The division of the world by meridians of longitude into zones and sectors where the same time is kept. The sun crosses each meridian of longitude at its local noon, four minutes later it reaches the next degree of longitude and crosses it also at its local noon. Without some agreed on system of time, all clocks in the world would show different times according to their longitude. The world had been divided into zones of 15 degrees of longitude within which all clocks keep the same time.