Time zones

The Earth turns 360 degrees in 24 hours, so every 15 degrees of longitude moves noon by one hour.

Why 15 degrees

Noon, in the old sense, is the moment the Sun is highest in the sky. Because the Earth spins from west to east, noon sweeps westward round the planet, and places farther east see it first. A full turn is 360 degrees in 24 hours, so the Sun appears to move 360 ÷ 24 = 15 degrees of longitude every hour, or one degree every four minutes. Before railways, each town set its clocks by its own noon, and the clocks of two towns a hundred kilometres apart could differ by several minutes.

Standard time zones replaced that patchwork. The ideal plan divides the globe into 24 bands, each 15 degrees wide and centred on a meridian that is a multiple of 15. The band around Greenwich keeps Coordinated Universal Time, written UTC, and every other zone is described as an offset from it: UTC+1, UTC+9, UTC−5 and so on. The figure shows that ideal plan. The real map is messier, because each country decides its own time by law and borders zigzag around coastlines and provinces.

Korea and its neighbours

South Korea keeps UTC+9, nine hours ahead of Greenwich. Nine hours times 15 degrees is 135 degrees, so Korean clocks follow the Sun at 135 degrees east, the meridian that also sets the time in Japan. Seoul itself is at about 127 degrees east, eight degrees west of that line. At four minutes per degree, the Sun reaches its highest point over Seoul about 32 minutes after it does over 135 degrees east, which is why noon by the Sun in Seoul comes at about 12:32 on the clock, before small seasonal shifts are added.

China spans about 60 degrees of longitude, wide enough for five zones, and before 1949 it did use five. Today the whole country keeps Beijing time, UTC+8, which makes China the largest country by area with only one time zone. Russia went the other way and has 11 time zones, the most of any country in one connected piece of land. The 50 United States cover six zones, from Hawaii to the East Coast.

Not every offset is a whole hour. India keeps UTC+5:30 across the whole country, and Nepal, its neighbour, keeps UTC+5:45, so crossing that border moves the clock by just 15 minutes. Many countries also shift their clocks by an hour in summer, which is called daylight saving time; the United States, for example, moves forward on the second Sunday in March.

The International Date Line

If you travel east and set your clock forward an hour every 15 degrees, by the time you have gone all the way round you will have added 24 hours, a whole day. Something has to take that day back. That is the job of the International Date Line, which runs roughly along the 180 degree meridian in the middle of the Pacific. Cross it going west and the date moves forward one day; cross it going east and it moves back. The line is not fixed by any treaty. It bends round island groups so that each country can keep a single date.