**Pi **(noun “pye”)

This is a mathematical constant — a number whose value never changes. Symbolized by Greek letter π, pi is the ratio of a circle’s circumference — the distance around the outer edge — to its diameter, the length across its center. That ratio is always pi to one. The number pi is often shortened to 3.14 or 3.14159. But this number is actually infinite. One number cruncher calculated pi out to *50 trillion* digits. The digits after the decimal place, though, go on forever, into infinity.

Mathematicians can use pi to determine a circle’s circumference from its diameter. A circle one meter in diameter, for instance, will have a circumference of 3.14159 meters. If it measures two meters in diameter, the circumference will be 6.28318 meters.

Pi also can be used to calculate the area of a circle. The radius of a circle is one half its diameter. The area is equal to pi times the radius squared. That one-meter-diameter circle, then, has an area of 0.79 square meter. Scientists might use this equation to determine the size of craters on the moon, for example, or to calculate where a spacecraft travels.

But some mathematicians think that pi is too confusing. Instead, they would like to use tau — or τ. Tau is the ratio between a circle’s circumference and its radius. Tau is equal to 2π. Like pi, tau is also a constant and infinite.

Mathematicians like tau because it is easier to use to calculate a circle’s radians — the unit for measuring angles — like the angle at the tip of a slice of pizza. If they succeed in replacing pi, then instead of celebrating pi day on March 14 (3/14), people might instead celebrate tau day on June 28 (6/28). But there might not be as many tasty baked goods.

#### In a sentence

Scientists gave an exoplanet the nickname “Pi Earth” because it orbits around its star once every 3.14 days.