Transcript:

So, how do we measure time accurately? Today, we can do it with cesium clocks, which are atomic clocks. And those clocks are accurate to one second over 3,000 years. That's pretty accurate. And this accuracy, it can maintain over 6 million years. So, it is the gold standard for time measurement.

Except, now that we can measure this time precisely, we have to account for the imperfections of passage of time on Earth. So why is there imperfections? Because the axis of Earth is tilted, and as it is spinning, there is a wobble. So the North Pole moves by like 7 inches, which doesn't seem a lot, but it's significant.

That wobble—what it does is, it changes the duration of a day by about a millisecond. That is, 10^-3 seconds. And this seems to be a small number, but accumulates over time. Which means every once so often, a leap second has to be added to keep the atomic clock and the passage of time or the day in sync.

And the way it is done is all the data from the atomic clocks is brought to one place. And then the data is analyzed, and then the corrected time is sent back. Except, this takes one month. Which means the correction of the measurement happens one month after it has been measured.