How many math lovers live in New York City? It’s a tough count to make, but the Museum of Mathematics made progress at its first anniversary celebration on Thursday, December 5.

With a mission to illuminate the math that permeates our day-to-day lives, the Museum of Mathematics, or MoMath, wasn’t about to waste its birthday on just another party. But by what founder Glen Whitney calls a “cosmic coincidence,” a date just one week before the museum’s December 12 anniversary held some special mathematic significance.

“Math shows up in ways we don’t realize,” Whitney says, “and the Flatiron building is a great example.”

Anyone can see that the iconic building is triangular. But it’s a very special triangle—a 5-12-13 right triangle, illustrating the Pythagorean theorem. It states that one can determine the length of the longest side of a right triangle (the hypotenuse) by adding the squared lengths of the two shorter sides and taking the square root of the result. In other words, as every middle school student learns, *a*^{2}+*b*^{2}=*c*^{2}.

“People celebrate Pi Day on 3-14,” Whitney says, “So we’re celebrating Pythagoras Day on 5-12-13.”

And to celebrate, MoMath enthusiasts came out in full force to honor the museum’s mission—in this case, by literally illuminating a mathematics equation. To prove that the Pythagorean theorem is correct, Whitney had museum members lined up around the building, holding glowsticks end to end so the outline was illuminated. If the theorem is correct, he said, they’d be able to predict the number of glowsticks used on the hypotenuse by counting those used on the other two sides. The 2,000-year-old equation came through, with the glowstick totals coming out to 75, 180, and 195—a perfect example of *a*^{2}+*b*^{2}=*c*^{2}. You can check the math here.

Those 450 participants were just the first to respond (“the most enthusiastic of our math enthusiasts,” Whitney says,), but around 2,000 came out to watch the theorem in action. “We’ve had a great year,” says co-director Cindy Lawrence, “But who would have thought that 2,000 people would come out to celebrate math?”

The crowd of 2,000 was lively, with triangle-themed attire to spare. Paige Breisacher, a 20-something professional who’s been a MoMath member for a year, had right triangles drawn on her face. “Well my favorite triangle is the Sierpinksi triangle,” she says, “but if I can't have that I'll take an ordinary equilateral triangle instead.” But she couldn’t resist joining the event, even though right triangles are only her third favorite.

“That’s what we’re all about,” Whitney says, “Showing that math is fun, and that you can play with it.”

*Image credits: MoMath (top 2 images); Rachel Feltman (lower 2 images)*