Before the late nineteenth century, each town set its clocks by the position of the sun, so the time in one town could differ from the time in a town only a short distance away. This did not matter much when people traveled slowly. When railways appeared, however, the differences caused confusion, because timetables were difficult to write. In 1884, an international meeting chose the line through Greenwich in London as the starting point for measuring time and position, and the idea of standard time zones gradually spread around the world. Today there are many time zones, and a traveler flying a long way east or west must change the time on a watch. Some large countries use several zones at once. After a long flight, people often feel tired and confused for a few days. This feeling is known as jet lag.

Question: What is the main idea of the passage?