Comment by DiogenesKynikos | Hacker News Reader

Comment by DiogenesKynikos | Hacker News Reader

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DiogenesKynikos

Small changes in the rotation rate of the Earth (primarily due to seismic activity and the tidal interaction with the Moon, I think) are the reason why sidereal (or Solar) time diverges from time as kept by an atomic clock (the atomic clock has errors too, but they're orders of magnitude smaller).

This is how you get different definitions of "Universal Time":

* UT1 = sidereal time, based on measurements of the angles of distant quasars.

* UTC = atomic clock time, with offsets added in whenever it gets too far (about 1 second) from UT1.

One final fun thing about UTC. Why is it UTC?

* Coordinated Universal Time = CUT ?

* Universal Coordinated Time = UCT ?

* Temps Universel Coordonné = TUC ?

UTC was chosen precisely because it doesn't correspond to either the English or French acronym.

> UTC was chosen precisely because it doesn't correspond to either the English or French acronym.

That's fascinating, I assumed it was a French acronym. "Universale Temps Coordinaté" (I do not speak French, but I have seen Kentucky Fried Chicken signs in Québec!) :)

Regardless of the political process of selection, I am glad things worked out the way they did. The other acronym possibilities are really unpleasant in English.

A few comments:

* Sidereal time is different from solar time; sidereal time measures the Earth's rotation relative to the stars, while solar time measures it relative to the Sun, which moves relative to the stars. Thus, UT1 and sidereal time have different rates.

* Wikipedia's page on day length fluctuations (https://en.wikipedia.org/wiki/Day_length_fluctuations) has a discussion of the influences involved. It also has a handy graph, from which we can see that the average day length is around 2 milliseconds shorter than in the 1970s (hence why we had a leap second every year from 1973-1979, but the last leap second was over four years ago).

* There's also UT2 and UT0. UT2 has a corrections for seasonal variations, while UT0 is derived from raw observations, without correcting for polar motion like UT1. These are rarely used.

I lumped Solar and sidereal time together because they're both essentially dependent on the Earth's rotation. Their rates are different by ~1 day/year, of course.

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