How do we actually know what the time is? Is there some “master clock” that all time zones are based on? And if so, what does THAT clock refer to?

Question

EDIT: I believe I have kicked a hornet’s nest. Did not expect this to blow up! But I am still looking for the “ur time”. the basis for it all. Like, maybe the big bang, or something.

In: 5399

Answer 1

Answer: We used to base time on astronomical events. A day used to be the amount of time that it takes for the sun to come back to the exact same position in the sky.

So we would take that length of time, divide it in 24 hours, divide each hour in 60 minutes and divide each minute into 60 seconds. In fact, the definition of a second used to be 1⁄86400th of a day.

24 × 60 × 60 = 86400

In the mean time, we kept creating ever more accurate clocks. The mechanical pendulum was replaced by a diapason, then by a quartz crystal, then by caesium 133.

So now, instead of measuring down from a day, we measure up from a caesium atom.

The catch is that the Earth’s rotation is not consistent. The 2011 earthquake in Japan [was strong enough to delay the day by 1.8 microsecond](https://www.space.com/11115-japan-earthquake-shortened-earth-days.html).

So on the following New Year’s Eve, we corrected the 1.8 microseconds during the twelve strokes of midnight.

So we not have clocks that are so stable that the Earth gets out of sync with them.