Just seven years ago, Americans emptied their homes for a chance to witness a total solar eclipse, an astronomical event hailed as a once-in-a-lifetime opportunity.
Back then, you might have heard this was the first to cut across the United States from coast to coast in a century — and it was. But lately, you might have heard there's another total solar eclipse slicing through North America on April 8, 2024.
So what gives?
Rest assured, skeptics, the 2024 eclipse is not crying wolf. There is another coming — the last major eclipse to cross the continent for 20 years — and the experience will be different from that of 2017, said Jenna Samra, a physicist at the Harvard-Smithsonian Center for Astrophysics.
The total solar eclipse this April will last longer, could appear darker, and the corona should look even more reminiscent of a pointy crown around the moon. Also, many more people live where it will be safe to remove protective eclipse glasses for the minutes when the sun is completely blocked.
"When people say the eclipse is a once-in-a-lifetime event, of course that's not strictly true. Actually, there's a solar eclipse every couple of years, if you can go anywhere in the world to see it," Samra told Mashable. "What makes them rare is appearing in a certain location on Earth."
A total solar eclipse occurs when the moon comes between the sun and Earth in space. During that brief period when the sun is entirely hidden, the sky will darken to twilight.
When the sun is concealed, people have the unusual chance to observe the sun's corona glowing around the edges of the moon. The corona, the outer layer of the sun's atmosphere, is normally washed out by the much brighter solar surface.
The moon is expected to first cast its shadow on Mexico's Pacific coast at 11:07 a.m. PT, according to NASA. The corridor of the moon's shadow, the so-called "path of totality," will arc from Texas to Maine, entering Canada through Southern Ontario and exiting on the Atlantic coast of Newfoundland at 5:16 p.m. NDT. The rest of the U.S. mainland will experience a partial eclipse, meaning the moon will only obscure a chunk of the sun.
The sun, moon, and Earth align like this to form a total solar eclipse every one to three years. But the shadow of the moon usually traces a path over oceans and desolate corners of the world, said Kelly Korreck, a heliophysicist and NASA's program manager for the 2024 eclipse.
"In one particular location, to get that solar eclipse (again), it's between 400 and 1,000 years on average," Korreck told Mashable. "So it's once-in-a-lifetime in a specific spot, but it's not necessarily once-in-a-lifetime for the planet."
There are, of course, outliers: Carbondale, Illinois, for example — a city that was in the moon's shadow during the eclipse on Aug. 21, 2017 — will also, coincidentally, be in the path of totality in April.
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The 2024 eclipse will last longer
The moon's shadow on Earth's surface was 71 miles wide during the 2017 eclipse, making the longest duration of total sun coverage two minutes and 41 seconds. But the 2024 eclipse will span 122 miles, Korreck said, providing an eclipse lasting up to four minutes and 27 seconds.
The shadow size changes based on the configuration of the moon's orbit around Earth and Earth's orbit around the sun, both slightly oval. The sun's size appears to change ever so slightly — enough to matter for an eclipse, said Michael Zeiler, an eclipse cartographer and co-founder of GreatAmericanEclipse.com, a resource for solar eclipses around the world.
"It all depends on the geometry," he told Mashable. "If the sun's apparent size to us is relatively small, or about the smallest it gets, and the moon is about the largest that it gets, then you will have the longest possible total solar eclipse, which has a theoretical maximum of about seven minutes and 30 seconds."
The 2024 eclipse could appear darker
Setting aside weather conditions, the wider path of totality is also the reason some solar eclipse observers could be treated to a darker sky, Zeiler said, allowing people to see more stars against the backdrop.
If a person stood in the center of the narrower path in 2017, then went to the center of the broader 2024 path this April, the sky could appear darker the second time around. The duration of the eclipse and the level of darkness are related.
"If you're in the center, then you're a farther distance away from sunlight. That's what it boils down to — how far you are from the edge of the shadow," he said.
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The corona will look 'spikier' in 2024
The corona, that hazy white glow that will appear when the moon blocks the sun, means "crown" in Latin. And because of where the sun is in its 11-year solar cycle, eclipse watchers are more likely to see a corona surrounded by pointy peaks, much like the name evokes.
"The shape of the corona that you see on Eclipse Day, it will become seared into your memory," said Zeiler, who has witnessed 11 total solar eclipses around the world. "When you look at a photograph, you'll instantly say, 'Oh yeah, that's the 2024 eclipse.'"
The solar cycle, caused by changes in the sun's magnetic fields, affects activity on the sun's surface. At the beginning and end of the cycle, that activity is at its calmest. But solar activity ramps up, peaking in the middle of the cycle, causing the sun to roil with giant eruptions.
The cycle will have nearly reached its maximum point during the 2024 eclipse. By contrast, the sun was closer to solar minimum in 2017, revealing a more ordered corona.
"The shape of the corona that you see on Eclipse Day, it will become seared into your memory."
"The corona will just look much more, I guess the word I can say is, 'spiky,'" Samra said. "It has to do with the way the magnetic field is. The spikes that we see are actually essentially plasma — hot, ionized gas — and the corona is following the magnetic field lines."
Astronomers and eclipse chasers are excited about the chance to see some wild fireworks in the corona. Those include solar prominences — wispy pink tendrils that extend into the corona. Some prominences resemble bright lakes of fire around the moon's rim.
But the highlight could be witnessing a coronal mass ejection, plasma spewed from the corona, which might look like a bubble or blob. Small CMEs have been reported during totality in recent times, but no major eruptions. Some historians believe the eclipse that crossed Spain in 1860 had a giant one, Zeiler said. Paintings and sketches from that event show a large hook-like feature launching out of the corona.
More people live in the 2024 eclipse's path
Motivated people will travel to the path of totality for an eclipse, but many who observe it will simply be those lucky enough to already live in the track of the moon's shadow. In 2017, that corridor fell over an estimated 12 million residents.
But this time in 2024, the path will arc toward the more densely populated East Coast, casting shade over about 31 million residents — over 2.5 times more people.
Some experts predict that because the eclipse will be in so many citizens' backyards, it will be the most-viewed astronomical event in American history — even if it's not a once-in-a-lifetime moment, per se.
"If you get lucky enough that it's not a-once-in-a-lifetime event, then just seize that," Samra said. "You're not gonna get sick of seeing it — like, no way. You're always going to wish you had longer."
