Math and dessert lovers marked Pi Day on March 14, a date chosen to match the first three digits of pi, one of the best-known mathematical constants. Pi represents the ratio of a circle’s circumference to its diameter and is commonly written as approximately 3.14159, with its digits continuing indefinitely. For many people, the constant is a familiar tool for school calculations such as the area of a circle or the volume of a cylinder, but researchers say its use extends far beyond basic geometry.

The holiday traces back to Larry Shaw, a physicist at the Exploratorium science museum in San Francisco, who created Pi Day in 1988. Sam Sharkland, program director of public programs at the museum, said Shaw 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,” a sentiment Sharkland described while reflecting on working with Shaw before he died in 2017.

At the Exploratorium, the celebrations have grown well beyond a small staff event that initially centered on pie. Over time, the museum’s Pi Day drew “a grand procession” where hundreds of visitors marched around a pi shrine, each carrying a digit. Sharkland said attendees often arrive early to claim their favorite digit for the parade, including a woman who has the symbol tattooed on her neck and marches near the front each year with a pi flag.

The program also includes a specific timing signal: the celebration begins at 1:59 p.m., marking the next three digits of pi. From there, the museum highlights how the constant appears in different scientific fields, including space exploration and medical research.

Pi in outer space and spacecraft engineering

In aerospace and mechanical engineering, Artur Davoyan said pi is so foundational that it is difficult to isolate a single use case. Davoyan, a professor at the University of California, Los Angeles, described pi as 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,” reflecting his view that the constant underpins many physical models engineers rely on.

Davoyan said pi’s role extends to systems involving round or cyclical or repeating properties, such as radio waves. He added that even when shapes appear non-circular—like squares or irregular blobs— they can be broken down into smaller circles and calculated using pi. His research focuses on developing propulsion systems intended to help spacecraft travel more quickly to distant parts of the solar system to gather and send back information to Earth.

To illustrate the idea, Davoyan pointed to NASA’s Voyager 1 and Voyager 2, which launched in 1977 but did not reach interstellar space until 2012 and 2018, respectively. He said NASA has to calculate Earth’s exact position in its orbit around the sun and design antennas for communication using pi. He also said scientists use pi again when receiving and breaking down complex signals beamed back to Earth.

Davoyan also described what happens when a signal is unfamiliar. “Say aliens send something to us, something that we don’t know how to deal with,” he said. “So the very first thing that you would do, you would try to split it into simple functions… and turns out that when you do this operation, you will naturally have pis in it.”

Pi in tiny fluids, cancer research and rapid testing

Pi also shows up, experts say, when working with small amounts of fluid. Dino Di Carlo, chair of the bioengineering department at the UCLA Samueli School of Engineering, described research that creates particles out of polymers designed to act as tiny test tubes for cells. He said these structures help researchers examine cells closely and study their functions and what’s inside them.

Di Carlo said pi is used in calculating how to form droplets, in surface tension calculations that help define how droplets break up, and in methods to control the size of the resulting volumes. He described using this approach to identify antibodies—small proteins that fight diseases in the body—that could block signals put out by cancer cells.

Beyond droplet formation, Di Carlo said pi is also part of calculations tied to how liquids flow through tubes and barriers. He pointed to an example involving a take-home COVID-19 test, where a fluid sample slowly flows sideways. He said he used similar properties to devise a new Lyme disease test that can be completed in 20 minutes, compared with the longer time frames of days or weeks previously.

As Sharkland and others highlighted during Pi Day programming, pi’s presence in daily education may come first, but the underlying mathematics continues to reappear in the practical work of scientists and engineers. Di Carlo summed up the constant by saying, “As an engineer and scientist, (pi) is just a part of life,” and adding, “Maybe I’ve taken it for granted.”