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About Time: Is the June 30th Leap Second the Last?
universetoday.com ^
| on June 23, 2015
| David Dickinson
Posted on 06/23/2015 12:59:23 PM PDT by BenLurkin
Chances are, youll soon be hearing that were tacking on an extra second to the very end of June 30th, though the reason why is a bit more complex...
The whole affair was addressed in 1956 by the International Committee for Weights and Measures, which defined what was known as the ephemeris, or astronomical second as a fraction1/31,556,925.9747th to be preciseof the tropical year set at noon on January 1st 1900.
Now, this decision relied on measurements contained in Simon Newcombs 1895 book Tables of the Sun to describe the motion of the Earth. Extrapolating back, a day was exactly 86,400 modern seconds long
in 1820.
In the intervening 195 years, the modern day is now about an extra 1/500th (86,400.002) of an SI second long. In turn, the SI second was defined in 1967 as:
The duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the Cesium-133 atom.
Now, physicists love to have an SI definition that isnt reliant on an artifact. In fact, the pesky holdout known as the kilogram is the last of the seven SI base units that is based on an object and not a constant that anyone can measure in a lab worldwide. Simply locking a second at 1/86,400th of a mean solar day would mean that the second itself was slowly lengthening, creating its own can of worms
So the leap second came to be, as a compromise between UT1 (Astronomical observed time) and UTC (Coordinated Universal Time), which defines a day as being comprised of 86,400 SI seconds. These days, the United States Naval Observatory utilizes observations which include quasars, GPS satellites and laser ranging experiments left on the Moon by Apollo astronauts to measure UT1.
(Excerpt) Read more at universetoday.com ...
TOPICS: Science
KEYWORDS:
1
posted on
06/23/2015 12:59:23 PM PDT
by
BenLurkin
To: BenLurkin
2
posted on
06/23/2015 1:00:34 PM PDT
by
zeugma
(The best defense against a bad guy with a gun is a good guy with a gun)
To: BenLurkin
Gadzooks. 11:59:60 is a real second
3
posted on
06/23/2015 1:06:16 PM PDT
by
BigEdLB
(They need to target the 'Ministry of Virtue' which has nothing to do with virtue.)
To: BenLurkin
Thanks for posting this! I’ve been very worried that I might miss this leap second! That would be a tragedy of enormous proportion.
4
posted on
06/23/2015 1:07:02 PM PDT
by
Fiddlstix
(Warning! This Is A Subliminal Tagline! Read it at your own risk!(Presented by TagLines R US))
To: BenLurkin
Great, now I have to reset my digital watch............
5
posted on
06/23/2015 1:07:50 PM PDT
by
Hot Tabasco
(War IS the answer! Peace activists never liberated anything or anyone....)
To: BenLurkin
Gps users need to be aware.
6
posted on
06/23/2015 1:10:05 PM PDT
by
enraged
To: All
Energy may not be created or destroyed (Issac Newton). All of this wind energy is resulting in the earth slowing up (the energy has to come from somewhere...), so we will be doing a lot more of this.
7
posted on
06/23/2015 1:12:09 PM PDT
by
bennowens
To: BenLurkin
The difference between UTC, GPS, Loran-C, and TAI time:
leapsecond.com
8
posted on
06/23/2015 1:21:52 PM PDT
by
Yo-Yo
(Is the /sarc tag really necessary?)
To: BenLurkin
To: BenLurkin
Biden is in charge of measuring the duration of a second?
10
posted on
06/23/2015 1:31:27 PM PDT
by
Paladin2
(Ive given up on aphostrophys and spell chek on my current device...)
To: Hot Tabasco
Who wears watches any more?.............................
11
posted on
06/23/2015 1:55:47 PM PDT
by
Red Badger
(Man builds a ship in a bottle. God builds a universe in the palm of His hand.............)
To: BenLurkin
Now I can get my work done.
To: BenLurkin
13
posted on
06/23/2015 2:55:34 PM PDT
by
broken_arrow1
(I regret that I have but one life to give for my country - Nathan Hale "Patriot")
To: BenLurkin
As I was walking down the street one day
A man came up to me and asked me what the time was that was
On my watch, yeah
And I said,
Does anybody really know what time it is
I don’t
Does anybody really care
Care
If so I can’t imagine why
About time
We’ve all got time enough to cry
Oh no, no...
14
posted on
06/23/2015 3:49:20 PM PDT
by
outofsalt
( If history teaches us anything it's that history rarely teaches us anything.)
To: enraged
Gps users need to be aware. Nope. GPS time does not include leap seconds.
If you compare UTC time and GPS time, you'll see that GPS is 20_ seconds behind.
15
posted on
06/23/2015 4:08:08 PM PDT
by
zeugma
(The best defense against a bad guy with a gun is a good guy with a gun)
To: zeugma
Funny. Every manufacturer of equipment that we use sent me info on how to deal with it. Sub centimeter accuracy survey grade gps. Umm yes it does matter.
16
posted on
06/23/2015 7:22:56 PM PDT
by
enraged
To: enraged
Funny. Every manufacturer of equipment that we use sent me info on how to deal with it. Sub centimeter accuracy survey grade gps. Umm yes it does matter. Only if you have two different times that you are measuring by. GPS does not add leap seconds. So after the next leap second is added, you'll have to add another second to GPS time to match whatever time system you are using.
Here's a graphic of the time as of a few minutes ago (for me)
- Local time is the date/time reported by your PC (as seen by your web browser). If your PC clock is accurate to a second then the other time scales displayed above will also be accurate to within one second.
- UTC, Coordinated Universal Time, popularly known as GMT (Greenwich Mean Time), or Zulu time. Local time differs from UTC by the number of hours of your timezone.
- GPS, Global Positioning System time, is the atomic time scale implemented by the atomic clocks in the GPS ground control stations and the GPS satellites themselves. GPS time was zero at 0h 6-Jan-1980 and since it is not perturbed by leap seconds GPS is now ahead of UTC by 16 seconds.
- Loran-C, Long Range Navigation time, is an atomic time scale implemented by the atomic clocks in Loran-C chain transmitter sites. Loran time was zero at 0h 1-Jan-1958 and since it is not perturbed by leap seconds it is now ahead of UTC by 25 seconds.
- TAI, Temps Atomique International, is the international atomic time scale based on a continuous counting of the SI second. TAI is currently ahead of UTC by 35 seconds. TAI is always ahead of GPS by 19 seconds.
Other ways of referencing time:
- Atomic Time , with the unit of duration the Systeme International (SI) second defined as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium 133. TAI is the International Atomic Time scale, a statistical timescale based on a large number of atomic clocks.
- Universal Time (UT) is counted from 0 hours at midnight, with unit of duration the mean solar day, defined to be as uniform as possible despite variations in the rotation of the Earth.
- UT0 is the rotational time of a particular place of observation. It is observed as the diurnal motion of stars or extraterrestrial radio sources.
- UT1 is computed by correcting UT0 for the effect of polar motion on the longitude of the observing site. It varies from uniformity because of the irregularities in the Earth's rotation.
- Coordinated Universal Time (UTC) differs from TAI by an integral number of seconds. UTC is kept within 0.9 seconds of UT1 by the introduction of one-second steps to UTC, the "leap second." To date these steps have always been positive.
- Dynamical Time replaced ephemeris time as the independent argument in dynamical theories and ephemerides. Its unit of duration is based on the orbital motions of the Earth, Moon, and planets.
- Terrestrial Time (TT), (or Terrestrial Dynamical Time, TDT), with unit of duration 86400 SI seconds on the geoid, is the independent argument of apparent geocentric ephemerides. TDT = TAI + 32.184 seconds.
- Barycentric Dynamical Time (TDB), is the independent argument of ephemerides and dynamical theories that are referred to the solar system barycenter. TDB varies from TT only by periodic variations.
- Geocentric Coordinate Time (TCG) is a coordinate time having its spatial origin at the center of mass of the Earth. TCG differs from TT as: TCG - TT = Lg x (JD -2443144.5) x 86400 seconds, with Lg = 6.969291e-10.
- Barycentric Coordinate Time (TCB)is a coordinate time having its spatial origin at the solar system barycenter. TCB differs from TDB in rate. The two are related by: TCB - TDB = iLb x (JD -2443144.5) x 86400 seconds, with Lb = 1.550505e-08.
- Sidereal Time, with unit of duration the period of the Earth's rotation with respect to a point nearly fixed with respect to the stars, is the hour angle of the vernal equinox.
17
posted on
06/23/2015 7:58:52 PM PDT
by
zeugma
(The best defense against a bad guy with a gun is a good guy with a gun)
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