NASA's Science Visualization Studio has illustrated the answer to this question with an animated video.
To someone on the moon during a lunar eclipse, the Earth would appear to be surrounded by a bright-red ring of fire.
The image above is taken from NASA's animation, which actually illustrates the precise appearance of Earth and the moon during the total lunar eclipse that occurred September 27, 2015.
But apart from the position of Earth's continents, this week's lunar eclipse will appear more or less the same from the moon's perspective.
Here's why.
What gives total lunar eclipses an orange-red color
Total lunar eclipses and total solar eclipses are essentially the reverse of one another.
However, their appearances are very different (whether you're observing them from Earth or its natural satellite).
During a total solar eclipse, the moon passes between Earth and the sun, casting a small, dark shadow on our planet. For those watching on Earth, the ring of the sun's light surrounding the moon looks colorless because the moon has no atmosphere. (Atmospheres, similar to glass lenses, can refract sunlight.)
Earth is surrounded by a blanket of air, though, and this lens-like refraction is ultimately why lunar eclipses make the moon look orange-red.
By volume, about 80% of Earth's atmosphere is made of nitrogen gas, or N2, and most of the rest is oxygen gas, or O2. Together, these gases take white sunlight — a mix of all colors of the spectrum — and scatter around blue and purple colors. Human eyes are much more sensitive to blues than purples, which is why the sky looks blue and the sun yellow to us during daylight hours.
During a sunset or sunrise, sunlight reaching our eyes has passed through a lot more atmospheric gas, and this effectively filters out the blues and makes the light appear orange or even red.
A similar thing happens during a lunar eclipse. Earth's atmosphere bends and focuses the sun's light into a glowing, cone-shaped shadow called the umbra.
The red color is never quite the same from one lunar eclipse to the next due to natural and human activities that affect Earth's atmosphere.
"Pollution and dust in the lower atmosphere tends to subdue the color of the rising or setting sun, whereas fine smoke particles or tiny aerosols lofted to high altitudes during a major volcanic eruption can deepen the color to an intense shade of red," David Diner, a planetary scientist at NASA's Jet Propulsion Laboratory, wrote in a blog post in 2010.
What Earth looks from the moon during a total lunar eclipse
Roughly 240,000 miles away at the moon, the Earth would look quite stunning during a lunar eclipse.
"If you were standing on the moon's surface during a lunar eclipse, you would see the sun setting and rising behind the Earth," Diner wrote. "You'd observe the refracted and scattered solar rays as they pass through the atmosphere surrounding our planet."
On the moon, you'd see the sunrise and sunset of Earth connected together in a roughly 25,000-mile loop. And on the ground around you, normally drab-gray lunar dust, or regolith, would look a bit orange-red.
Earth's color-tinted umbra is always out there — if you had enough cash and a spaceship, you could fly into it anytime you wanted.
However, the moon's slightly tilted orbit means that it only passes through our planet's shadow only about twice every 11 months.
Where and when to see Friday's total lunar eclipse
The coming eclipse will happen during what's called a "micro" moon – the opposite of a super moon. This happens because the moon's orbit isn't perfectly circular, so it appears larger at times and smaller at others during its roughly 29-day-long orbit around Earth. In this case, it will look a bit smaller.
North America will be out of luck during the lunar eclipse, since the moon will be below the horizon. You can still watch the phenomenon on a live webcast, though.
If the weather cooperates, most of eastern Africa, the Middle East, and central Asia should see the full and total lunar eclipse. Scientists in Antarctica should also have a great view.
Europe, eastern Asia, Australia, Indonesia, and other regions will enjoy a partial lunar eclipse, where the moon passes partly through Earth's shadow.
The partial eclipse begins when the moon first touches the penumbra, or outer shadow, of Earth. According to NASA, that should happen at 17:14 Universal Time on July 27.
The total eclipse — when the moon is fully inside the red-hued umbra of Earth — starts at 19:30 UT and ends at 21:13 UT. That's a full 1 hour 43 minutes, which is just four minutes shy of the longest total lunar eclipse possible, according to EarthSky.
The partial eclipse will resume immediately afterward, as the moon starts to leave Earth's shadow. The whole event will be over at 23:28 UT (which might technically be early on July 28, depending on where you live).
See NASA's animation below of a total lunar eclipse from the moon.
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