Date line explanation & funny effects.

Imagine a person who gets into a plane which doesn't need to stop to get petrol and which can fly as fast as you want. Imagine also that the person in the plane has got two clocks which can keep track of both hour and date. Both clocks are marking the same hour and date at the departure of the plane,(local date at the airport.) The person in the plane has been told to not to touch the first clock and to maintain every hour in the second clock the local time of wherever he goes. Now, let's see what can happen:

Suppose the plane can go at a speed of 15 grades of longitude per hour, and that it is going in eastern direction. Every hour, the person will have to add one more hour to the second clock, so the second clock will advance 2 hours while the first clock advances only one. After 24 hours, he will arrive to the same airport with both clocks marking the same hour, but with one day of difference! More or less, this was the funny thing that happened to William Fog in Julius Verne's novel Turning around the world in 80 days. It doesn't matter at what speed you go. If you turn the world going to the east, the difference between the two clocks will be the number of turns you have had in days.

If the plane goes in western direction, it would even be more funny, because besides the plane would always see the sun in the same position (an eternal sunset could be seen), every hour, the person would have to set the second clock to one hour less. The clock would be doing something like going from 7:00 to 8:00 minute by minute and then coming back to 7:00 again. The second clock would be always marking the same date, month and year although the plane was planing for a century! This other funny experience happened to the first Spanish and Portuguese sailors who wanted to have a ``holiday'' trip around the world. After some months of going in their fragile ships, in western direction, fighting against the abysmal monsters and surviving to diseases and lack of food, they arrived to their homes having lost one of their days of their lives. I suppose they were so happy for having arrived home that the strange price of loosing one day of their lives wouldn't seem too expensive.

British people, who always want to establish the standards for the rest of the world, thought about how the problem of loosing and earning days could be solved. They arrived to this artificial solution, which is sometimes difficult to understand, but that works properly. In the middle of the Pacific Ocean at 180 degrees of longitude (they did not want to have the funny effects of their solution near Britain), the "date line" was established and the following rule was set: If you pass through the date line in eastern direction, please subtract one whole day from your clock. If you pass the date line in western direction, then please add one whole day to your clock. This way, the funny problems explained before were solved. The plane which goes in eastern direction has to increase one hour every hour (funny words, aren't they?), but when the plane crosses the date line, 24 hours should be subtracted. When that plane arrives to the airport, now both clocks will mark the same date and the same hour. The same effect can be seen if the plane goes in western direction, one hour will be subtracted every hour but 24 hours will be added every day.

However, more funny things can still happen with this system, and that's the problem that will occur to Robert Cailliau when he flies from California to Sidney. His plane will depart on Friday evening and he will arrive on Sunday morning having spent just a few hours in the plane, so he will have lost the Saturday! The explanation about this is as following:

Consider the world and two lines that divide it in two pieces. First line is already known. It is the date line and it is statically fixed in the middle of Pacific Ocean. The other line is the line of midnight hour. This other line is moving around the world. If you cross it, you will also change your date. So the world is divided in two pieces both of them delimited by the two lines. The date in both pieces is different. If in the first piece (I say pieces and hot halves) it is Friday, in the other piece it is Saturday. There is always one day of difference between them. Take care of both pieces are growing or shrinking while the day advances, but I think it's much better understood if you look at the following pictures:

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