Getting ready for the solar eclipse

On August 21, a solar eclipse will be viewable across the United States. Those who want to take a look will need eye protection, though. Looking directly at a solar eclipse without such protection can damage the eyes.

Eeriness creeps in. Colors change and shadows sharpen. The last minutes before a total eclipse of the sun triggers a primal reaction, says astronomer Jay Pasachoff.

“You don’t know what’s going on,” says Pasachoff. “But you know something is wrong.”

Millions of people will encounter this reaction on August 21, 2017. That’s when a total eclipse of the sun will sweep across the continental United States. This will be the first eclipse to grace the country since 1979. And it will be the first since 1918 to bring a total, if temporary, blackout coast-to-coast. Its path will be roughly 120 kilometers (75 miles) wide. Created by the moon’s shadow, this so-called totality will pass through 12 states, from Oregon to South Carolina.

Researchers and the public alike have been gearing up to make the most of this rare spectacle. After all, U.S. communities won’t get another chance until 2024.

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eclipse path
On August 21, a solar eclipse will race across the United States.

The mystery of the eclipse

A solar eclipse occurs when the moon passes between the sun and our planet, casting its shadow on Earth. For someone sitting on Earth viewing the eclipse (with some kind of eye protection), it appears that the moon nearly blots out the sun. A total solar eclipse occurs about once every 18 months.

Eclipse enthusiasts will travel from all over the world to experience up to nearly three minutes of midday twilight and glimpse the sun’s corona (Koh-ROH-nah). The outer layer of the sun, it is an intensely hot, ionized gas, or plasma. During a total eclipse, this seldom-seen halo of light will frame the blacked-out sun.

eclipse corona
During a solar eclipse, the moon blocks out the sun. That allows people to see the sun’s corona (as visible in this picture from a 1999 eclipse).

At its sight, “People cheer and people cry,” Pasachoff says of Williams College in Williamstown, Mass. And he should know. He’s already witnessed 33 total solar eclipses and 30 partial ones.

People like him will often have to travel to the far ends of the Earth to experience an eclipse. That’s because eclipses that pass over densely populated portions of the planet are fairly rare. This explains why the 2017 event is so special. Millions will be able to experience it firsthand, often without leaving home.

For scientists, an eclipse offers more than an interesting experience, though. It’s a time to study the corona. Though some of it is visible all of the time to a few telescopes in space, the region where the corona meets the surface is masked by the sun’s intensity. “Only on days of eclipses can we put together a complete view of the sun,” Pasachoff explains. For researchers, the 2017 eclipse is thus another chance to connect what they see on the surface of the sun to what’s happening in the outer reaches of its corona.

One enduring mystery relates to temperatures on the sun. The sun’s surface is a relatively balmy 5,500° Celsius (nearly 10,000° Fahrenheit). But the corona is millions of degrees hotter. Scientists still aren’t really sure why. “The consensus is that the sun’s magnetic field is responsible,” says Paul Bryans. “But it’s not clear how,” he adds. Bryans…

Eclipses come in many forms

solar eclipse
solar eclipse

A solar eclipse is one of Nature’s most awesome spectacles. It occurs when the moon passes in front of the sun (as seen from Earth).

Amazing things happen in the heavens. In the hearts of distant galaxies, black holes swallow stars. Once every 20 years or so, on average, a star somewhere in our Milky Way galaxy explodes. For a few days, that supernova will outshine entire galaxies in our night sky. Near our solar system, things are thankfully quiet.

Nevertheless, awesome events happen in our neighborhood too.

Eclipse means to overshadow. And that’s exactly what happens during a solar or lunar eclipse. These celestial events take place when the sun, moon and Earth briefly make a straight (or very nearly straight) line in space. Then one of them will be fully or partially shrouded by another’s shadow. Similar events, called occultations and transits, occur when stars, planets, and moons line up in much the same way.

Scientists have a good handle on how planets and moons move through the sky. So these events are very predictable. If the weather cooperates, these events easily can be seen with the unaided eye or simple instruments. Eclipses and related phenomena are fun to watch. They also provide scientists with rare opportunities to make important observations. For instance, they can help to measure objects in our solar system and observe the sun’s atmosphere.

Solar eclipses

Our moon is, on average, about 3,476 kilometers (2,160 miles) in diameter. The sun is a whopping 400 times that diameter. But because the sun is also about 400 times further from Earth than the moon is, both the sun and moon appear to be about the same size. That means that at some points in its orbit, the moon can entirely block the sun’s light from reaching Earth. That’s known as a total solar eclipse.

This can happen only when there is a new moon, the phase that appears fully dark to us on Earth as it moves across the sky. This happens about once per month. Actually, the average time between new moons is 29 days, 12 hours, 44 minutes and 3 seconds. Maybe you’re thinking: That’s an awfully precise number. But it’s that precision that let’s astronomers predict when an eclipse will occur, even many years ahead of time.

So why doesn’t a total solar eclipse occur each and every full moon? It has to do with the moon’s orbit. It is slightly tilted, compared to Earth’s. Most new moons trace a path through the sky that passes near to — but not over — the sun.

Sometimes the new moon eclipses only part of the sun.

The moon creates a cone-shaped shadow. The totally dark part of that cone is known as the umbra. And sometimes that umbra doesn’t quite reach Earth’s surface. In that case, people along the center of the path of that shadow don’t see a totally darkened sun. Instead, a ring of light surrounds the moon. This ring of light is called an annulus (AN-yu-luss). Scientists call these events annular eclipses.

annular eclipse
Ring-like annular eclipses (lower right) occur when the moon is too far from the Earth to completely block the sun. In the early phases of this eclipse (proceeding from upper left), it is possible to see sunspots on the face of the sun.

Not all people, of course, will be directly in the center path of an annular eclipse. Those in line with only a portion of the shadow, its antumbra, will see a partially lit moon. The antumbra is also shaped like a cone in space. The umbra and antumbra are lined up in space but point in opposite directions, and their tips meet at a single point.

Why won’t the umbra reach Earth every time there’s a solar eclipse? Again, it’s due to the moon’s orbit. Its path around Earth isn’t a perfect circle. It’s a somewhat squished circle, known as an ellipse. At the closest point in its orbit, the moon is about 362,600 kilometers (225,300 miles) from Earth. At its furthest, the moon is some 400,000 kilometers away. That difference is enough to make how big the moon looks from Earth vary. So, when the new moon passes in front of the sun and is also located in a distant part of its orbit, it’s won’t be quite big enough to completely block the sun.

These orbital variations also explain why some total solar eclipses last longer than others. When the moon is farther from Earth, the point of its shadow can create an eclipse lasting less than 1 second. But when the moon passes in front of the sun and is also at its closest to Earth, the moon’s shadow is up to 267 kilometers (166 miles) wide. In that case, the total eclipse, as seen from any one spot along the shadow’s path, lasts a little more than 7 minutes.

The moon is round, so its shadow creates a dark circle or oval on Earth’s surface. Where someone is within that shadow also affects how long their solar blackout lasts. People in the center of the shadow’s path get a longer eclipse than do people near the edge of the path.

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shadow diagram
Partially lit portions of Earth’s shadow are known as the penumbra and antumbra. The cone-shaped umbra is completely dark. The shadows of all celestial objects, including the moon, are divided into similar regions.

Partial eclipses

People completely outside the path of the moon’s shadow, but within a few thousand kilometers on either side of it, can see what’s known as a partial solar eclipse. That’s because they’re within the partially lit portion of the moon’s shadow, the penumbra. For them, only a fraction of the sun’s light will be blocked.

Sometimes the umbra completely misses the Earth but the penumbra, which is wider, doesn’t. In these cases, no one on Earth sees a total eclipse. But people in a few regions can witness a partial one.

eclipse shadow
The moon’s shadow on Earth’s surface during a total solar eclipse, as seen from the International Space Station on March 29, 2006.

On rare occasions, a solar eclipse will start and end as an annular eclipse. But in the middle of the event, a total blackout occurs. These are known as hybrid eclipses. (The change from annular to total and then back to annular…

Seeing Earth From Between Saturn’s Rings, Nearly a Billion Miles Away

Earth and the moon (on the left) from between Saturn's rings.
Earth and the moon (on the left) from between Saturn’s rings.

The Cassini Orbiter is about 5,000 pounds (minus its fuel, which is all gone, and the Huygens probe it dropped off on Titan in 2004) of science that’s been orbiting Saturn for nearly 13 years. It is, by any objective take, a vanishingly small speck in the vastness of space, and one of the subtle feats of its 12 sensors—including an ultraviolet imaging spectrograph, plasma spectrometer, and cosmic dust analyzer—is reminding…

The Fireball That Killed the Dinosaurs Could Help Us Find Life on Other Planets

When David Kring of the University of Arizona gave a presentation at the Lunar and Planetary Science Conference in 1991, he didn’t expect a packed crowd for his talk on the petrology of the Chicxulub Structure in the Yucatan, Mexico. Normally, Kring knew, impact-cratering sessions were presented in the smallest room—the miserable Room D, a shoebox on the second floor. But the magnitude of his announcement attracted scientists across fields and disciplines, so he was bumped up to the main room.

Kring had been investigating a place called the Yucatán-6 borehole, and he and his team had discovered shock quartz and impact melt fragments in two thumb-sized bits of rock that were over half a mile beneath the surface of the Earth. This was evidence that the hole, thought for a very long time to be a volcanic center, was actually an impact structure. And not just any “impact structure,” and not just any crater―but the crater of all craters on Earth. The one behind the death of the dinosaurs 66 million years ago.

Last year, Kring was part of an expedition in which scientists drilled into Chicxulub to investigate how the disastrous collision of fireball and Earth that killed the dinosaurs also created the conditions for life to begin anew. Last month, Kring and his colleagues returned to the Lunar and Planetary Science Conference to present their findings from the new core samples they took on that expedition. The results provide new clues about how life may have begun on Earth about 4 billion years ago—and point us towards how and where we can look for life across the universe.


Back in the early 1990s, Kring knew what he was looking for—a crater of the size and magnitude that would provide evidence of catastrophic extinction—but he didn’t know where to look. “It was a race to find the impact site,” Kring tells mental_floss, “and we had made a discovery of this very thick impact ejecta deposit in Haiti, which pointed us to [the Yucatan].”

Impact ejecta is what’s blasted from the Earth or other body when a meteor crashes into it. In this case, a giant chunk of the Earth was blown a thousand miles away. Until the Haiti discovery, people were looking all over the planet for the crater. But now they had a target region. Meanwhile, Petroleos Mexicanos, an oil company, had drilled down into what they thought was a “geophysical anomaly” in the Yucatan―a salt dome, maybe, where there might be oil. That’s when Kring and his colleagues re-examined samples collected from the site and realized there were features consistent with an impact.

That the Yucatan site was still intact to be found wasn’t a given. In the last 65 million years, half of the seafloor has…

50 years ago, continental drift began to gain acceptance

map of Earth
FULL OF PLATES Earth’s outer crust is composed of more than a dozen large pieces, known as tectonic plates, which bump or slide against each other.
Science News cover from April 29, 1967

Drifting theories shake up geology

Continental drift, a theory often considered amusing but rarely important, seems about to become the focus of a revolution in geology. At the least, it has already split the geological community into those who find the evidence…

Look Up! A Huge Asteroid Is Whizzing Past Earth This Week

Grab your telescope and get ready to duck, because an enormous space rock will be zooming through the sky the night of April 19.

Okay, fine. You won’t actually need to duck. The path of asteroid 2014 JO25 will be a near miss in space terms only, arriving in the sky 1.1 million miles from where we stand. That’s about four and a half trips to the Moon.

While it may not be the most dramatic flyby, 2014 JO25’s appearance is worth celebrating. Discovered just…

A Villain-Themed Land Didn’t Make It Into Disney World

Disney World is known as “The Happiest Place on Earth” for a reason. The characters, scenery, and rides exude the squeaky-clean cheeriness the brand is famous for, but something much darker was once planned for the Orlando, Florida destination. According to Movie Pilot, a land celebrating the villains of Disney never made it past the proposal stage.

The area, dubbed “The Dark Kingdom,” would have looked like a very different version of the Disney World that fans are familiar with. Filling the role of Cinderella’s castle…

Pees (Peace) On Earth – I Didn’t Know Galactus’ Real Name Was Calvin?!

When an intergalactic being sets its sights on conquering planet Earth it typically does so to exploit our planet for its resources or to enslave humanity and use us as slaves. But Galactus is an intergalactic being of immense size, with an even more immense appetite, and he sees Earth as only one thing- food. Thankfully Galactus doesn’t go number one or two where he eats, so we don’t have to worry about the…

NASA Puts the Planet Up for Adoption in Time for Earth Day

If you’re looking to feel a deeper connection to the planet you call home, NASA has good news. As reports, the space agency is putting Earth up for adoption one 55-mile-wide section at a time.

The project launched on April 6 in anticipation of Earth Day on April 22. Unlike other programs that invite you to symbolically adopt a panda or a star, this process doesn’t require a donation. Just type in your name and NASA will assign you one of 64,000 adoptable locations that cover the globe. The areas are divided into hexagonal tiles, each accompanied by Earth…

How Earth got its moon

full moon
full moon

The story of our moon’s origin does not add up. Most scientists think that that the moon formed in the earliest days of our solar system. That would have been back around 4.5 billion years ago. At that time, some scientists suspect, a Mars-sized rocky object — what they call a protoplanet — smacked into the young Earth. This collision would have sent debris from both worlds hurling into orbit. Some of the rubble eventually would have stuck together, creating our moon.

Or maybe not.

Astronomers refer to that protoplanet as Theia (THAY-ah). Named for the Greek goddess of sight, no one knows if this big rock ever existed — because if it did, it would have died in that violent collision with Earth.

moon Theia
Early Earth and a smaller protoplanet called Theia may have collided long ago, many scientists think. That would have hurled debris from both into space. In this simulation, red particles escaped the system, yellow formed the moon and blue fell to Earth.

And here’s why some astronomers have come to doubt Theia was real: If it smashed into Earth and helped form the moon, then the moon should look like a hybrid of Earth and Theia. Yet studies of lunar rocks show that the chemical composition of Earth and its moon are exactly the same. So that planet-on-planet impact story appears to have some holes in it.

That has prompted some researchers to look for other moon-forming scenarios. One proposal: A string of impacts created mini moons largely from Earth material. Over time, they might have merged to form one big moon.

“Multiple impacts just make more sense,” says Raluca Rufu. She’s a planetary scientist at the Weizmann Institute of Science in Rehovot, Israel. “You don’t need this one special impactor to form the moon.”

But Theia shouldn’t be left on the cutting room floor — at least not yet. Earth and Theia could have been built largely from the same type of material, new research suggests. Then they would have had a similar chemical recipe. There is no sign of “other” material on the moon, this explanation argues, because nothing about Theia was different.

“I’m absolutely on the fence between these two opposing ideas,” says Edward Young. He studies cosmochemistry — the chemistry of the universe — at the University of California, Los Angeles. Determining which story is correct is going to take more research. But the answer could offer profound insights into the evolution of the early solar system, Young says.

Mother of the moon

Earth’s moon is an oddball. Most other moons in our solar system live way out among the gas giants, such as Saturn and Jupiter. The only other terrestrial planet with orbiting moons is Mars. Its moons, Phobos and Deimos, are small. The leading explanation for them is that likely were once asteroids. At some point, they were captured by the Red Planet’s gravity. Earth’s moon is too big for that scenario. If the moon had come in from elsewhere, it probably would have crashed into Earth or escaped and fled into space.

An alternate explanation dates from the 1800s. It suggests that moon-forming material flew off of a fast-spinning young Earth. (Imagine children tossed from an out-of-control merry-go-round.) That idea fell out of favor, though, when scientists calculated the spin speeds required. They were impossibly fast.

In the mid-1970s, planetary scientists proposed the giant-impact hypothesis. (Later, in 2000, they named that mysterious planet-sized body as Theia.) The notion of a big rocky collision made sense. After all, the early solar system was like a game of cosmic billiards. Giant space-rock smash-ups were common.

But a 2001 study of rocks collected during NASA’s Apollo missions to the moon cast doubt on the giant-impact hypothesis. Research showed that Earth and its moon were surprisingly alike. To figure out a rock’s origin, scientists measure the relative abundance of different forms of oxygen. Called isotopes (EYE-so-toaps), they are forms of an element with different masses. (The reason they differ: Although each has the same number of protons in its nuclei, they have different numbers of neutrons.)

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isotope moon
The mix of oxygen isotopes inside moon material and meteorites called enstatite chondrites (shaded yellow) is surprisingly similar to that of Earth rocks (blue line). Other solar system materials are largely composed of different isotopic mixes.

Scientists can use amounts of various isotopes as something like fingerprints at a crime scene. Rocks from Earth and its moon, the scientists found, had seemingly identical mixes of oxygen isotopes. That didn’t make sense if much of the moon’s material came from Theia, not Earth. Rufu and her colleagues modeled the impact on a computer. From that they calculated the chance of a Theia collision yielding a moon with an Earthlike composition. And it was very slim.

Studies have been done of other elements in moon rocks, such as titanium and zirconium. They, too, suggest that Earth and its moon…