About Time: Is the June 30th Leap Second the Last?

Chances are, you’ll soon be hearing that we’re 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 fraction—1/31,556,925.9747th to be precise—of the tropical year set at noon on January 1st 1900.

Now, this decision relied on measurements contained in Simon Newcomb’s 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 isn’t 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.

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)

What time is it?

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.

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