It is now universally agreed that atomic clocks provide a much better measure of time thanĭoes the Earth's rotation, yet for human comfort, we would still like time to stay in sync with the Earth's rotation. A UT1 day is now a couple of milliseconds longer (on average) than is a TAI day thanks to the slowing of the The TAI second is based on the Earth's average rotation rate between 1750Īnd 1892. There are exactly 86400 TAI seconds in a TAI day. TAI (International Atomic Time), which you didn't ask about, measures time according to a number of atomic clocks. There are exactly 86400 UT1 seconds in a UT1 day. Scaled by a factor of (one mean solar day)/(one sidereal day), with small adjustments for polar motion. UT1 (Universal Time 1) measures the Earth's rotation with respect to the distant stars (quasars, nowadays), Pretty much the same is stated (much more clearly and eloquantly) in this physics.stackexchange answer (slightly edited):
UTC is maintained via leap seconds, such that DUT1 remains within the range:Įssentially, UTC is a compromise devised to satisfy the needs of two communities of users:
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. On the subject of the difference between UTC and UT1, Wikipedia has this to say: But note that the duration of a UTC second is always identical to the duration of a TAI second. 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. 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. 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. Given that the angular speed of the Earth's rotation is of uniform,Ĭonstant flow) causes UTC to diverge from the solar time (which is tied to physical realities and thus "dirtier"), it has to beĪdjusted once or twice a year with the addition or removal of a second. Unlike UT1) flows uniformly at a constant "rate" (driven by TAI). TAI flows uniformly by design but is unconnected to anyīy combining TAI and UT1 we get UTC (Coordinated Universal Time). Worldwide, and provides the exact speed for our clocks to tick. TAI time (International Atomic Time) is a time scale that combines the output of some 400 highly precise atomic clocks it is actually measuring the sidereal day and subsequently adjusted with a mathematical formula to arrive at the solar day.
This is simply sayingĪgain (with different words) that UT1 does not flow uniformly.Īlso note that even though UT1 tracks the mean solar day, it is actually measured using distant X-ray sources, Minute or second (understood as fractions of a UT1 day) are not the same either. It therefore also follows that the exact duration of a UT1 hour, Given the various subtleties and physical world complexities governing the Earth's rotation, the exact duration ofĪ UT1 day is not always the same. This is the essence of astronomical, or solar time. Meridian, when the Sun is at its zenith, it's 12 o' clock noon, by definition. Relative to the observer as such it is influenced by vagaries in the Earth's rotation and does not flow uniformly. UT1 time (Universal Time) is also known as astronomical time or solar time and is determined by the position of the Sun UTC is really a crude and fundamentally flawed attempt to compromise Only TAI would make sense to an alien civilization.īetween UT1 and UTC, UT1 is the most anthropomorphic of the two. Only TAI (atomic time) is a true time scale and measures Physics time.
The same can be said about all civil notions of time in general (as customarily used throughout history or enshrined in legal time definitions) UT1 and UTC are not really time scales, they do not really denote time, they just measure the Earth's angleĪnd keep tally of its revolutions and rotations around the Sun. The below notes capture my understanding of the different nuances in the definition of UT1, TAI and UTC times.
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