The most important number in the universe is arguably … pi. Why? Because it defines circles, spheres and wave motion from atomic to galactic scales.

Pi is the ratio of a circle’s diameter to its circumference. If the diameter is 1, the circumference is pi.

And pi, you may remember, is 3.1415926535 … etcetera, going on forever and never repeating.

It was first recognized 4,000 years ago, and since then, mathematicians have been trying to calculate it.

In 250 BC, the Greek mathematician Archimedes got to 3.14 and change.

1,600 years ago, Chinese mathematician Zu Chongzhi took pi to nine decimals, so accurate that if you used it to calculate Earth’s circumference today, you’d be within a quarter of an inch.

400 years ago, Newton invented calculus, which allowed him to calculate pi to hundreds of digits. Another mathematician proposed the Greek letter pi for the word “perimetros” or “perimeter.” And the name stuck.

Today pi is used in math, science, engineering, architecture, construction and manufacturing, building space ships and planning space flight and much more.

It’s so valuable that congress declared March 14—3.14—National Pi Day. Which coincidentally—or maybe not—is Albert Einstein’s birthday.