Time Today

The History of Timekeeping

Course 2 · Chapter 8 · Capstone: Time Today

The Edinburgh gun that began this course is still fired every day at one o'clock, but nobody now sets a watch by it. Your phone knows the time to a fraction of a second without being asked, and so does the cash machine, the railway signal and the trading floor. This closing chapter explains how that agreement is kept, why the world sometimes adds a second to its clocks, how satellites and networks spread the time, and what the whole story from shadow to atom tells us about how people agree on things.

Who Keeps the Time?

No single clock keeps the world's time. Instead, many national laboratories run atomic clocks, and their readings are combined into an international average.

International Atomic Time (TAI)

A steady scale made by combining a large number of atomic clocks around the world. It counts seconds and does not follow the Earth.

Coordinated Universal Time (UTC)

The civil time used worldwide. It runs at the same rate as atomic time but is kept within a second of the Earth's rotation by occasional adjustments (check the details).

Local time

UTC plus or minus a whole number of hours (sometimes half or quarter hours) set by each country, plus daylight saving where used. In Britain this is Greenwich Mean Time in winter and British Summer Time in summer.

In the United Kingdom, the National Physical Laboratory keeps national time and contributes to the international average (check). The International Bureau of Weights and Measures near Paris combines the data from laboratories around the world (check).

The Leap Second

The Earth's rotation is slightly irregular and slowing over long periods, so a day is not exactly 86,400 atomic seconds. To keep UTC close to the position of the Sun, an extra second was occasionally added at the end of 30 June or 31 December, starting in 1972 (check). There have been about twenty-seven such additions (check the count).

A debate about the leap second Leap seconds are a nuisance for computers, because a minute with 61 seconds breaks many assumptions, and several outages have been blamed on them (check). Metrologists have voted to phase out leap seconds in the future, allowing UTC to drift further from the Sun's time, with the details and date still to be settled (check the current position). The question echoes the debates of earlier centuries: how much should the clock bend to the sky, and how much should the sky bend to the clock?

Spreading the Time

Method	How it works	Typical accuracy
Radio time signals	Broadcast from a national laboratory to radio-controlled clocks	Around a millisecond or better (check)
Internet time (NTP)	Computers ask servers for the time and correct for the delay	Milliseconds to tens of milliseconds (check)
Satellite navigation	Each satellite carries atomic clocks and broadcasts its time	Tens of nanoseconds, in the best cases (check)
Fibre links	Time and frequency sent along dedicated optical fibres between laboratories	Even better than satellites over short distances (check)

Longitude, completed Chapter 6 showed that finding position at sea depends on time. Satellite navigation completes that story: a receiver works out its position by comparing the tiny differences in the time signals arriving from several satellites. A time error of a microsecond is a position error of hundreds of metres, since radio signals travel about 300 metres in a microsecond (check). The chronometer problem has become a problem of nanoseconds.

What Depends on Exact Time Now

Financial trading. Transactions must be time-stamped to fractions of a second to settle disputes and meet regulation.
Power grids. The timing of alternating current across a network must be coordinated.
Mobile networks. Base stations need shared time to avoid interfering with each other.
Science. Astronomers combine telescopes across continents, and physicists test their theories with ever more precise clocks.

The Story as a Whole

Looking back across the course, a few patterns stand out.

Theme	Where it appeared
Each device fixed a weakness of the last	Sundial to water clock; verge to pendulum; pendulum to chronometer
Demand drove precision	Prayer, markets, navigation, railways, computing
Agreement mattered more than accuracy	Railway time; time zones; the Gregorian reform
Politics shaped the calendar	The Roman priests; the papal bull; the 1752 change
Legends cluster around "firsts"	Galileo's lamp; the lone inventor of the clock; the Harrison underdog tale

Reading a claim about time When you meet a claim such as "the oldest clock", "the first accurate watch" or "the day the calendar lost eleven days", ask four questions. Which definition is being used? What is the earliest surviving evidence? Is a legend filling a gap? And which calendar and time standard was the writer using? These four checks will serve you well in the later history of any invention.

Exercises

Exercise 1

In four sentences, explain the difference between atomic time, UTC and local time, and say why a leap second is needed.

→ Solution

Exercise 2

A satellite navigation receiver has a clock error of 2 microseconds. Using the figure of about 300 metres per microsecond, estimate the position error. Then say why the satellites carry atomic clocks while receivers do not.

→ Solution

Exercise 3

Choose any three of the eight chapters and write one sentence each on what problem the chapter's main idea solved and what new problem it left. Then write two sentences on which advance you think mattered most.

→ Solution

Course complete You have followed timekeeping from a stick in the ground to a clock that would lose less than a second in the age of the universe (check). The next time you glance at a phone, or hear the one o'clock gun from Edinburgh Castle, you are looking at the end of a very long chain of agreements.

Quick Reference

TAI: steady atomic scale
UTC: atomic rate, kept near the Sun by leap seconds
Local time: UTC plus a zone offset and any daylight saving
Leap seconds: added since 1972; being phased out
Distribution: radio, internet, satellite and fibre
One microsecond: about 300 metres of signal travel
Pattern: each device fixed a weakness; agreement mattered as much as accuracy
Check: the leap second count and future, accuracy figures, the organisations' roles