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# World Time Zones and the International Date Line

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World Time Zones and the International Date Line

OBJECTIVES Identify what time zone we live in
Given a city calculate the time in that specific time zone based on the time in Pensacola, FL Explain how time zones are divided (15 degrees per time zone 15/360 = 24 time zones) Identify where Greenwich Mean Time is located Identify where the International Date Line is located

Vocabulary

Time Zones Areas of the Earth divided into 15 degree zones that share the same time.

World Time Zones

Greenwich Mean Time The time zone that the prime meridian is located in. Named after Greenwich, England where the imaginary line runs through

International Date Line
An imaginary line on the surface of the Earth running north to south and separates one calendar day from the next. This is located in the Pacific Ocean at longitude line 180 degrees.

International Date Line

Introduction The Earth is currently divided into 24 major time zones so that everyone in the world can be on roughly similar schedules (noon being roughly when the sun is highest in the sky).

What are time zones? We have time zones in order for people to take the best advantage of daylight hours. The Earth is almost a perfect sphere and has a 360 degrees Each time zone is 15 degrees Total of 24 Time Zones

Why Do We Have Time Zones?
We measure the day by saying that the sun is at it's highest point at 12 o'clock, midday. Because the earth spins, as it travels around the sun, one half of the earth is in darkness and one half in light, all the time. This brings us to a problem. If it is midday where you are, on the opposite side of the earth, it is dark, so what time is it on the opposite side of the world?

HOMEWORK ASSIGNMENT Students will:
Determine  what time it in the following cities if it is 4pm in Pensacola, FL (*Hint use the time zone map on the resource page).                 - New York City, NY                - Paris, France                - Moscow, Russia                - Sidney, Australia                - Rome, Italy

Day and Night Before time keeping and clocks were invented, people used to know approximately what time it was by watching the sun. If you look out of the window, at different times of the day, you will see that the sun is in a different place each time you look. This is the same everywhere in the world. Once clocks were invented, they simply had to look at a clock to know what time it was. The earth spins on an imaginary line known as its axis, this movement is known as rotation. While it is rotating on its axis, it is also traveling around the sun, this movement is known as revolution. Although the earth is revolving around the sun, the sunlight only falls on the part of the earth facing the sun. So, the part facing the sun has day and the part away from the sun has night. As the earth rotates different parts are exposed to the sun, and put in the shade. There are 360 degrees of longitude on the earth, so what the scientists did was divide these by 24, that means that there are 15 degrees of longitude in every time zone. If you travel more than that, you have to adjust your watch. You may be wondering why time zones are measured in degrees of longitude and not miles or kilometers.

Who came up with the idea of time zones?
The idea to divide the Earth into time zones was proposed by the Canadian railway planner And engineer Sir Sanford Fleming ( ) in the 1870’s.

When were time zones first used?
Time zones were first used on November 18, 1883 by railroads in order to make their schedules less confusing. World time zones were determined in 1884, at an international conference in Washington, D.C. Each of the 24 world time zones are about 15 degrees wide and differ by one hour.

Early Time Keeping Until about 100 years ago, each city set its clocks to local time -- noon was the time when the Sun was at its highest in the sky, as viewed from each city. Neighboring cities needed to set their clocks differently. For Example, when it was 8:00 in New York City, it was 8:12 in Boston (Boston is about 3 degrees east of New York). Before modern transportation and communication, this difference didn't really matter.

A Need for Synchronized Schedules
Once railroads were built, this became very awkward. Train schedules needed to be written using common time settings that everybody agreed to, so the U.S. railroad companies adopted the idea of time zones. This was soon extended internationally, with the world being divided into 24 time zones, each one a long strip from North Pole to South Pole, about 15 degrees of longitude wide. All the people in one time zone would set their clock the same way.

How to Calculate Time Zones

How to Calculate Time Zones
In order to calculate time zones one should first determine what time zone they are in. For example Pensacola, FL is in the United States Central Time Zone (which is six time zones west of GMT or Time Zone 0 and is represented on the map on the previous slide by -6).

Locate it on the previous map. For Example Naples Italy is in Time
Next determine the location you wish to find out what time it is. Locate it on the previous map. For Example Naples Italy is in Time Zone +1 of Greenwich Mean Time (GMT Time Zone 0). Simply count the time zones between the two places and add that many time zones if you count to the east or subtract the number of time zones if you count to the west. The difference between -6 time zone (United States Central Time) and +1 time zone (Naples, Italy) is 7 time zones to the east which means you would add 7 hours to determine the time difference between Pensacola, FL and Naples, Italy. So if it is 1pm in Pensacola, FL it is 8pm in Naples, Italy.

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