324 lines
21 KiB
Plaintext
324 lines
21 KiB
Plaintext
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C I R C U M L U N A R
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T R A N S M I S S I O N S
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Issue One May 2021
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==============[ What's the Deal with Leap Seconds? ]===============
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================[ A Brief Overview of Timescales ]=================
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by solderpunk
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ASTRONOMICAL SECONDS
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Why is a second as didn't, so you're "why is a day as long
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long as it is, and not stuck reading this, as it is?" - one day
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a little shorter or a instead: a brief, is the time it takes
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little longer? This incomplete, possibly the Earth to complete
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is a seemingly simple slightly inaccurate a single revolution
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question which leads overview based on my about its axis. And
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down a deep and own characteristically since there are 60
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delightfully twisted obsessive reading on seconds in a minute,
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rabbit hole. It's the topic over the 60 minutes in an hour
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something that I wish past week or so. and 24 hours in a day,
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Neal Stephenson had a second is simply one
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written an epically For most of the time 86,400th of the time
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long, inexplicably that the concept of it takes the Earth to
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compelling 1990s Wired the second has been rotate once. Or, if
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article about, in the around, its length has you like, a second is
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spirit of his "Mother been defined the time it takes for
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Earth, Mother Board" implicitly by that of the Earth to rotate
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or "In the Kingdom of the day. Everybody one 240th of a degree,
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Mao Bell". But he knows the answer to out of the full 360.
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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End of story, right? observatories tracking enough to notice
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the movement of this). In fact, the
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Well, no. This is a distant radio sources Earth's rotation is
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perfectly sensible way across the sky as the slowing down. Very
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to define time - for Earth rotates. This slowly, of course.
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some applications, is easier than making Every century, a
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it's the best way to precision measurements complete rotation
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do it. This of the sun, but is takes about 2
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astronomically still measuring the milliseconds longer
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defined time scale is exact same thing. than it used to. The
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still in use today in rate of slowing down
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certain contexts. The EARTH IS A NICE PLACE is not steady. Some
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official name of its TO LIVE, BUT IT'S NOT years the change is
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modern incarnation is THE BEST CLOCK more and other years
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Universal Time, or UT it's less. In fact,
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(technically, there The problem with an even though the
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are a few subtly astronomical overall trend is one
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different variants, definition of the of slowing down, some
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denoted UT0, UT1 and second is this: the years the rotation
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UT2, but we'll gloss Earth doesn't actually actually speeds up.
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over that here). The rotate at a perfectly The dynamics of the
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official determination constant rate (it process are
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of UT nowadays is wasn't until the 19th complicated, and we
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based mostly on century that we could can't make accurate
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measurements made at build clocks accurate long term forecasts.
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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Gravitational small, random was going to be late
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interaction between fluctuations in their to lunch because of
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the Earth and the moon duration over long the Earth's unsteady
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is the primary driver, time spans. If you rotation. But by the
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but the movement of define the second by 20th century, sci-
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tectonic plates and looking into the entific and techno-
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friction between skies, no two seconds logical progress
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Earth's surface and are exactly the same. meant these tiny
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its atmosphere and fluctuations started
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oceans have their say, That's a pretty to matter, as we
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too. The Indian Ocean inconvenient property began measuring
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Earthquake in 2004 was for the official natural phenomena and
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powerful enough to definition of a building machines
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shorten the length of fundamentally which operated on
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a day by 2.68 important unit like very small time
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microseconds. There the second to have. scales. A 10 mega-
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are even periodic For most of the time hertz radio
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variations in the rate this definition was oscillator, for
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of rotation that we used, the fluctuations example, has a period
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just don't understand were smaller than we of 0.0000001 seconds
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the cause of yet. But could reliably --only 100 nano-
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the take home message measure. Certainly, seconds! Gigahertz
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is that, whatever the they weren't enough to radiation, which is
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causes, astronomical have an impact on important in radio
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seconds actually have everyday life. Nobody astronomy and was
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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used for communi- Fortunately, in the The new atomic second
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cations and radar 1950s, atomic clocks was defined such that
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during WWII decades were invented which it had the same
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before it came to kept time better than length as the
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underpin modern any previous astronomical second
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technology like GPS, mechanism. I'll gloss in use before it, as
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WiFi, and mobile data right over the far as measurements
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networks, has periods details, but suffice at the time could
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measured in it to say, we came up tell, but it had the
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*picoseconds*. Even with a new way to added bonus that the
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very, very small define the second length of the second
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variations in the which involved was then fixed and
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length of a second are measuring the unchanging. Caesium
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enough to make the properties of caesium atoms at a given
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measured frequency of atoms instead of temperature "vibrate"
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radio waves change, looking at things (very loosely
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even if the *actual* moving through the speaking) at a
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frequency is fixed. sky. In 1967, the frequency which, as
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Modern technological relatively young far as we can tell,
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society simply International System is completely and
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couldn't be built (or SI, for the French perfectly stable, and
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using a wobbly clock "Système which can be measured
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like the Earth. International") of very accurately in a
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units redefined the sufficiently advanced
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ATOMIC SECONDS second on this basis. laboratory.
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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With the arrival of until now is a little other hand, is
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atomic seconds, a new less than 40 seconds, fundamentally
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time scale was also but it will continue divorced from it.
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defined: International to grow, without Thousands of years in
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Atomic Time (or TAI, limit. And while the the future, there
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for the French "Temps perfectly uniform will come a day when,
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Atomique Inter- seconds of TAI make it according to TAI, the
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national"). At mid- the perfect tool for sun rises in
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night on January 1st some tasks, this drift Greenwich at
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in 1958, TAI and UT apart from UT makes it midnight.
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were perfectly problematic for
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synchronised. Ever others. If you go This isn't just an
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since then, they have outside at noon UT in abstract concern for
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slowly but surely Greenwich, England (or the distant future.
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drifted apart. The anywhere else at 0 In the late '50s when
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seconds of TAI are of degrees longitude), TAI was defined, it
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perfectly unchanging the sun will *always* was still common for
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length (as measured by be high in the sky. ships at sea to
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averaging hundreds of This is true today and figure out where they
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atomic clocks all over it will be true in a were by using a
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the world), but the thousand years, sextant to record the
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seconds of UT Because UT is position of the sun
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fluctuate with the fundamentally linked above the horizon at
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Earth's rotation. The to the Earth's a certain time and
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accumulated drift up rotation. TAI, on the consulting a printed
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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table of conversions. increasingly less International de
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For this purpose, accurate as TAI l'Heure", but today
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ships carried the most drifted further out of the torch has been
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accurate clocks they synch with the Earth passed to a
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could afford, and and the stars. This combination of the
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compared them meant that the "new International Bureau
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regularly against true and improved" TAI time of Weights and
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UT time using time scale wasn't actually Measures, or BIPM,
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signals broadcast by an improvement for for the French
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radio stations all everybody. "Bureau International
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over the world. des Poids et Mesures"
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Celestial navigation COORDINATING CHAOS and the International
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works very well when Instead of Earth Rotation
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using a timescale broadcasting two Service, who have the
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which is tightly different time signals gall to abbreviate
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linked to Earth's for different the *English* version
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rotation, and hence purposes, which could of their name and go
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the position of things easily lead to by IERS) defined yet
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in the sky. But if the confusion, on January another time scale,
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radio time signals 1st in 1960 the powers in an attempt to
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switched to that be (back then achieve the best of
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broadcasting TAI that was the both worlds and make
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instead of UT, International Time everybody happy.
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celestial navigation Bureau, or BIH, for Enter Coordinated
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would become the French "Bureau Universal Time, or
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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UTC - at last, allowing scientists technical
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something normal and engineers around compromises, using
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people have heard of! the world to ugly hacks. It cannot
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calibrate their be any other way, as
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The abbreviation UTC instruments and UTC is a stubborn
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is a strange reliably measure time attempt to reconcile
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compromise between intervals and two desirable but
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the English frequencies very fundamentally
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abbreviation CUT and precisely. But incompatible
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the French whereas TAI is properties of a
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abbreviation TUC (for destined to drift timescale: perfectly
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"Temps Universel ever further away regular seconds, and
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Coordonné"). This is from UT, to the synchronisation with
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somewhat fitting, chagrin of sailors a spinning globe
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because UTC itself is and astronomers, UTC whose rate of
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a strange compromise is kept synchronised rotation is
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time scale between UT closely enough with unpredictably
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and TAI. Like TAI, UT that it allows irregular.
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UTC is an atomic time seafarers to perform
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scale. Every second celestial navigation The precise nature of
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of UTC is exactly as with sufficient the ugly hack
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long as any other, accuracy for safe underlying UTC has
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using the SI standard ocean passage. This changed somewhat
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second based on synchronisation is since it was first
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caesium atoms, achieved, like all defined, but for
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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almost 50 years now, not absolutely fixed rather than removals.
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starting in 1972, the at 86,400 by They don't happen on
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ugly hack of choice definition. There a regular,
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has been the leap almost always *are* predictable basis,
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second. The way it 86,400 seconds in a like leap years
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works is this. The UTC day, but 86,401 (which are an
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difference between and 86,399 are also adjustment for the
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UTC and UT - a allowed when fact that the time it
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quantity denoted DUT necessary to keep the takes the Earth to
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- is carefully time scale locked to orbit the sun once,
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monitored. Any time the movement of the defining a year, is
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it looks like that sun across the sky. not perfectly
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difference is on divisible by the time
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track to exceed 0.9 So far, there have it takes the Earth to
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seconds, in either been 27 leap seconds rotate once, defining
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direction, UTC is defined, although UTC a day). Because the
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kicked back into and ATI are today Earth's rate of
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alignment by either exactly 37 seconds rotation fluctuates
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inserting or removing apart - the other 10 randomly, sometimes
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a single second on seconds come from slowing down and
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one particular day. hacks applied before sometimes speeding
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This makes UTC the leap seconds were up, astronomers need
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*only* time scale established in 1972. to be actively on the
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where the number of All of them to date lookout for excessive
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seconds in a day is have been insertions values of DUT. When
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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it's decided a leap precise time technology means that
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second is needed - synchronisation celestial navigation
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it's the IERS who should be caught by at sea is now rarely
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makes that call - surprise when a leap a matter of life or
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they are announced at second rolls around. death (although some
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least six months in In practice, it's not sailors still
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advance. They're always so simple. appreciate the
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allowed to occur on relative simplicity
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either June 30th or INCREASING of the technology it
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December 31st, and IMPLEMENTATION BURDEN relies on), removing
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are inserted or AND AN UNCERTAIN some of the argument
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deleted at midnight FUTURE for making sure UTC
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UTC (which is the stays in lock step
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middle of the day in Leap seconds have with the Earth's
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some time zones, of always had their rotation. At the same
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course) on those critics, but at the time, the internet
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days. At the time of time they were has come along: a
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writing, the last adopted, their massive network of
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leap second happened benefits arguably computers talking to
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on December 31st, balanced their each other, with the
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2016. In principle, associated hassle. 50 frequent need for
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six months is enough years later, this hack activity on one to be
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advance warning that is starting to show synchronised with
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nobody doing anything its age. The advent of activity on another
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which depends on cheap and reliable GPS (hence tools like the
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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Network Time Protocol, added to software as Some systems, of
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NTP). Computer an afterthought - if course, forget to do
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programmers *hate* they are added at all. anything at all.
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leap seconds, for the Some systems represent Because all the leap
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same reason they hate the extra second using seconds to date have
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Daylight Saving Time: the timestamp been insertions
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they complicate time 23:59:60, but others rather than removals,
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calculations (you instead repeat the it's a safe bet that
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can't accurately timestamp 23:59:59 there's plenty of
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calculate the number twice (since some software out there
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of seconds between two software will fail to which has worked
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UTC timestamps without parse a timestamp correctly so far but
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consulting a table of ending in :60). Other will fail the first
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when previous leap systems "smear" the time a second is
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seconds were inserted) leap second out over removed. And the
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and are a frequent longer time periods, Earth's rotation is
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cause of confusion and like 24 hours, to going through a bit
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errors, when one avoid problems of a fast phase right
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system implements them associated with sudden now, so the first
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differently from discontinuities. This negative leap second
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another its trying to just leads to a whole might be looming on
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interoperate with. day of small, slowly the horizon.
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varying errors
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Affordances for leap compared to The software
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seconds are often non-smearing systems. interoperability
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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situation at the time option. It's one thing cause are only likely
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of a leap second is to shut down the New to get worse. The
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bad enough that York Stock Exchange atomic definition of
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several major stock for an hour, but Air second likely isn't
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exchanges simply Traffic Control has to going away any time
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agreed to voluntarily stay up 24/7. It's no soon, though, and
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shut down for an hour surprise that that means that
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around midnight UTC in increasingly many getting rid of leap
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2016, rather than risk voices in the tech seconds entirely
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problems by continuing industry are calling means abandoning the
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to trade during the for leap seconds to be millennia old notion
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leap second. Given abolished. Plenty of that the way we
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that a number of major people are very represent time is
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web services, unhappy with that intimately linked
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including Amazon, idea, of course, not with the natural
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Instagram, Netflix and to mention there's no cycle of night and
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Twitter, experienced consensus on what to day. Assuming we're
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outages around this do instead. not willing to do
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time, this was that, there are only
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probably not a bad It's far from clear two alternatives:
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idea. Of course, what the future holds coming up with a new
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simply shutting time for the leap second. ugly hack which is
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critical services off As software continues somehow less
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for every leap second to eat the world, the problematic, or
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isn't always an headaches leap seconds giving up on a "one
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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size fits all" time recommend the article time scales like
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scale. "The leap second: its Ephemeris Time (ET),
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history and possible GPS Time (GPST) and
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BUT WAIT, future", which you can Terrestrial Time
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THERE'S MORE! easily find on the web (TT), and discover
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(full citation below), that the SI system of
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If you think this along with, as always, units defined the
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story has been following Wikipedia second based on
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needlessly fiddly and links wherever they something other than
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complicated, rest take you. Along the Earth's rotation when
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assured I have skipped way you can learn the it was established in
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over a tonne of difference between 1960, seven years
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details. If you're UT0, UT1 and UT2, meet before the caesium
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actually interested to other exciting definition was
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learn more, I highly astronomic and atomic adopted.
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- - -[ REFERENCES ]- - -
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Nelson, R., McCarthy, D., Malys, S., Levine, J.M., Guinot, B.,
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Fliegel, H., Beard, R., & Bartholomew, T. (2001). The Leap Second
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- Its History and Possible Future. Metrologia, 38, 509-529.
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"Time-nuttery 101" by Ole Petter Ronningen
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https://efos3.com/TimeNut.html
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"UTC might be redefined without Leap Seconds" by Steve Allen
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https://ucolick.org/~sla/leapsecs/timescales.html
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