If you have a mechanical watch or clock, you can hear it ticking off the seconds — 86,400 of them every day.

The second has been the most basic unit of time for millennia — since long before it could even be measured. It originated in ancient Babylon. Astronomers used the number 60 as the base for keeping time and for measuring angles in the sky. It’s divisible by many other numbers, so it was easy to work with.

In this system, each smaller division was one-sixtieth the size of the larger one. So one-sixtieth of an hour became a minute. And one-sixtieth of a minute became a “second minute” — indicating that it was the second position in the list of time.

No one could really measure so small a unit of time, though, until the invention of mechanical clocks in the 16th and 17th centuries.

Not long after that, scientists realized they needed a precise definition for a second, which is a key unit for many measurements and equations. Originally, their definition was based on Earth’s rotation. But that rate isn’t constant. Hurricanes, earthquakes, and other disturbances can change it by a tiny bit.

So in 1967, they adopted a new definition. It uses the vibrations of a cesium atom that’s zapped with a laser. It’s measured with atomic clocks. So one second is the amount of time it takes an atom to “vibrate” nine billion, 192 million, 631 thousand, 770 times — the length of a single “tick” on the clock.

Script by Damond Benningfield