We all know that our planet takes about 24 hours to complete the rotation of 360 °, so that at each longitude is a different time. In the past, each country had its own hour National: The entire nation is conformed hour calculated for the meridian passing through the capital. In 1893 was adopted the system of time zones, created in 1859 from Italian Filopanti Quirico.
Set as the zero meridians the meridian of Greenwich, have defined other 23 meridians, spaced 15 ° apart. The 24 meridians are meridians to the average center of many segments of the Earth’s surface, each of which is a time zone that, by convention, the time has its own central meridian. When it is noon in Greenwich, in the west zone is 11, because due to the rotation of the earth here comes the sun with a time delay. In the east zone are the 13, because here the sun first appeared.
The boundaries between neighboring time zones, of course, for the most part coincide with the boundaries of states and not with the straight lines of the meridians; this convention is intended to prevent time change within the same country. However, there are exceptions due to other reasons: France and Spain, for example, would be the first time, that of Greenwich, but use the time zone of Germany and Italy because of the close relations that bind to these countries.
Russia has as many as 11 different time zones, followed by Canada with fewer than 6 time zones, Brazil and Australia with 3 with 4 time zones. There are other countries with exceptions such as India and Australia central shaft and China which uses the same time everywhere.
In summer, in addition, many countries introduce daylight saving time, which precedes the one hour zone.
The meridian of Greenwich is opposite to the line of the change of date. Whoever crosses the line coming from the east to the west increases the date by one day.