Can you guess — what is 1 second?

On this keyboard, each key plays a tone of a different duration.

How precise is your sense of time?

Press each key and try to estimate which tone lasts exactly 1 second.

Check your answer

Lift the panel below this text. Underneath, you will see the exact duration of the tone produced by each key.

What is one second, really?

The second is the basic unit of time — and perhaps the one that feels the most “natural,” because we think we can senseit.

But in science, the second is defined with extraordinary precision:
since 1967, one second has been the duration of 9,192,631,770 oscillations of the electromagnetic radiation corresponding to a transition between two energy levels of the caesium atom.

Before that, the second was based on the motion of the Earth around the Sun — but Earth does not move perfectly uniformly, and the length of a day changes by thousandths of a second!
This is why today, precise timekeeping is done by optical atomic clocks, which would take more than 10 billion yearsto drift by just one second.

Did you know?

• The human brain “updates” the world about 20 times per second — which is why films at 24 frames per second look smooth.