When the moon slips in front of the sun on April 8, many places will hold eclipse viewing parties. The Alamo in San Antonio, Texas, for example, will hand out special viewing glasses shaped like the historic building.
"We encourage everyone to come to the Alamo," says Alamo Trust spokesperson Jonathan Huhn, who notes that past astronomical events have drawn thousands to the Alamo's plaza. "We're hopeful to have another 5,000 people out in front of the shrine of Texas liberty to witness this beautiful celestial event."
But the Alamo is just outside the so-called path of totality—that's the ribbon of land that stretches through 13 states, from Texas to Maine, that will see a total solar eclipse. During a total eclipse, the sun is completely obscured by the moon.
"We're not in the 100% area of totality," says Huhn, who says he thinks the sun will be around 99.9% obscured. "It's very, very close."
Close but no cigar, according to eclipse experts.
"I would never tell someone, '99% is close enough.' That's definitely not the case with a solar eclipse," says Michelle Nichols, who directs public observing programs at the Adler Planetarium in Chicago.
"Drive those last few miles to get into the path of totality," she urges. "People go to total eclipses to get the full experience, and 99% will not get you the full experience."
"It's 100% or nothing," agrees Fred Espenak, a retired NASA astrophysicist who has experienced 30 total solar eclipses. "There's such a radical, dramatic difference between a 99% partial and a 100% total. There's no comparison."
During a total eclipse, the sky darkens suddenly and dramatically. The temperature drops. Stars come out. Beautiful colors appear around the horizon. And the once-familiar sun becomes a black void in the sky surrounded by the glowing corona — that's the ghostly white ring that is the sun's atmosphere.
"It seems supernatural," says Espenak. "It is so far beyond the scope of normal, everyday existence that it seems dream-like or hallucinogenic."
A partial solar eclipse offers none of that magic, according to Rick Fienberg, the project manager for the American Astronomical Society's solar eclipse task force.
"Even at 99%, it gets no more dark than on a sort of typical overcast day," says Fienberg. "You can have a 75% or 80% partial solar eclipse, and if you didn't know it was happening, you might not notice because the environment changes so little."
That's because the sun is just so incredibly bright that even a tiny exposed sliver can light up the sky — or hurt your eyes if you're not wearing protective glasses. Only during the brief total eclipse phase when the sun is completely covered (which varies depending on your exact location but could be around four minutes) is it safe to look up toward the sun without special eye protection.
"The sun is about a million times brighter than the full moon," explains Angela Speck, an astronomer at the University of Texas at San Antonio. So if 99.9% of the sun is obscured, she says, there will still be "a thousand times more light than the full moon, and so it's still bright."
The last time parts of the United States got to see a total solar eclipse, in 2017, the path of totality went over a lot of rural areas, says Nichols. This time, the path cuts through more urban areas.
That means while around 32 million people live in the path, many more live a short distance away.
For them, the difference between seeing a partial eclipse and seeing a total one may mean going across town.
"If you get right up to the path, but don't go that last, you know, a few hundred yards into it, you're going to have a very, very deep partial eclipse," says Fienberg. "It will definitely get noticeably dark, but not near as dark as it will get at 100%. And you will not see the solar corona."
Popular places located just outside the path of totality will have to decide what kind of event to hold that day, if any.
The San Antonio Zoo is on the side of the city that won't see a total eclipse. Unlike the nearby Alamo, it will focus on having pre-eclipse activities on the day before.
"We are hosting an Eclipse Prep event on Sunday instead of an event on Monday," zoo spokesperson Hope Roth told NPR by email, adding that astronomers will attend and free solar glasses will be available. "We will encourage guests to visit the zoo, grab their glasses, and have a fun time while preparing for the Eclipse the following day."
The historic Cincinnati Observatory, which frequently holds sky gazing parties, will only see a 99.7% partial eclipse. Staffers there thought hard about whether or not to hold an eclipse celebration.
"For the longest time leading up to this eclipse, we had been committed to being closed," says executive director Anna Hehman, "because totality, if the skies are clear, is within an hour of us."
Eventually, they decided to hold an event for those members of their community who, for whatever reason, couldn't travel to the path of totality.
"While we encourage everyone to head to totality if possible, we love that people want to be at the Cincinnati Observatory for out-of-this-world events like this one. So, if you can't head to totality, please join us," the observatory's website says.
After all, a partial eclipse is still an interesting celestial event that people have observed since antiquity, even if it's not as dramatic.
"The buzz about solar eclipses is usually focused on totality, and witnessing a total eclipse is worthy of the hype. But the vast majority of people won't be in the path of totality," astronomy educator Shauna Edson, at the National Air and Space Museum, told NPR in an email.
That's why the museum is holding an eclipse celebration on the National Mall in Washington, D.C., which will see an 89% partial eclipse.
"This eclipse falls during cherry blossom season, so a lot of people will be visiting DC, and the Museum wanted to provide a space where they could enjoy the eclipse together," Edson noted, adding that participants will be able to enjoy the crescent-shaped shadows that form under trees and try out different methods for viewing the partial eclipse.
After April 8, the contiguous United States won't see another total solar eclipse for 20 years. And the one that's coming in 2044 will only be visible from the less-populated states of North Dakota, South Dakota and Montana.
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