MOORHEAD – A day like this will not be repeated, at least not any time soon.

Today, mathematicians, engineers and astronomers are all celebrating the little number that never ends. And while March 14 always gives them an excuse to eat pie, today is no ordinary Pi Day.

“This is considered the Pi Day of the century,” Concordia College math professor Bill Tomhave said.

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Why is 3/14/15 so special? The date matches 3.1415, the starting digits of pi: the ratio of circumference to diameter and a number that has fascinated humans for millennia.

“The most famous early approximation of pi was by Archimedes around 220 B.C.,” said Douglas Anderson, chairman of Concordia’s math department. “His approximation could be used by an engineer today; it’s good enough.”

More than 2,000 years later, YouTube abounds with videos of kids reciting thousands of digits. About 50,000 viewers have watched an animated video singing 300 of them as the numbers float through space. 3.1415926535 …

At Concordia on Friday – a day early so students could participate – the math department set out a paper string of numbers and 30 pies. “It’s part of our budget,” Anderson said.

At the Minnesota State University Moorhead planetarium today, there will be a countdown to pi second – 9:26:53 a.m.

The occasion is momentous for those of us who rely on pi every day. Don’t think that’s you? Think again.

When you phone a friend (or listen to the radio, or watch ‘The Bachelor’)

Whenever you flip on your favorite TV channel or radio station, you should probably thank pi.

Sound travels in waves, meaning it’s mathematically expressed with functions derived from circles. You don’t need to know that to hear your show, but someone else probably used pi to make your products work.

Take radio. When you tune in to a given station, you’re actually tuning in to a particular frequency, or cycles per second.

“You think of going around a circle over and over again, you’re repeating the same process over and over again, and because a circle has pi in it, those functions have pi in them, too,” Anderson said.

That same line of reasoning can be applied to other situations in which sound travels and you receive the correct message.

“Why does your friend sound like someone you recognize when you’re talking on the phone?” Anderson said. “Those mathematical formulas that give you those frequencies, they also help maintain the integrity of the initial content.”

**When you look at stars**

Pi is critical to understanding our universe.

Planets are relatively spherical, their orbits are close to circular and the light from stars spreads out in a sphere.

“We’ve got circles in astronomy all over the place,” said MSUM planetarium coordinator Sara Schultz. “You would be hard-pressed to not find pi almost anywhere you look.”

Schultz lists numerous examples: Pi is in the equation that determines a star’s energy. Pi can determine how fast you’re moving as the Earth spins on its axis. And pi is necessary to calculate distances between points in the Milky Way.

“The concept is fairly simple,” Schultz said. But “it is mind-boggling, a little bit, to think it just has so much to do with so many things that we just take for granted.”

At the planetarium’s “Pi in the Sky” event, which starts at 9:15 a.m. today, kids will eat pie and pizza pie, compete in a pi-off memorization contest and learn some of those applications.

“If you really stop to think about it, it really is incredible,” Schultz said.

**When you buy a heater**

Like sound, heat travels in a way that can be described using pi.

Specifically, engineers use the Fourier series to describe things that propagate, such as heat and sound. That series is too complex to describe here, but it relies on trigonometric functions, which come from the unit circle and therefore involve pi.

“It’s amazing, really, but the circle is such a fundamental shape,” Anderson said.

Joseph Fourier, a French mathematician, created the series to solve the heat equation, and today his work allows engineers to predict how a heat source will behave.

That said, even they don’t use the series, or pi, every day.

“Somebody’s already done the calculations,” said Jim Nelson, a mechanical engineer with Obermiller Nelson Engineering, which does a variety of work including heat generation and transfer projects. “I don’t have time to design every piece from the ground up.”

**When you log on**

In the Bible, pi is estimated at 3, no decimals.

In 2014, pi was calculated out to 13.3 trillion decimal places, a computation that took 208 days.

How cyclical: It takes a computer to calculate pi, and pi to build a computer.

“It’s meaningful to pretty much everyone here,” said Dan Olsen, vice president of sales and marketing at Packet Digital, which designs computer chips to improve power efficiency.

At the Fargo company, pi is used to calculate areas of chips, which have to be large enough to handle a given amount of power.

Pi also plays a role in the mechanical engineering of computers – their size, shape and air flow. Those who work in supercomputers measure their capabilities by how many digits of pi they can compute.

“We don’t use it explicitly every day, but we’re probably more dependent than we actually realize,” Olsen said. “It lends itself to every engineering discipline, probably every science discipline, every math discipline. It’s kind of a constant in the universe.”