TIMELINE From the sky to the wrist.

At sunrise the obelisk’s shadow is endless and points west. As the sun climbs, it shortens and swings round.

Its tip draws a different line on the ground every day: straight at the equinoxes, curved the rest of the year.

The Egyptians divided daylight into twelve hours, summer and winter alike: temporal hours, of varying length.

Change the season: in Thebes a summer hour lasts some 68 of our minutes, and a winter hour about 52.

A vessel with a hole near its foot. The water runs out through it, and the level falls with the night.

But the jet weakens as the water falls: with less height above it, it runs out more slowly.

That is why the Karnak vessel flares upwards: there is more water to empty per hour at the top, and the level falls at almost the same pace all night.

Try a straight vessel: the hour marks spread apart at the top and crowd together at the bottom.

A weight on a cord turns the clock’s wheels. The last of them is the crown wheel, its saw teeth standing on its rim like a crown.

In front of it stands a staff with two pallets. One stops a tooth; the wheel pushes it and turns the staff.

The foliot turns with the staff. As the upper pallet lets its tooth go, the lower one catches a tooth on the far side: the wheel escapes half a tooth at each beat.

To regulate it, the foliot’s weights are moved: the further out they hang, the harder the bar is to turn, and the slower it beats.

Freshly wound, the mainspring presses its coils round the arbor and pulls with all its strength.

A gut cord carries that pull to the fusée, a cone cut with a spiral groove. From about 1650 a chain begins to replace it, as it lasts longer.

While the spring is strong the cord pulls on the small radius; when it is weak, on the large one. Like a lever that grows longer.

Take the fusée away: without it, the force on the train falls with the spring, and the watch changes rate through the day.

The Earth turns 15° an hour. Between two meridians of this globe lies one hour.

A clock that keeps the time of the home port tells, at the ship’s noon, how far the Earth has turned since: four minutes for every degree.

On 18 November 1761 HMS Deptford sails from Portsmouth for Jamaica with John Harrison’s H4 aboard, in the care of his son William.

After 81 days and 5 hours at sea, H4 is only 5 seconds slow: 1.25 minutes of longitude, about one nautical mile.

The balance swings and the escapement keeps it going. In a pocket watch, upright all day, gravity pulls the balance to one side.

Breguet mounts the whole escapement in a cage that keeps turning: the errors of each upright position cancel out every turn.

The wheel below is fixed: the escape pinion rolls round it as the cage turns.

The cage’s pointer marks the seconds of your clock. Stand it upright, as in a pocket, and watch it keep turning.

Before time zones, every town lives by the time of its own meridian: four minutes’ difference for every degree to the west.

A train from Boston to St Louis crosses almost twenty degrees: its timetable is written in one time and read in another.

On 18 November 1883 the railways of the United States and Canada adopt time zones: each clock takes the time of its standard meridian.

Switch to zone time: the clocks of each band agree, and between one band and the next lies exactly one hour.

A pocket watch: crown and bow at twelve, to hang it from a chain.

In the First World War, reading the time without letting go of anything matters: the case grows lugs for a strap.

The crown moves down to three, where the fingers reach it without taking the watch off.

And the case leaves the circle: it is drawn for the wrist from the start, like the Santos of 1904 or the Tank.

The pusher at two starts the chronograph. The same pusher stops it.

The central seconds hand, independent of the time of day, measures anything: a dive, a flight, a lap.

The counter at three adds up the minutes, up to half an hour.

The pusher at four returns the hands to zero, but only once the chronograph is stopped. Try it.

A quartz tuning fork rings 32,768 times a second when a current drives it.

A circuit halves that frequency fifteen times over: 32,768 is 2 to the power of 15.

The last stage gives one pulse a second, and a stepping motor jumps the hand.

Take the slider to real speed: the crystal becomes a hum, and only the last stages can be told apart by eye.

The barrel stores the energy: a mainspring that unwinds over days.

The train shares it out: the centre wheel turns once an hour, and the fourth wheel once a minute, carrying the seconds hand.

The escape wheel and the lever let it go in equal beats, like the foliot seven hundred years before.

The balance and its spring keep the beat: 28,800 vibrations an hour, eight a second. And everything goes back in its place.

Six lasers slow a cloud of caesium atoms to a few millionths of a degree above absolute zero, and toss it a metre upwards.

The cloud rises, crosses a microwave cavity, slows, and crosses it again on the way down.

If the microwaves oscillate at exactly caesium’s frequency, most of the atoms come back changed in state.

A laser makes them glow and a detector measures the light. It is repeated, tuning the frequency until they glow brightest: that frequency defines the second.