Time zones

Spin the globe again from west to east: that is how our planet turns on its axis, making one full turn every 24 hours. A full turn is 360°, so the Earth turns 15° every hour (15 × 24 = 360). That also means 1° takes 4 minutes (60 minutes ÷ 15).

Moving east from the Prime Meridian, the meridians go 0°, 15°E, 30°E, 45°E, and so on. Each 15° adds one hour of local time: if it is 12 noon at Greenwich, it is 1 pm local time at 15°E, 2 pm at 30°E, and so on. Going west it is the other way round: 11 am local time at 15°W, 10 am at 30°W.

The book gives a lovely example. Two friends talk on the phone late one afternoon, one in Porbandar in Gujarat and one in Tinsukia in Assam. The friend in Assam says the sun has set and it is dark; the friend in Gujarat is astonished, because where he is it is still full daylight. Both are right: they are about 30° of longitude apart, which is two hours of local time.

We could work out the local time of any place this way. But it would be very awkward for a country to use many local times: if a class is fixed for 4 pm, a child in the east and a child in the west would arrive at different moments. That is why most countries use one standard time, based on a meridian passing through them.

Indian Standard Time (IST) is set by the meridian at 82½°E, which passes near Mirzapur in Uttar Pradesh. IST is 5 hours 30 minutes (5.5 hours) ahead of the time at Greenwich, called Greenwich Mean Time (GMT). So both friends on the phone read the same IST on their watches, even though the sun is doing something different at each of their windows.

All these standard times are arranged in time zones, which roughly follow strips of 15°. The lines between the time zones are not fully straight, because they have to fit each country's standard time, so they tend to follow international borders. Geography draws the 15° lines; countries bend them.

Not every country has just one standard time. Russia, Canada and the USA are too wide for a single time zone. The USA has six time zones and Russia has 11, so travelling across Russia from east to west you would reset your watch 10 times!

The line opposite the Prime Meridian, at about 180° longitude, is the International Date Line. The +12 and −12 time zones touch at this line. If you cross it by ship or plane, you change the date: travelling eastward you subtract a day (say, from Monday to Sunday); travelling westward you add a day. It is only about at 180°, because it bends in places so that no country is split into two different days.