How long is a second?

Illustration of Droste effect on a black clock face with white hands and numbers.

​​There are 24 hours in a day, 60 minutes in an hour, and 60 seconds in a minute — so surely a second is just 1/(24 x 60 x 60), or 1/86400, of a day, right? Well, it turns out that defining time isn't that simple.

We're used to thinking of a second as a fixed increment of time, but this small unit has changed several times over the centuries.