Geography
Why are there different time zones?
Quick answer
Time zones exist because Earth spins, so the Sun is high in the sky at different times in different places. Earth is split into zones about an hour apart so that clocks match the daylight.
In one sentence
Time zones are regions of the world that share the same clock time so that noon is roughly when the Sun is highest.
Explained
Earth spins once every 24 hours, so the Sun is highest over different places at different times. When it is midday where you are, it is already afternoon farther east and still morning farther west. If everyone used the same clock, people in some places would eat lunch in the dark. So we divide the world into time zones.
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A full turn is 360 degrees, and it takes 24 hours, so Earth turns 15 degrees every hour. That gives about 24 zones, each roughly 15 degrees of longitude wide. The starting point is the prime meridian, which passes through Greenwich in London. Time there is called UTC, or Coordinated Universal Time. Zones are written as the difference from it: Lagos is UTC+1, Cairo UTC+2 (in winter), Mumbai UTC+5:30 and Tokyo UTC+9. New York is UTC−5 in winter. East is ahead because the Sun reaches places in the east first.
Example: it is 12 noon in London (UTC+0).
- Lagos (UTC+1): 1 pm.
- Tokyo (UTC+9): 9 pm.
- New York (UTC−5): 7 am.
Real zones do not follow straight lines. They bend around country borders and regions so neighbours can share a clock. India uses one time for the whole country, with a half-hour offset. China is so wide that it could have five zones, but it uses just one. Russia has 11, and France, counting its overseas territories, has the most.
Somewhere in the Pacific Ocean, near the 180 degree meridian, is the International Date Line. Cross it going west and the date jumps forward a day, and going east it falls back a day. Some countries also move their clocks an hour in summer, called daylight saving time, but many places, including Nigeria, never do.
Time zones matter for phone calls, flights and video chats. Try the 24-hour clock when planning, and read about day and night to see why the Sun reaches places at different times.
Examples
| City | Zone | Time when it is 12:00 in London |
|---|---|---|
| Lagos | UTC+1 | 1 pm |
| Cairo | UTC+2 | 2 pm |
| Mumbai | UTC+5:30 | 5:30 pm |
| Tokyo | UTC+9 | 9 pm |
| New York | UTC−5 | 7 am |
Common mistakes
- Subtracting for places east. East is ahead, so you add hours.
- "Time zones follow straight lines." They follow borders.
- Forgetting daylight saving. The difference can change by an hour in some seasons.
Quick quiz
Time zones
Check what you know with 4 quick questions.
Question 1 of 4
About how many degrees of longitude make one hour of time difference?
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Telling the time practice
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Latitude and longitude are imaginary lines on Earth. Latitude measures how far north or south of the Equator you are; longitude how far east or west.
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We have day and night because Earth spins once around every 24 hours. The side facing the Sun has daylight, and the side facing away is in darkness.
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