Why it is night in Tokyo
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What you will learn
By the end, you can explain time zones and work out the time in a city from how far east or west of Greenwich it is.
Rui in London calls Yuki in Tokyo at 12 noon, Greenwich time. Yuki answers in the dark, ready for bed: it is 9 o’clock at night there. Nobody’s clock is wrong. Earth is a spinning ball, and the Sun can only light one side of it at a time.
Read the explanation
Earth spins once every 24 hours, so the Sun seems to rise in the east and set in the west, and at any moment half the planet has day while the other half has night. A full turn is 360° of longitude in 24 hours, so the Sun’s position moves 15° of longitude every hour, and the world is divided into time zones roughly 15° wide. The starting line is the Prime Meridian through Greenwich, London: places east of it are ahead, places west are behind.
- 15° of longitude = 1 hour. East of Greenwich, clocks are ahead; west of Greenwich, they are behind.
- Tokyo is at about 140°E: 140 ÷ 15 is about 9, so Japan uses Greenwich time + 9 hours.
- Countries bend the zone lines to fit their borders, and many move clocks forward in summer, so the real gap can differ by an hour from the longitude rule.