NORTH ANDOVER — There is only a brief moment when it is safe to look at the sun during a total eclipse.
That occurs when the sun is completely obscured by the moon, which will be true at around 3 p.m. during the eclipse April 8, but only for people standing in a 100-mile wide swath across northern New England.
For everyone else the eclipse will be partial and their best bet is to wear a pair of eclipse glasses, 500 of which will be available at StevItens Memorial Library starting Monday.
“The Stevens Memorial Library, like many other Massachusetts libraries, is only able to offer a limited number of eclipse glasses,” said Library Director Kathleen Keenan. “Because of the limited supply, we’ve set some rules.”
The eclipse glasses are provided by an organization called Science-Technology Activities and Resources for Libraries (STAR), which partnered with the Space Science Institute to help make 5 million pairs available free of charge to 10,000 libraries across the country.
The effort to find some glasses may be worth it, because the next total eclipse to visit the lower 48 American states won’t be until Aug. 12, 2045.
But STAR is now out of glasses and recommends visiting the American Astronomical Society at eclipse.aas.org/eye-safety/viewers-filters to find places, approved by the society, where a pair might be purchased. STAR also has a map on its website showing other libraries that they have supplied with glasses.
“Many libraries learned with a past eclipse, the demand was much higher than could be met so many people were disappointed,” Keenan said. “You might check to see what other area libraries or organizations also have eclipse glasses.”
Eclipse glasses have filters that absorb the different kinds of radiation present in sunlight, said John Schroeder of the Institute for Industrial Art and History in North Andover.
“Forty years ago those probably cost several hundred dollars,” he said. “Now they’re made out of plastic and they cost 20 cents.”
There are even cheaper ways to look at the sun indirectly, such as punching a pinhole in a piece of cardboard and holding it above a sheet of paper, where an image of the eclipsed sun will appear. A variety of do-it-yourself options are described in an “Eclipse Booklet” at STAR’s website.
The Institute will hand out its own free supply of 30 eclipse glasses at a presentation at the library Saturday, April 6, at 10:30 a.m. Registration is recommended at www.stevensmemlib.org.
But the focus of their demonstration on eclipses will be using a magic lantern, a 19th century version of slide projectors.
Magic lanterns were a popular form of mass entertainment, almost the equivalent of television today, and were typically used to show images of national parks, or downtowns in major cities, Schroeder said. Some of the slides are mechanical and simulate motion, for instance of rotating planets, and are set in motion by turning a crank.
“We have two sets of astronomical slides that date from the 1870s to the 1890s,” Schroeder said. “They start off by showing different ideas of the solar system.”
There are also slides depicting solar eclipses, which occur when the moon blocks the sun, and lunar eclipses, when the earth comes between the sun and the moon and the latter turns dark. Schroeder will also show a lantern slide depicting the transit of Venus.
“Venus is closer to the sun than earth, so about every 120 years, Venus crosses the sun and we can see it from earth,” Schroeder said. “That’s an eclipse, but not in a traditional sense. It was used to determine how far the sun is away from earth.”
Schroeder has a background in chemistry, and has spent most of his life studying how light travels through the earth’s atmosphere.
The larger point of next Saturday’s presentation, he said, is to show that science has a history, and our understanding of eclipses has evolved along with everything else we know about the world.
“Cell phones wouldn’t have been possible if there had never been a steam engine,” he said.
His talk at the library will even travel farther back than the 19th century, to explore how ancient cultures interpreted eclipses.
Some believed that eclipses occurred when a dragon devoured the sun, and others believed that if there was an eclipse during a rainstorm, the rain would be poisonous.
“It was the best science of the time,” Schroeder said.