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Sunlight arriving from one side, and noon marked on the turning Earth The Earth is drawn as a large plain circle towards the right of the picture, seen from high above the north pole. Four long lines with arrow heads come in from the left and run right up to the circle, showing sunlight arriving from a very great distance. The half of the circle facing away from the light is shaded, and a dashed line down the middle separates the lit half from the dark half. A short bold spoke sticks out from the point that faces the light most directly, ending in a solid dot: that one place is the one having noon. Three smaller dots sit at other places round the circle, waiting their turn, and a small curved arrow beside the circle shows which way the Earth carries them.

Noon Is Not the Same Moment Everywhere

About 13 minutes

Call somebody far away and the first thing anyone says is what time it is at their end. Morning there, evening here.

It is easy to hear that as a rule people invented — as though the world agreed to hand out different numbers to different countries, the way countries have different flags. It is not. The difference is real, and you could check it with a stick in the ground.

Noon is a fact about the sky

Forget clocks for a moment.

Noon means the moment the Sun reaches its highest point in your sky. Before it, the Sun is still climbing. After it, the Sun is on its way down. At noon, shadows are at their shortest for the whole day, and they point straight along a north–south line.

That is not a reading. It is something happening above your head, and you can watch it. A stick in the ground will tell you when it arrives — that is exactly what a sundial is watching.

And here is the thing that makes time zones unavoidable: the Sun cannot be highest everywhere at once. The Earth is a ball, and the Sun lights one side of it. If the Sun is directly over one place, then there are places where it is only just rising, places where it is setting, and places on the far side in the middle of the night.

Noon belongs to one place at a time on the turning Earth The Earth is drawn as a large circle seen from high above the north pole. Six long parallel lines with arrow heads arrive from the left and stop at the circle, showing sunlight coming from a very great distance. The half of the circle nearest the light is left plain and the half facing away is lightly shaded, with a dashed line down the middle dividing the two. Four places are marked with solid dots. The one facing the light most directly is labelled noon. The one at the top of the circle, on the dividing line, is labelled sunrise. The one at the bottom, also on the dividing line, is labelled sunset. The one facing directly away from the light is labelled midnight. A curved arrow round the outside shows which way the Earth turns, carrying every place through each of those four in turn. sunlight noon sunrise sunset midnight the way it turns
Sunlight arrives on one side. Noon belongs to the strip of the world facing it, and everywhere else on the lit half is on its way into noon or on its way out.

Fifteen degrees an hour

The Earth turns all the way round once a day. That is 360 degrees in 24 hours, which comes out at a very tidy number:

360 ÷ 24 = 15 degrees every hour

So a place 15 degrees of longitude west of you gets its noon exactly one hour after you get yours. Thirty degrees west, two hours. The Sun does not move across the countries — the countries turn underneath it, and noon crawls westward at a steady fifteen degrees an hour, all day, every day, for ever.

One place carried round the Earth from noon to midnight The Earth is drawn as a circle seen from high above the north pole, with sunlight arriving as parallel arrows from the left. A solid triangle outside the circle points at the one spot facing the light most directly, and that spot is labelled noon. It never moves. Four labels round the circle mark noon at the left, sunrise at the top, sunset at the bottom and midnight at the right. A single marked place sits on the circle. As the hours are added the whole circle carries that place round, away from the noon marker, past sunset at the bottom, and on towards midnight on the far side. The sunlight and the noon marker stay exactly where they were the whole time. sunlight noon sunrise sunset midnight Nothing about the Sun moves. The ground does.

Turn the Earth by the hour and watch which place is under the noon line.

Nothing about the Sun changes. The arrow stays exactly where it is. The only thing moving is the ground, and that is enough to give every place on it a different noon.

Every town used to keep its own time

For most of history this caused nobody any trouble, because every town simply set its clock by its own Sun. A town 20 kilometres to the east had its noon about a minute earlier, and its clocks ran a minute ahead, and nobody minded — you could not travel fast enough for a minute to matter.

Then came railways, and a railway needs a timetable.

A timetable is a promise about a moment, made to people in dozens of towns that all disagree about what moment it is. A train leaving at noon leaves at a different instant in every station on the line. It was a genuine mess, and occasionally a dangerous one on single-track lines where two trains were kept apart by the clock.

So the railways fixed it by force. British railway companies started running everything on London's time from the 1840s, and people called it railway time. In the United States the railroads carved the country into standard zones and switched over on 18 November 1883. In 1884 an international meeting agreed to count longitude from Greenwich, in London, which is why the map is numbered the way it is today.

Clock time did not grow out of the Sun. It was imposed on top of it, by trains.

Why does a place 15 degrees west of you have its noon an hour after yours?

Which is why the zone map is such a mess

If zones followed the fifteen-degree rule exactly, the world map would be 24 neat stripes.

Look at a real one and it is nothing like that. The boundaries wander around mountain ranges, jump along national borders, and bulge hundreds of kilometres out of their way. Some places use offsets that are not whole hours at all: India runs half an hour off, and Nepal runs three quarters of an hour off, on its own for the whole world.

That is because a zone is a decision, not a measurement. A country picks its clock, and it usually picks whatever keeps the country on one clock, because a nation split across two times is a nuisance every single day.

China takes this furthest. It spans around 60 degrees of longitude — four hours' worth of turning — and runs all of it on a single clock set for Beijing in the east. So in the far west of the country, the Sun reaches its highest point at about three in the afternoon, and sets close to midnight in summer.