https://mperdikeas.github.io/utc-vs-ut1-time.html
UTC vs. UT1 time (and other nuances)
The below notes capture my understanding of the different nuances
in the definition of UT1, TAI and UTC times.
TL;DR
UT1 and UTC are not really time scales, they do not really denote
time, they just measure the Earth's angle and keep tally of its
revolutions and rotations around the Sun. The same can be said about
all civil notions of time in general (as customarily used throughout
history or enshrined in legal time definitions) Only TAI (atomic
time) is a true time scale and measures Physics time. Only TAI would
make sense to an alien civilization. Between UT1 and UTC, UT1 is the
most anthropomorphic of the two. UTC is really a crude and
fundamentally flawed attempt to compromise between UT1 and TAI.
Solar time
UT1 time (Universal Time) is also known as astronomical time or solar
time and is determined by the position of the Sun relative to the
observer; as such it is influenced by vagaries in the Earth's
rotation and does not flow uniformly. On a given meridian, when the
Sun is at its zenith, it's 12 o' clock noon, by definition. This is
the essence of astronomical, or solar time.
Given the various subtleties and physical world complexities
governing the Earth's rotation, the exact duration of a UT1 day is
not always the same. It therefore also follows that the exact
duration of a UT1 hour, minute or second (understood as fractions of
a UT1 day) are not the same either. This is simply saying again (with
different words) that UT1 does not flow uniformly.
Also note that even though UT1 tracks the mean solar day, it is
actually measured using distant X-ray sources, i.e. it is actually
measuring the sidereal day and subsequently adjusted with a
mathematical formula to arrive at the solar day.
Atomic time
TAI time (International Atomic Time) is a time scale that combines
the output of some 400 highly precise atomic clocks worldwide, and
provides the exact speed for our clocks to tick. TAI flows uniformly
by design but is unconnected to any astronomical observations.
Coordinated time
By combining TAI and UT1 we get UTC (Coordinated Universal Time).
UTC, like TAI (and unlike UT1) flows uniformly at a constant "rate"
(driven by TAI). Given that this restriction (i.e. of uniform,
constant flow) causes UTC to diverge from the solar time (which is
tied to physical realities and thus "dirtier"), it has to be adjusted
once or twice a year with the addition or removal of a second. Given
that the angular speed of the Earth's rotation is slowing down over
time, this means that TAI is getting slowly ahead of UT1 which is
precisely why so far seconds have always been added, not removed from
UTC. Note that this may sound counter-intuitive but the addition of a
leap second is actually slowing down UTC to bring it more inline with
UT1. Leap seconds are always added as the last second of a day so
once or twice a year a UTC day had 86401 UTC seconds instead of
86400. But note that the duration of a UTC second is always identical
to the duration of a TAI second.
On the subject of the difference between UTC and UT1, Wikipedia has
this to say:
The time correction DUT1 (sometimes also written DUT) is the
difference between Universal Time (UT1), which is defined by
Earth's rotation, and Coordinated Universal Time (UTC), which is
defined by a network of precision atomic clocks.
DUT1 = UT1 - UTC
UTC is maintained via leap seconds, such that DUT1 remains within
the range:
-0.9 s < DUT1 < +0.9 s.
Essentially, UTC is a compromise devised to satisfy the needs of two
communities of users:
* Astronomers and navigators who like a time connected with the
angle of the Earth's rotation in space
* Physicists and engineers who prefer a perfectly uniform time
scale and ascribe no value to maintaining a link between the
value of time and astronomical phenomena
An alternative (but equivalent) rendition
Pretty much the same is stated (much more clearly and eloquantly) in
this physics.stackexchange answer (slightly edited):
UT1 (Universal Time 1) measures the Earth's rotation with respect
to the distant stars (quasars, nowadays), scaled by a factor of
(one mean solar day)/(one sidereal day), with small adjustments
for polar motion. There are exactly 86400 UT1 seconds in a UT1
day.
TAI (International Atomic Time), which you didn't ask about,
measures time according to a number of atomic clocks. There are
exactly 86400 TAI seconds in a TAI day. The TAI second is based
on the Earth's average rotation rate between 1750 and 1892. A UT1
day is now a couple of milliseconds longer (on average) than is a
TAI day thanks to the slowing of the Earth's rotation rate.
This creates a fundamental problem. It is now universally agreed
that atomic clocks provide a much better measure of time than
does the Earth's rotation, yet for human comfort, we would still
like time to stay in sync with the Earth's rotation. How to
accomplish this?
GMT (an archaic term that is deprecated except in Great Britain)
has now become another just another name for the time zone
UTC+0h. Prior to 1972, GMT was the de facto standard that
attempted to keep universal time and atomic time in sync. The old
GMT adjusted the length of the last minute of every day to keep
the two disparate concepts of time in sync. The BBC incorporated
these daily adjustments in their broadcasts of the "six pips".
The US and Canada did much the same with their radio-based time
broadcasts. The daily adjustments used in GMT were becoming ever
more problematic with an ever more connected and ever more
precise world. These problems motivated the replacement of GMT
with UTC.
UTC (Coordinated Universal Time) is the modern successor to GMT.
A UTC second is by definition always exactly equal to one TAI
second, but like the old GMT, a UTC day is not necessarily 86400
seconds long. The difference between the old GMT and UTC is that
in lieu of the small daily adjustments used in the now deprecated
GMT, the adjustments to UTC are infrequent and are always exactly
one TAI second. These are leap seconds. The predictability and
the current close match between UT1 and TAI means that leap
seconds can be announced well in advance, only have to occur on
June 30 or December 31, but can still keep UTC and UT1 within 0.8
seconds of one another. (Aside: This will not remain the case in
the not too distant future.)
The rationale for switching to intermittent leap seconds was that
doing so would keep time uniformly except for those intermittent
leap second boundaries, and the problem introduced by those leap
second boundaries could be easily circumvented due to the
announcement of those jumps well in advance of when they would
occur. This has turned out to not be the case; a number of
different computer systems (e.g., Microsoft, Unix, and MacOS)
have problems with leap seconds. There are moves afoot to get rid
of leap seconds.
An insightful perspective
An insightful comment is found in the following paper:
Precision time and the rotation of the Earth, Dennis D. McCarthy
Transits of Venus: New Views of the Solar System and Galaxy,
Proceedings IAU Colloquium No. 196, 2004; c 2004 International
Astronomical Union
... where the author says (in the conclusions section):
[we should] acknowledge that UT1 is essentially a measure of the
Earth's rotation angle to be determined astronomically, and
improve the accessibility of this information to users by
electronic means.
... and also in the discussion section where the author comments in
response to a question that:
Dennis McCarthy: Well, I didn't have a chance to say it, but I
think we need to get used to the fact that UT1 - UTC is just a
way of expressing an angle, the Earth's rotation angle. It's no
longer a time, UT1 is not really a time, it's a way to express
the Earth's rotation angle and it should not be thought of as a
real time scale.
On the hairiness of civilian time
... or "can somebody please blow my brains out"
See this horror story