UTC vs. UT1 time and other nuances (2020)

TAI is also a kind of approximation, since time elapses differently depending on gravitational pull, due to relativistic effects. It is defined as the physical time elapsing on a geodesic, an imaginary surface covering the earth at roughly sea level with equal gravitational pull at all points.<p>In reality, physical time elapses at slightly different “rate” depending on where you’re located on the surface of the earth (or possibly not on the surface of the earth), also due to nonuniformities in the earth’s mantle affecting the gravitational field.<p>TAI is taken as the average between the 400-or-so contributing atomic clocks, adjusted for their relative height above sea level, and possibly other factors, and taking into account the signal propagation time between the clocks.<p>Compared to the time in the reference frame of the sun, for example, (which may be taken as a solar-system wide time-keeping standard) TAI wiggles around that solar time according to earth’s yearly cycle around the sun.<p>Of course, those variations are much smaller than DUT1 (at least close to earth’s reference frame).

&gt; Essentially, UTC is a compromise devised to satisfy the needs of two communities of users:* Astronomers and navigators, * Physicists and engineers<p>I think author is wrong about astronomers, for whom UTC is an unwanted complication. Astronomers use sidereal time, which is unrelated to ther apparent motion of the sun. For short intervals, physicists and engineers may as well use atomic time.<p>The WP article on UTC has a section titled &quot;Rationale&quot;, but doesn&#x27;t explain what problems&#x2F;compromises UTC was supposed to address. It&#x27;s worthy of note, however, that of the three bodies involved in the first version of UTC, two were national naval observatories.

How in the world did astronomers &amp; their software developers manage to push off onto the rest of the software world (and society at large) the atrocious concept of leap seconds.

After reading the comments, it seems to me a theme is that everyone thinks somebody else is mistaken or misunderstanding the subject. This is not a complaint, but an observation on how difficult and slippery this subject is! I, at least, hardly understand it at all.

The author argues that only TAI would make sense to an alien civilization, but I don&#x27;t think that&#x27;s true at all. I think they miss the fact that it also is only defined within the reference frame of the Earth and also that it depends on many technically subtle aspects.<p>Especially if one only wants to communicate times between the civilizations, such as meet here at 13:00 in exactly three years from now. This is much easier to communicate based on rotation angles of the Earth than on a atomic time scale that would need to be transferred very precisely to be useful at all.

Unfortunately this continues the perception that UTC&#x2F;GMT is a time zone. It isn&#x27;t. A time zone is a geographical area where all the clocks show the same time, but the time that they show may vary - for instance, despite Microsoft&#x27;s insistence, the UK is not in the &quot;UTC timezone&quot;.<p>This may seem pedantic, and <i>most</i> of the time it&#x27;s not important to understand the difference. But when this misunderstanding bites, it will take your leg off at the knee.

Even more confusing, the time printed by our computers may appear to be UTC, but is probably POSIX (or Unix) time. It is very close to UTC but ignores leap seconds. <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Unix_time" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Unix_time</a><p>During a positive leap second, the POSIX second repeats itself (an alternative mental model could be that the given POSIX-second lasts 2 UTC-seconds). During a negative leap second, the given POSIX-second disappears (or alternatively, the given POSIX-second lasts 0 UTC-seconds).<p>On my Ubuntu box, the `date +%s` command prints the number of POSIX seconds since 1970-01-01, not the number of UTC seconds since 1970-01-01. To get the number of UTC seconds, we must add the number of intervening leap seconds since 1970-01-01.<p>I get a headache every time I have to think about this.