Big question
Why are there time zones?
Time zones exist because the Earth is round and spinning, so the Sun rises and sets at different moments in different places. Splitting the world into roughly 24 zones lets each region keep its clocks matched to when the Sun is actually overhead, so midday stays close to midday.
One Spin, Many Sunrises
The Earth spins all the way around once roughly every 24 hours, and only the half facing the Sun has daylight at any moment. If every country used exactly the same clock time, the Sun would be high in the sky in one place while it was pitch dark in another, even though both said '3 p.m.' Long ago, towns each kept their own local time based on the Sun, which worked until trains and telegraphs connected distant places and people needed timetables that made sense across hundreds of kilometres. Dividing the world into time zones, each about 15 degrees of longitude wide, was a practical compromise between keeping clocks tied to the Sun and keeping them simple to use.
Fifteen Degrees Per Hour
Because the Earth is a sphere that turns 360 degrees in 24 hours, it rotates about 15 degrees every hour. Mapmakers used this to draw time zones roughly 15 degrees of longitude apart, so that travelling into the next zone usually means moving your clock forward or back by one hour. Real borders are not perfectly straight, though: countries bend the lines to keep a whole nation, state or island on one single time, which is why some zones bulge oddly around coastlines and political borders instead of following neat vertical stripes on a map.
A Line Where the Date Changes
Time zones are measured out from a starting line at Greenwich, England, called the Prime Meridian, and they run in both directions around the globe. Head east and clocks move forward hour by hour; head west and they move back. Follow either direction all the way around and the two sets of hours meet on the opposite side of the planet, roughly along the International Date Line in the Pacific Ocean. Crossing that line means jumping forward or backward a whole calendar day, which is why a plane can sometimes appear to land the day before it took off.
Try it yourself
Next time you video call someone in a different country, ask what time it is for them. Work out how many hours apart your clocks are, and notice whether they are having breakfast while you are having dinner.
Questions children ask
How many time zones are there in the world?
There are about 24 main time zones, roughly one for every 15 degrees of longitude around the Earth, though some countries add half-hour or odd offsets. A few very large countries, like Russia and the USA, span several zones at once.
What is the International Date Line?
The International Date Line is an imaginary line running mostly through the Pacific Ocean where the calendar date changes by one day. Crossing it travelling west skips a day forward, and crossing it travelling east repeats a day.
Do all countries use time zones based on the Sun?
Most countries base their time zone on longitude and the Sun, but some choose a single time zone for the whole country for convenience, even if it stretches wide. China, for example, uses one official time zone despite spanning a huge area.