Issue OneAutumnThe Measured Hour

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Seven short chapters and a timeline on the long project of catching time: the dripping jar, the swinging weight, the ticking box that crossed an ocean, and the atom that finally agreed to keep the count.

A single-issue digital magazine.
Set in Fraunces and Source Serif. Printed on nothing. Read it slowly; the subject rewards it.
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Contents
  1. The shadow and the jarp. 2
  2. A weight that falls in stepsp. 5
  3. The pendulum learns to countp. 8
  4. Mr Harrison goes to seap. 11
  5. The railway abolishes noonp. 15
  6. Quartz, and time in the pocketp. 18
  7. The caesium secondp. 21
  8. Milestones: a timelinep. 24
On the type The masthead uses the optical size, softness and "wonk" axes of Fraunces. Body copy is Source Serif 4 with old-style figures.
Chapter one

The shadow and the jar

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Before there were clocks there was the sun, and the sun is a poor timekeeper at night. The earliest fix was the shadow stick, a gnomon planted upright so that its shadow swept a marked ground. Egyptian shadow clocks survive from around 1500 BCE, and obelisks did the same job at civic scale. The trouble was the dark, and cloud, and winter. So people turned to water. A clepsydra, literally a "water thief," is a vessel with a small hole: fill it, and the falling level reads the passing hours. Egyptian examples date to the reign of Amenhotep III, and their inner walls carry separate scales for different months, because a night in winter is longer than a night in summer, and the hours were stretched to fit.

Chapter two

A weight that falls in steps

A falling weight on a cord will spin a drum as fast as friction allows, which is useless for telling time. The genius of the medieval clockmaker was to interrupt the fall. The verge and foliot escapement, appearing in Europe in the late thirteenth century, lets a toothed crown wheel advance one tooth at a time as a horizontal bar rocks back and forth. Each rock is a beat. The beat was not very regular, and early tower clocks drifted by a quarter of an hour a day, but they did something no sundial could: they struck. Bells at Norwich, Milan and Salisbury rang the hours to whole towns, and the equal hour, sixty minutes long in every season, quietly replaced the old stretched one.

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“The clock, not the steam engine, is the key machine of the modern industrial age.”
Lewis Mumford, Technics and Civilization, 1934
Chapter three

The pendulum learns to count

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Galileo noticed, or is said to have noticed while watching a lamp swing in Pisa cathedral, that a pendulum's period depends on its length and hardly at all on how wide it swings. He sketched a pendulum clock late in life but never built one. Christiaan Huygens did, in 1656, and patented it the next year. The difference was startling. A good verge clock lost minutes a day; Huygens's pendulum clock lost seconds. For the first time a machine kept time well enough to need a minute hand, and soon a second hand. Huygens also worked out the mathematics of the thing, publishing Horologium Oscillatorium in 1673, and in doing so turned clockmaking from a craft into a branch of physics.

Chapter four

Mr Harrison goes to sea

A pendulum is no use on a ship. It swings with the deck, and it stops in a squall. Yet a sailor who knows the exact time at a home port, and can compare it with local noon, knows his longitude. Britain's Longitude Act of 1714 offered up to twenty thousand pounds for a practical method, and a Yorkshire carpenter named John Harrison spent the rest of his life on the answer. His first three sea clocks were large, balanced machines. The fourth, H4, finished in 1759, was a watch five inches across. On a voyage to Jamaica in 1761 it lost about five seconds in eighty one days. The Board of Longitude quibbled for years; Harrison was eighty before Parliament paid him most of what he was owed.

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A ship carrying a good clock carries its home port with it.
Editor's note on the marine chronometer
Chapter five

The railway abolishes noon

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Until the 1840s every town kept its own time, set by its own noon. Bristol ran about ten minutes behind London, because the sun reaches Bristol ten minutes later. That was fine for stagecoaches and hopeless for timetables. The Great Western Railway adopted London time along its whole line in 1840, and by 1847 most British railways had followed, using time telegraphed from Greenwich. Britain made Greenwich Mean Time legal for the whole country in 1880. The United States was a harder case: at one point it had dozens of railroad times. On November 18, 1883, the railroads imposed four standard zones on the continent, and the following year the International Meridian Conference in Washington fixed Greenwich as the world's prime meridian.

Chapter six

Quartz, and time in the pocket

A quartz crystal, squeezed, produces a voltage; given a voltage, it flexes. Cut it to the right shape and it will ring at a frequency that barely changes with temperature or age. Warren Marrison and J. W. Horton built the first quartz clock at Bell Telephone Laboratories in 1927, and by the 1930s observatories were using quartz to check the rotation of the Earth itself. The crystals were shoebox-sized and hot. Shrinking them took forty years. On Christmas Day 1969 Seiko put the Astron on sale in Tokyo, the first quartz wristwatch, at roughly the price of a small car. Within a decade the price had collapsed, the Swiss mechanical trade had been gutted, and a factory watch kept better time than any chronometer Harrison ever made.

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Chapter seven

The caesium second

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Every clock so far had borrowed its standard from the Earth: a second was simply a fraction of a day. But the Earth wobbles and slows, and by the 1950s quartz clocks could see it doing so. The answer was to stop measuring the planet and start measuring an atom. Louis Essen and Jack Parry ran the first accurate caesium atomic clock at Britain's National Physical Laboratory in 1955. In 1967 the General Conference on Weights and Measures redefined the second as the duration of 9,192,631,770 periods of the radiation from a particular transition of the caesium 133 atom. That number has not changed since. Today's best caesium fountains would drift by about one second in a hundred million years, and the world's civil time, UTC, is a weighted average of several hundred such clocks.

Appendix

Milestones, left to right

Thirty five centuries in one strip. Red marks are the chapters above; the rest are stops along the way.

Scroll sideways
Egypt1500 BCE

Shadow clocks and clepsydrae

Gnomons mark the day; water clocks with seasonal scales take over at night.

Han China132

Zhang Heng's water-driven sphere

A clepsydra turns an armillary sphere in step with the sky.

Kaifeng1092

Su Song's astronomical tower

A water-wheel escapement drives a clock tower forty feet high.

Europe1280s

The verge escapement

Weight-driven tower clocks begin striking the hours in English and Italian towns.

Nuremberg1510

The mainspring

Coiled steel replaces the falling weight; the clock becomes portable.

The Hague1656

Huygens's pendulum clock

Accuracy improves roughly sixty-fold in a single stroke.

London1675

The balance spring

Huygens and Hooke each claim the spiral spring that makes a watch keep time.

Jamaica run1761

Harrison's H4 at sea

About five seconds lost in eighty one days, enough to fix longitude.

Britain1847

Railway time

Greenwich time is telegraphed down the lines; local noon begins to disappear.

North America1883

Four standard zones

The railroads reset the continent's clocks on the day of two noons.

Washington1884

The prime meridian

Twenty five nations vote Greenwich the zero of longitude.

Bell Labs1927

The quartz clock

Marrison and Horton ring a crystal and count the vibrations.

Teddington1955

The caesium clock

Essen and Parry run the first accurate atomic standard at the NPL.

CGPM1967

The second is redefined

9,192,631,770 periods of caesium 133 radiation, and no longer a slice of the day.

Tokyo1969

Seiko Astron

The first quartz wristwatch goes on sale on Christmas Day.

Worldwide1972

UTC and the leap second

Civil time is tied to atomic time, with occasional one-second corrections.

Orbit1978

The first GPS satellite

Atomic clocks in orbit turn timekeeping into position finding for everyone.

Laboratories2010s

Optical lattice clocks

Strontium and ytterbium clocks outperform caesium; a redefinition is under discussion.

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