There is a special clock in your house. It is designed to tell time for other places in the world where your relatives live. You are in Virginia and your time is different from California (where your older brother lives), Hawaii (where your grandparents live), and England (where your aunt lives). You are always reminded to look at the special clock before you call your family members. This will let you know the time at the place you will call.

The clock shows that if it is 6:00 PM in Virginia, then it is 3:00 PM in California, noon in Hawaii, and 11:00 PM in England. Timekeeping was quite different before time zones were used.

Before 1884, every city would have its citizens set their watches and clock to noon for their city - that is, when the sun is at its highest point above the city. This was a major problem for railroads. Riders had to change their watch every time they would arrive at another city. This averaged out changing one minute on your watch for every twelve miles traveled! Beginning in 1884, a system of (time) zones was set up for the entire world.

The Earth can be thought of as a rounded ("not perfectly round") sphere. Looking down on a sphere, you see a circle and know that a circle contains 360 degrees. Scientists also know the sun appears to move 15 degrees across the sky every hour. (Of course, we know the Earth is actually rotating which makes the sun appear to do the moving)

If you travel 15 degrees in one hour, how long would it take you to go through all 360 degrees on the Earth? You must travel the distance of 15 degrees 24 times to move through the entire 360 degrees (travel around the earth). We do have 24 hours in a day that is matched with 24 time zones around the Earth. At the end of a day, the sun really has appeared in all 24 time zones. An "additional" 25th time zone is created in the Pacific Ocean by the International Date Line. This line is discussed later in this article.

Longitude lines on Earth are used to separate time zones, which are 15 degrees wide. Sometimes, a longitude line would split a state - or even a city - in half. This could place the two parts of the state or city within two different time zones. Most times, the longitude line at the edge of the time zone will go "around" a state along its border. This way, an entire state or city would be in the same time zone. These time zones extend from the North Pole all the way to the South Pole. That is why Virginia residents have the same time as people near the Panama Canal or in Miami Beach.

Time zones begin at 0 degrees longitude. This line is called the Prime Meridian. Time in this zone is called Greenwich Mean Time (GMT - named for the city of Greenwich, England where you find the Prime Meridian). This time zone is about 75 degrees from your time zone for Virginia. 75 degrees divided by 15 degrees equals 5 hours (75¿ 15 = 5). This means that time in England is 5 hours different. Because they are east of your location, you would add five hours to your clock to know their correct time.

Therefore, if it is 6:00 PM in your house in Virginia, then it is 11:00 PM in England (you add 5 hours to 6:00 PM). Places west of you subtract hours from your time.

The United States is so large that several time zones must be used. They are Atlantic, Eastern, Central, Mountain, Pacific, Alaska, and Hawaii-Aleutian. As an example, if it is 6:00 PM in the Eastern time zone, then it is 5:00 PM in the Central Time Zone, 4:00 PM in the Mountain Zone, and 3:00 PM in the Pacific Time Zone. It is noon in Hawaii (6 hours "behind" you). Even these time differences are affected by something called "Daylight Saving Time."

Daylight Saving Time (DST) is a special situation. We have used some variation of DST since 1918. Its purpose is debated but it does provide you with more daylight. In DST, the sun sets later in the summer, which means we really do have more sunlight.

The idea for DST is credited to Benjamin Franklin. He (and others) felt we would use less candles (less resources) if the daylight lasted longer. DST has you move your clock ahead one hour in April and back one hour in October. Hawaii, Arizona, and most of Indiana do not change their clocks to observe DST. Countries close to or on the equator do not observe DST.

A unique situation occurs along the longitude line at 180 degrees away from the Prime Meridian. This is the International Date Line. You will change days as soon as you cross the line. If you cross it going west, you will add a day. Going east across it means you lose a day. This line is in the Pacific Ocean and care was taken to make sure it does not go through landmasses.

It is still possible that two people may have watches that do not agree. That is not necessarily because of time zone problems. Time zones allow everyone on Earth to have a similar day with the sun rising in the morning and setting during evening hours.