For a few minutes in the middle of the day Aug. 21, the sky will go completely dark over parts of the country. Thousands of people will do what countless others have done before them spanning across history; look up, and marvel at the celestial body that gives life to all on Earth.
A total solar eclipse is rare, and the August event will mark the first time since 1918 that an eclipse has been visible in the United States from coast to coast.
A total solar eclipse occurs when the moon aligns between the Earth and the sun. The shadow directly under the moon, called the umbra, is the total eclipse. The moon will also have a penumbra, which is a lighter shadow around the umbra. This is viewed as a partial eclipse. The path of the moon’s umbra as it moves across Earth is called the path of totality. The total eclipse will be visible along a path spanning from Salem, Oregon, through the heartland to Charleston, South Carolina.
The Merrimack Valley isn't in the path of totality, meaning sky gazers in New England will only see a partial eclipse. About 60 percent of the sun will be eclipsed by the moon in Massachusetts.
For those in the path of totality, the totality — when the sun is completely blocked out by the moon — is about a two-minute event, flanked by the moon slowly moving from in front of the sun.
Peak viewing time in the Merrimack Valley is around 2:45 p.m., when the moon will be in front of about 60 percent of the sun, but viewers will probably see the moon start to creep across the sun starting at around 1:30 p.m.
A natural laboratory
The unusual conditions spawned by eclipses make them natural laboratories for scientists.
Local universities are taking advantage of the Aug. 21 eclipse to send teams to locations along the path of totality to take atmospheric readings.
UMass Lowell Center for Space Science and Technology has assembled an "eclipse team" to measure how the atmosphere changes during an eclipse.
As the moon moves in front of the sun, the atmosphere cools off, mimicking sunset.
"What we're expecting to see as the cold spot moves across the country, is as the cold spot shrinks, hot atmosphere should be rushing into it," said Tim Cook, a physics professor at UML.
The team is comprised of the science and tech center's director Supriya Chakrabarti, Cook, research scientist Susanna Finn, astronomer Silas Laycock, and grad students Saurav Aryal and George Geddes.
They will split into two groups, one in Jackson, Wyoming, and one in Carbondale, Illinois, where they will deploy instruments called spectrographs they built themselves that separate light waves into a spectrum. A camera records the results.
"This is a fairly new instrument; it's unique in its ability to take readings even when the sun is out," said Cook.
As with most scientific discoveries, Cook is prepared to analyze data they gather even if it's not what they expected.
"Even if we get it wrong, it will still give us clues," Cook said.
Merrimack College Physics professor Ralph Pass also is planning a trip to the path of totality.
"I'm going to just enjoy it," said Pass, who is going with his grandson to southern Idaho. "Solar eclipses are totally overwhelming. They're very moving experiences, and I'm going to have a camera with me and take some pictures."
As the sky darkens, birds come out, and the entire horizon glows like the sun is setting. The temperature drops, and stars come out.
Pass said local viewers should find anything that creates a pinhole effect and projects the image of the sun on the ground. Or, just go under a tree where the leaves don't block the light completely.
"You'll be able to see the crescent shape where the moon is over the sun," Pass said.
And for solo viewers like Dick Ciampa, in Salem, Mass., a deck is sufficient.
"My plan is to watch it from my deck ... and if anyone is walking by let them have a look with the solar glasses," he said.
Ciampa, 67, said the last time he saw a total eclipse was in the 1970s. That eclipse did not expand from coast to coast.
"It's kind of eerie since it gets dark," he said. "Animals and birds start getting ready to bed down, and it gets noticeably cooler."
In order to safely watch the eclipse, viewers outside the path of totality should use glasses made with special lenses to block out the sun and protect their eyes.
NASA recommends using a pair that meets an international safety standard approved in 2015. Ordinary sunglasses, however dark they may seem, are not sufficient to protect your eyes.
The space agency and the American Astronomical Society recommends consumers choose eclipse glasses made by five companies — Rainbow Symphony, American Paper Optics, Thousand Oaks Optical, TSE 17, and Baader Planetarium.
The proper lenses should meet what’s called “ISO 12312-2” (sometimes written as “ISO 12312-2:2015”) international safety standards, according to the group.
Public libraries across the Merrimack Valley have organized viewing parties, so amateur astronomers should check with their local branch for details.