It’s around 9 pm. It will soon be dark. It is sunset on the west coast of France. What this really means is that the earth has now turned to a position where France no longer gets any sunlight. France is in the shade.
Meanwhile it is light at other places on earth. In New York, it is late afternoon. There, it’s 3 o’clock. It will be another six hours before the earth has turned enough to put New York in the shade. So, the time is different at different places on earth, at exactly the same moment.
The earth is divided into time zones. The time zones follow the meridians of the earth, the imagined lines going from the North Pole to the South Pole. Here, passing through Greenwich outside London, is the prime meridian. Right now, it’s 12 noon at Greenwich. Meanwhile, 15 degrees east, it’s one o’clock.
30 degrees east, it’s two o’clock. And 45 degrees east of Greenwich, it’s three o’clock in the afternoon. So, there are 15 degrees between every time meridian. From one time meridian to the next, the clock time differs by one hour. All around the earth, there are 24 time meridians, as many as there are hours in a day.
But the time meridians are drawn without regard to country borders or where people live, which is not very practical. Imagine you have a basketball team with some players who live on one side of a time meridian and others on the other side, where the time is different. Game day tomorrow. We meet at five o’clock for you living in West city, and six o’clock for you living in East city. But what about me?
I live in the city centre. Instead, the time zones follow the borders between countries and regions, like this. The time zone near the Greenwich meridian is the starting point. This is zero. We describe all the other time zones with signs and numbers.
Plus three here means that if it’s 12 noon in Greenwich, at the same moment it is three in the afternoon in Saudi Arabia and Madagascar. Minus 8 here means that when it’s 12 noon in Greenwich, it’s four in the morning in Los Angeles. On the map of the time zones we can see that large countries, like Russia and the USA, extend over several time zones. When it’s four in the morning in Los Angeles, it’s seven in New York, even though both cities belong to the same country. The usual thing is that the clock time in the countries of the world differs by whole hours.
But some countries choose a time which is half an hour from their neighbouring countries. Therefore, if it’s 12 in Pakistan, it’s 12.30 in India, and 12.45 in Nepal. Here, in the middle of the Pacific ocean, there is a special time boundary. West of it, it says plus 12, and east of it, it says minus 12. If the time is 12 at night in Greenwich, then it’s 12 noon both at the Gilbert Islands here and at Baker Island here.
The difference is that they have different dates. On the 1st of january at noon on the Gilbert Islands, at the same moment it’s noon on the 31st of December on Baker Island. This is the International Date Line. If you sail across it eastbound, the clock is turned back 24 hours, to yesterday. What?
Are we travelling in time now? No, listen, we’re travelling across the date line. Today is now yesterday. Is it yesterday today? Today is today.
But this is yesterday. Since we travelled ACROSS the date line. Understand? I was just joking. I understood already from the start.