A quarter century after the Sept. 11 terrorist attacks, millions of Americans still remember one striking detail from that morning: the sky.
Before the deadliest terrorist attack in U.S. history, New York City experienced what many would call, abrilliant, cloudless blue sky, a shade that would later become known as "Memorial Blue" and one of the most enduring symbols of remembrance.
Through its annual "Remember the Sky" campaign, the 9/11 Memorial & Museum is inviting people around the world to photograph the sky wherever they are on Sept. 11 and share the image on social media using the hashtags #NeverForget911 and #RememberTheSky.
Here's what to know about why blue became synonymous with remembering 9/11.
watching to remember Sept. 11 attacks
Why is blue used to recognize 9/11 victims?
Each year around Sept. 11, people wear shades of blue and landmarks across New York City glow in blue light to honor those killed in the terrorist attacks. The color, known as "Memorial Blue," is inspired by the clear, cloudless sky over New York on the morning of Sept. 11, 2001, a detail many Americans still vividly remember 25 years later.
The shade is also linked to "Trying to Remember the Color of the Sky on That September Morning," a permanent installation at the 9/11 Memorial & Museum. Created by artist Spencer Finch, the work features 2,983 blue watercolor squares, each representing a person killed in the Sept. 11 attacks or the 1993 World Trade Center bombing.
The installation reflects a central idea of remembrance: While people may recall the exact shade differently, they share a common memory of the day itself.
"Many remember the beauty of the clear blue sky on the morning of 9/11. But, our own perception of the color blue might not be the same as that of another person,"the museum's website reads. "However, just like our perception of color, our memories share a common point of reference."
Blue lights have become a symbol of remembrance
The color also pays homage to "Tribute in Light," the annual memorial that transforms the New York City skyline ever Sept. 11 since 2022. From a site near the 9/11 Memorial & Museum, 88 powerful blue lights project twin beams up to four miles into the sky, recreating the silhouette of the Twin Towers.
First unveiled six months after the attacks, the installation has become one of the most recognizable symbols of 9/11 remembrance, illuminating the sky from dusk to dawn every Sept. 11. Across the city, landmarks including the Empire State Building, Rockefeller Center and other landmarks also glow blue, extending the tribute far beyond Ground Zero.
Museum invites Americans to 'Remember the Sky'
As the nation marks the 25th anniversary of the attacks, the 9/11 Memorial & Museum is asking people nationwide to participate in a digital project called "Remember the Sky." Participants are encouraged to photograph the sky above them on Sept. 11 and share the images on social media using #NeverForget911 and #RememberTheSky.
By collecting photos from across the country, the museum hopes to create a shared moment of reflection while connecting younger generations to a day many know only through history.
As younger Americanslearn about 9/11 through classrooms and memorials rather than personal experience, the museum says the project aims to bridge memory and history, helping ensure the lessons and legacy of the attacks are not forgotten.
Greta Cross is a national trending reporter at USA TODAY. Story idea? Email her at gcross@usatoday.com.
This article originally appeared on USA TODAY: The meaning behind 9/11's 'Memorial Blue'
