 International Time Zones The world is divided into 24 time zones. Notice the clock on each time zone. Time zones are a measurement of the Earth's rotation. Each time the planet completes a full rotation, a single day is completed. As the Earth rotates, locations on the Earth move away from the sun into the darkness, until that night and day are trading places. Local time around the world is determined in relation to the prime meridian, which goes through Greenwich, England. Notice the degrees of longitude at the bottom of the map. The prime meridian is at zero degrees. The international date line is on the other side of the world at 180 degrees. When you travel west over this line, the new day begins. When you travel east over this line, you repeat the day. For example, when you travel from California to Australia over the Pacific Ocean, you lose a day. North America and South America lie in the Western Hemisphere, west of the prime meridian and east of the international date line. Europe, Asia, Africa and Australia lie in the Eastern Hemisphere, east of the prime meridian and west of the international date line. You can figure out the local time of a location by adding one hour for every time zone east of the prime meridian. You subtract one hour for every time zone west of the prime meridian. There may be local variations such as daylight savings time. This concludes this learning activity, International Time Zones.