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ELI5 Yahaaa

Noon cannot be everywhere at once

Why Is It a Different Day Somewhere Else?

Clocks are set by the Sun, and the Sun is only overhead in one place at a time.

The Earth, stripedinto zones Noon is wherever the Sunis highest
Twenty-four stripes, one for each hour of the turn.

  1. Step 1 of 5

    A day is one turn of the Earth.

    The Sun stays put The Earth turns Noon reacheseach place inturnEvery 24hours

    The Earth spins once in twenty-four hours, so every place passes under the Sun once. Noon is not a moment — it is a place.

  2. Step 2 of 5

    Dividing the turn gives the zones.

    1 360 degrees2÷÷ 24 hours3 15 degrees perzone

    Three hundred and sixty degrees divided by twenty-four hours is fifteen degrees an hour. Each stripe of fifteen degrees keeps one clock time.

  3. Step 3 of 5

    Before railways, every town kept its own noon.

    Local noon Every town slightlydifferentZone time One clock for a widestripe

    Clocks were set by the local Sun, so a town twenty miles east was minutes ahead. Timetables made that unworkable overnight.

  4. Step 4 of 5

    The zones bend around countries.

    Straight stripes What the maths saysReal zones What the map actuallydoes

    The stripes are not straight. Borders pull them about so a country can share one time, and a few zones sit half an hour off the rest.

  5. Step 5 of 5

    Somewhere the date has to change.

    1 Add an hour,and another2 All the wayround3 The date muststep back a day

    Go far enough east adding hours and you come back around. The line down the Pacific is where the arithmetic is forced to jump a whole day.

    Two islands in the Pacific sit a few miles apart and twenty-five hours away from each other.

The short version

Time zones divide the Earth's twenty-four-hour turn into stripes, so clocks stay roughly in step with the Sun.

Try it yourself

Work out the time in a city eight zones away by adding eight hours. If it goes past midnight, it is tomorrow there.

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