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Math enthusiasts and dessert lovers come together every March 14 to celebrate Pi Day, a date representing the first three digits of the mathematical constant pi.
Pi, the ratio of a circle's circumference to its diameter, is approximately 3.14159 — but its digits continue infinitely. While students often use pi to calculate the area of a circle or the volume of a cylinder, its applications extend far beyond the classroom and touch every aspect of our world.
The holiday was established in 1988 by Larry Shaw, a physicist at the Exploratorium science museum in San Francisco.
“He had a very open and expansive view of the world and saw an opportunity with this number, mathematical concept, to invite people into the joy of mathematical learning,” said Sam Sharkland, program director of public programs at the museum, who worked with Shaw before his passing in 2017.
What began as a small staff celebration featuring pie quickly grew into a large event where hundreds of visitors march around the pi shrine, each carrying a digit. Attendees often arrive early to claim their favorite digit for the parade. One woman, who has the pi symbol tattooed on her neck, attends every year and marches near the front carrying a pi flag, Sharkland noted.
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The celebration officially begins at 1:59 p.m., representing the next three digits of pi.
Here are some ways pi is used at the forefront of scientific discovery.
Pi in Outer Space
In the field of mechanical and aerospace engineering, pi is so fundamental that pinpointing a single use is difficult, said Artur Davoyan, a professor at the University of California, Los Angeles.
“Pi is part of literally every single formula that you would use to do any calculation, like for spacecraft motion, for materials and how they work, or propulsion systems,” Davoyan explained.
Anything round or exhibiting cyclical or repeating properties—such as radio waves—involves pi. Even squares or irregular shapes can be broken down into a series of smaller circles and analyzed using pi, he added.
Davoyan’s research focuses on developing new propulsion systems to send spacecraft faster to the outer reaches of the solar system to collect and transmit data back to Earth. He referenced NASA’s Voyager 1 and 2 probes, launched in 1977 but only reaching interstellar space in 2012 and 2018.
To communicate with these probes, NASA must precisely calculate Earth’s position in orbit and design antennas using pi. Scientists then use pi again to decode the complex signals sent back to Earth.
“Say aliens send something to us, something that we don’t know how to deal with,” Davoyan said. “The very first thing you would do is try to split it into simple functions... and it turns out that when you do this operation, you naturally encounter pi.”
Tiny Droplets of Pi
Pi also plays a crucial role in studying small volumes of fluids.
Dino Di Carlo, chair of the bioengineering department at UCLA’s Samueli School of Engineering, researches creating tiny polymer particles that act as miniature test tubes for cells. This technique is vital for closely examining cells and understanding their functions.
Pi is used in calculations for forming these droplets, determining surface tension that affects droplet breakup, and controlling their size, Di Carlo explained.
He is applying this method to identify antibodies—small proteins that fight disease—that could block signals emitted by cancer cells.
Pi is also essential in calculations involving fluid flow through tubes and barriers, such as the lateral flow of samples in at-home COVID-19 tests.
Using these principles, Di Carlo developed a new Lyme disease test that delivers results in 20 minutes, compared to days or weeks previously.
“As an engineer and scientist, (pi) is just a part of life,” Di Carlo said. “Maybe I’ve taken it for granted.”
