3.141592653589793238462643383279502884197169… We could go on.

Those are the first 43 digits of pi, an irrational number that plays a huge role in science and math here at NASA and everywhere else in the world. Today, March 14, scientists, mathematicians and students around the planet celebrate Pi Day, an unofficial holiday marking that the month and day are the first three digits of pi: 3.14.

What is pi?

It’s the ratio of a circle’s circumference—the length of its edge—to its diameter, which is the distance from one side of the circle to the other at its widest point.

Why do we care about pi for Earth science?

One big reason: Earth is round! When we’re calculating orbital paths of spacecraft and determining how many orbits it takes to image the entire planet, we use pi!

Happy Pi Day!