We all know that a single Earth day technically contains 86400 seconds. This has been known since the year 1000 when Muslim scholars subdivided the mean solar day into 24 hours and the corresponding minutes and seconds. But, as soon as our instruments improved, we discovered the Earth’s rotation period varied irregularly, at something like a fraction of a second each year. We will see how that is possible later in this video, but for now…imagine if our clocks were off by just a few seconds.

Over time, those few seconds can add up and cause significant discrepancies in timekeeping. That’s why scientists needed to define the concept of a “leap second,” to ensure that our clocks are as precise as possible.
To put it into perspective, a leap second is like a leap year, but on a much smaller scale. A leap year is added every four years to account for the fact that the Earth’s orbit around the sun isn’t exactly 365 days long. So just like how we add an extra day in February every four years, we also add an extra second to our clocks as needed to account for the minute changes in the Earth’s rotation.
Some people might think that being born on a leap year is unlucky since they only get to celebrate their actual birthday once every four years. But let’s be honest, they get to age four times slower than the rest of us, so maybe it’s not so bad after all!
But all jokes aside, the gradual slowdown of the Earth’s rotation is a real phenomenon that has important implications for our planet’s climate, geography, and even timekeeping. So let’s dive into the science behind it all, and explore what’s causing our planet to change its rotation velocity over time.