If I punch in on time when the leap second arrives, will it be shown as late for work?

January 1, 2017 can be regarded as a New Year with special significance. For China, which uses the Eastern Time Zone 8 as the standard time, we have ushered in the strange phenomenon of "7:59:60" . This extra second in the "tick-tock" sound is the leap second. So far, the world has implemented leap seconds 27 times. In the long years, what is the practical significance of these "extra time" that increases (or decreases) out of thin air?

01 How are leap seconds created?

There are currently two scientific systems for measuring time: "universal time" based on astronomical measurements of the Earth's rotation, and "atomic time" based on the period of atomic oscillations. In simple terms, leap seconds are an artificial product of the difference between Coordinated Universal Time and atomic time.

Universal time is a time scale based on the rotation of the Earth . In order to measure the rotation of the Earth, people selected two basic reference points on the celestial sphere, namely the vernal equinox and the mean solar position. The times determined by these points are called sidereal time and mean solar time respectively.

Although sidereal time corresponds to the angle of the Earth's rotation and meets the requirements of the time measurement standard based on the Earth's rotation, it cannot meet the needs of daily life and application because it is inconvenient to observe. People are accustomed to seeing the sun rise in the east and set in the west to determine the time, but the orbit of the Earth's revolution around the Sun is elliptical, so the apparent motion speed of the true Sun is uneven. In order to obtain a time measurement system based on the apparent motion of the true Sun and overcome its unevenness, people introduced an imaginary reference point - the mean Sun. It moves at a uniform speed on the celestial equator, and its speed is consistent with the average speed of the true Sun. Greenwich Mean Solar Time, which starts at 0 o'clock on the night of the mean solar day, is called Universal Time (UT) .

Copyright images in the gallery. Reprinting and using them may lead to copyright disputes.

Because the earth is a sphere, people around the world see different times of sunrise or noon sun. The time measured based on the meridian of the observer's location is called local mean solar time. At the same moment (instant), the local mean solar time measured by observers at different longitudes on the earth is different. As a result, there is no unified standard for time in different places. For the convenience of use, in the mid-19th century, some European and American countries began to adopt a unified national time, mostly based on the meridian of their capital or important commercial ports. However, with the development of long-distance railway transportation and ocean navigation, people's activities have greatly expanded, and there is still confusion in the use of their own local time in international exchanges. In the 1870s, someone proposed a proposal to divide time zones with unified standards around the world and implement zone timekeeping. The International Meridian Conference held in Washington on October 13, 1884 decided to use the meridian passing through Greenwich, southeast of London, England as the prime meridian as the starting point of the world's standard "time zone", called standard time . The division of world time zones is not only a need for scientific research, but also a need for world economic and political development.

Before the 1960s, universal time was widely used as the basic time measurement system. Under this timekeeping standard, "1 day" and "1 second" had strict conversion standards: 1 mean solar day was equal to 24 mean solar hours, which was equal to 86,400 mean solar seconds .

With the rapid development of modern science and technology, people's measurement of time is getting higher and higher in accuracy. Scientists have been exploring new time frequency standards, and atomic time determined by the atomic oscillation period has come into being.

In October 1967, the 13th General Conference on Weights and Measures held in New Delhi, India, officially defined the atomic time determined by the cesium atomic clock as the international time standard . The new second length is defined as: the duration of 9192631770 cycles of transition oscillation between two hyperfine energy levels of the ground state of the cesium 133 atom at sea level in a zero magnetic field is one atomic time second, and larger time units are obtained by accumulating seconds. With the definition of the second length, multiple atomic clocks that meet the definition of the second length distributed in countries around the world are combined to produce the International Atomic Time (TAI) .

Atomic time is generated by atomic clocks. The length of a second is very stable and can meet the requirements of high time interval uniformity, but its moment has no specific physical connotation. The moment of universal time corresponds to the specific position of the sun in the sky, reflecting the change in the position of the earth's axis when the earth rotates in space. It is not only closely related to people's daily life, but also has practical application value in scientific and technological fields such as geodesy, astronomical navigation and tracking measurement of space flight bodies. However, since the rotation rate of the earth is not very uniform, the length of a second in universal time will change slightly, and the daily speed difference can reach a few thousandths of a second.

In order to resolve this contradiction, the international community decided to adopt a " Coordinated Universal Time (UTC) " system for coordination. Whenever the time error caused by the rotation of the earth accumulates to close to one second with the atomic clock, the clock must be artificially increased or decreased by one second to re-coordinate UTC with the world time.

The length of a second in Coordinated Universal Time uses atomic time, and the deviation between the instant and the universal time is kept within ±0.9 seconds. When it exceeds 0.9 seconds, a 1-second adjustment is made to the Coordinated Universal Time. This one-second adjustment is called a leap second.

Copyright images in the gallery. Reprinting and using them may lead to copyright disputes.

02 What impact does the leap second have on my clocking in at work?

The implementation of leap seconds is related to the Earth's rotation period. If the Earth's rotation slows down, a positive leap second may occur, and if it speeds up, a negative leap second may occur. When a positive leap second is implemented, 23:59:60 will appear. When a negative leap second is implemented, the next second after 23:59:58 will be 0:0:0, that is, the second 23:59:59 will be skipped.

Due to the tides and the internal structure of the earth, the current trend of the earth's rotation is getting slower and slower, which is equivalent to the length of a universal second being longer than a second in atomic time. Therefore, the 27 leap seconds implemented so far are all positive leap seconds. Therefore, if you clock in on time when a positive leap second occurs, it will be shown as arriving at the company one second early. If a negative leap second occurs, it will be shown as being one second late.

You may be curious, why don’t you see any changes on your watch hands when a leap second occurs?

The reason is related to the timing principle of our watches. Most people use electronic watches, quartz watches or mechanical watches. The timing principle is to use the built-in oscillator of the watch to generate uniform seconds, and then accumulate seconds to achieve timing. When the watch leaves the factory, the worker has aligned the starting time of the watch with the national standard time. After using it for a period of time, people may find that the watch has a deviation of several seconds or even minutes from the national standard time. This is because the oscillator cycle in the watch is not accurate enough. It can be re-adjusted to align the time. Therefore, the time of ordinary watches is manually adjusted and aligned. If there is no manual time alignment operation when a leap second occurs, the watch pointer will not change at all. Because the oscillator cycle is not accurate enough, even if it is not adjusted when a leap second occurs, the leap second of 1 second may be submerged in the error and difficult to be detected. There is also a kind of watch that can receive radio signals in real time or regularly. It will automatically align with the national standard time through radio waves. When a leap second occurs, a leap second will occur.

03 Who needs a clock that is getting more and more accurate?

As people's activities expand, the demand for navigation increases and the accuracy becomes higher. Self-driving cars even need to be able to accurately locate within meters. From early direction judgment to current global satellite navigation, they are highly dependent on more accurate time. In terms of communication, time synchronization is the basis for the normal operation of the communication system. The synchronization performance of the time synchronization network is the key guarantee for high-bandwidth and low-latency communication, and it also requires high-precision atomic clocks and time synchronization technology as support.

Additionally, ultra-precise atomic clocks could help scientists detect dark matter, measure gravitational waves, and determine the exact shape of the Earth's gravitational field with unprecedented accuracy.

With the development of atomic clock technology, the frequency accuracy and stability of cesium atomic clocks, which are used as the definition of the second, have lagged behind microwave clocks and optical clocks by one to two orders of magnitude, restricting the acquisition of a more stable second. The call for an updated definition of the second is growing stronger, and optical clocks are likely to become the "new wave" of the definition of the second, replacing cesium atomic clocks. There is no conclusion on whether the definition of the second should use an optical clock of a certain element or a combination of multiple types. For this reason, my country has made early arrangements for the research of multiple types of optical clocks, including ytterbium atomic optical lattice clocks, calcium ion optical clocks, strontium atomic optical lattice clocks, aluminum ion optical clocks, ytterbium ion optical clocks, mercury atomic optical lattice clocks, mercury ion optical clocks, etc.

On October 31, the Long March 5B Y4 carrier rocket carrying the space station's Mengtian experimental module was ignited and launched from my country's Wenchang Space Launch Center. The Mengtian module carries the world's first space cold atomic clock composed of cold atomic strontium optical clocks.

Image source: Xinhua News Agency (Photo by Guo Zhongzheng)

At present, my country's optical clock research has reached the world's leading level in many fields. China is also the first country in the world to put an optical clock into space. The world's first strontium atomic optical clock is running on the Chinese space station. The world's longest and most accurate optical fiber time and frequency transmission network currently under construction will also make China's contribution to the redefinition of the second based on optical frequency transitions.

Expert: Liu Ya, researcher at the National Time Service Center of the Chinese Academy of Sciences

Produced by: Science Popularization China

The cover image of this article comes from the copyright library. Reprinting and using it may cause copyright disputes