Lunar eclipse: How Earth turns the ‘micro’ moon red with its shadow

The phases of a total lunar eclipse or blood
moon.
Russ Opdahl

• A total lunar eclipse, or blood moon, will
  happen overnight on Friday, July 27.
• The eclipse will be colored orange-red due to sunlight
  passing through Earth‘s atmosphere and bouncing off the
  moon.
• The eclipse is slated to last nearly 1 hour 43 minutes
  — the longest in about a century.
• North America won’t see the eclipse, since the moon will be below the
  horizon, 
  but anyone can
  watch via a live video webcast.

Huge swaths of Earth are in for a special astronomical treat in
late July: the
longest total lunar eclipse in roughly 100 years.

During the evening of July 27 and into the early morning of July
28, Earth will pass between the sun and the moon to cast a shadow
on our 4.5-billion-year-old satellite.

Earth’s shadow isn’t a dull gray, though.

It ranges from orange to an eerie blood-red hue if you’re right
in the middle, which is precisely where the moon will be this
time around.

Here’s that works.

How a total lunar eclipse colors the moon red

A total lunar eclipse and a total
solar eclipse are similar, if not the reverse of one another,
but their appearances are significantly different.

During a solar eclipse, the moon passes between Earth and the sun
to cast its shadow on our planet. The shadow is colorless because
the moon has no atmosphere to scatter or refract any sunlight.

Earth, of course, is a different story.

What
a total lunar eclipse would look like from the
moon.

NASA

Our planet’s nitrogen-rich atmosphere takes white sunlight, a mix
of all colors of the spectrum, and scatters around the blue
colors. This makes the sky appear blue during the day and the sun
yellow.

Around sunset and sunrise, the light reaching our eyes has been
more throughly scattered, so much that blues are nearly absent.
This makes the sun and its light appear more orange or even red.

Roughly 240,000 miles away at the moon, the Earth would look
quite stunning as the same air, like a big lens, refracts that
tinged light toward the full moon.

“If you were standing on the moon’s surface during a lunar
eclipse, you would see the sun setting and rising behind the
Earth,” David Diner, a planetary
scientist at NASA’s Jet Propulsion Laboratory, wrote in a blog post. “You’d observe
the refracted and scattered solar rays as they pass through the
atmosphere surrounding our planet.”

A
diagram of the Earth, moon, and sun during a total lunar eclipse
or “blood moon.”
Shayanne Gal/Business
Insider

This is why lunar eclipses are orange-red: All of that colored
light is focused on the moon in a cone-shaped shadow called the
umbra.

The moon is also covered in ultra-fine, glass-like rock dust
called regolith, which has a special property called “backscatter.” This bounces a lot of light back the
same way it came from, in this case toward Earth (Backscattering
also explains why full moons are far brighter than during other
lunar phase.)

So, when we’re looking at the moon during a total lunar eclipse,
we’re seeing Earth’s refracted sunset-sunrise light being bounced
right back at us.

A
total lunar eclipse on September 28, 2015.
Matt Cardy/Getty Images

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,” Diner
said.

This total lunar eclipse will also happen during what’s called a
“micro” moon, or 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.)

Where and when to see the total lunar eclipse

North America will be out of luck this year, since the moon will
be below the horizon. You can still watch on a live webcast, though, if you’re located there.

But 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.

A
map of locations where the total lunar eclipse of July 27 and 28,
2018, will be visible.

Fred
Espenak/NASA Goddard Space Flight Center

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 passes out of Earth’s shadow, and the whole event will be
over at 23:28 UT (early on July 28, depending on where you live).