Look at your calendar. You might not immediately see it, but today, December 16, 2020, is a unique day. To be more specific, it’s Pythagorean Theorem Day (also called Right Triangle Day)—and the next one won’t be for another five years.

## How can you tell if it’s Pythagorean Theorem Day?

As evidenced by Pi Day, mathematicians are no strangers to celebrating wonderfully weird holidays.

We celebrate Pythagorean Theorem Day when the sum of the squares of the month (mm) and day (dd) in a date equals the square of the year (yy) in the date. The celebration got its name from (surprise, surprise) the Pythagorean theorem. In case you needed a refresher, the theorem says that in a right triangle (a triangle that has one 90-degree angle), the square of the hypotenuse (or its longest side, which is the side opposite the right angle) is equal to the sum of the square of the other two sides (which are the sides that form the right angle).

Take today, for example. Following the mm/dd/yy format, you’d write the date as:

12/16/20

If you follow the dd/mm/yy format, you’d write it as:

16/12/20

Either way, the sum of the squares of the month (12) and day (16) is equal to the year (20) squared.

12² + 16² =20²

This means that next year, December 16 won’t be Pythagorean Theorem Day (as the date would be 12/16/21 or 16/12/21). In fact, the last time we celebrated this oddball holiday was on August 15, 2017 (8/15/17 or 15/8/17, because 8² + 15² = 17²). And the next time we’ll celebrate it will be on July 24, 2025 (7/24/25 or 24/7/25, because 7² + 24² = 25²).

## Who formulated the Pythagorean Theorem?

The Pythagorean theorem is a well-known mathetical statement formulated by Pythagoras (c. 570 – c. 495 BCE), a Greek mathematician and philosopher. (Legend has it that formulating the theorem was such a “Eureka!” moment for Pythagoras, he offered an ox—or a hundred oxen, as some claim—as gratitude to the gods.)

Pythagoras’ theorem was just one of the many parts of his legacy as a pioneering thinker. His contributions span not only mathematics and philosophy, but also music, astronomy, and medicine. One could say that he was a jack-of-all-trades. He also established the Pythagorean School of Mathematics in Cortona, a Greek seaport in Southern Italy.

Although the theorem bears Pythagoras’ name, the principle behind it was discovered more than a thousand years before his birth. Evidence exists of the theorem’s rules being applied during the Old Babylonian period (which ended at around 1595 BCE). The Baudhayana Shulba Sutra of India, which dates back to 800 BCE, has a list of Pythagorean triples, as well as a variation of the theorem.

## The ‘right’ way to celebrate Right Triangle Day

We may not be acutely aware of it, but we live in a world filled with geometric shapes. As a result, there are many ways in which we can observe the Pythagorean theorem in action in our daily lives.

When you go out, look at structures around you that form 90-degree angles as they stand perpendicular to the street.

Similarly, observe how trees form right angles as they stand perpendicular to the ground.

Feeling creative? Work on a scale model that features right angles.

You can also take control of the kitchen and prepare food shaped like right triangles. (Or you can just *buy *food shaped like right triangles, too!)

Whether it’s in architecture, the arts, or even in the food that we eat, Pythagoras’ theorem manifests in more ways than one. And for some reason, it just feels so…right.—*by Carolyn Grace Tongco and **Mikael Angelo Francisco*

#### References

- https://www.mathletics.com/blog/news/pythagorean-theorem-day/
- https://www.3plearning.com/blog/pythagorean-theorem-day/
- http://jwilson.coe.uga.edu/emt669/student.folders/morris.stephanie/emt.669/essay.1/pythagorean.html
- https://www.timeanddate.com/holidays/fun/pythagorean-theorem-day
- https://stemlearning.in/pythagorean-theorem-real-life-application/