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What Are Time Zones?
By Cindy Grigg
Time zones are based on the
position of the sun. We say that
12 o'clock noon is midday. The
sun is at its highest point,
directly overhead. The Earth
spins or rotates on its axis, like
the spinning of a top. As the
Earth spins, the sun looks like
it moves across the sky from
east to west.
It takes the Earth about 24
hours to make one rotation.
This causes day and night. When one side of the Earth is turned
toward the sun, it is day. On the other side of the Earth, it is nighttime.
The Earth's surface can be thought of as a giant circle. Circles can be
measured in degrees. One whole circle is made up of 360 degrees. It
takes the Earth 24 hours to turn in a circle. So when we divide 360
degrees by 24 hours, we see that the Earth rotates about 15 degrees
each hour.
Each time zone is set up having 15 degrees of longitude. Longitude
lines are the LONG lines you see on a globe. They run from the North
Pole to the South Pole. They are like the LONG sides of a ladder.
There are twenty-four time zones around the world.
Lines of longitude are also called meridians. This word came from
the Latin word for midday. The sun crosses the meridians or lines of
longitude at midday, or halfway between the times of sunrise and
sunset. This is where we get the abbreviations for time of A.M. and
P.M.
A.M. stands for "ante-meridian," which means "before midday" or
before the sun crosses the meridian (line of longitude). P.M. stands for
"post-meridian," which means "after midday" or after the sun has
crossed the meridian.
One special line of longitude is called the International Date Line. It
is the line of 180 degrees longitude. It is special because each day
begins at this line. On the east side of this line, it is late night on
Sunday. On the west side of this line, it is early morning on Monday.
When you travel west across the IDL, you gain a day. When you
travel east, you lose a day.
This line was chosen because it passes through the fewest land areas
on Earth where people live. It would be very confusing if you lived in
a country that was divided by the International Date Line. It would be
Sunday in one part of your country. It would be Monday in the other
part. To make sure this doesn't happen, the International Date Line is
not a straight line. If you look at a globe, you will see that this line
zigzags around any land areas. It does not pass through them.
Inside each time zone, most places have the same time. New York
City in the United States, in North America, and Lima, Peru, in South
America, have the same time. This is because they are both within the
same time zone or between the same two lines of longitude.
Most time zones follow the lines of longitude. Some cities have a line
of longitude passing through them. These cities have had to choose
one time zone or the other to follow. This makes life easier if the same
time is followed throughout the city.
The United States has four time zones for the 48 contiguous states.
These are the states that make up the continental U.S. Alaska and
Hawaii are farther west. They fall in between other lines of longitude.
They have two different time zones. So all 50 of the United States fall
into six different time zones.
What Are Time Zones?
Questions
1. Time zones are based on ______.
A. the government
B. clocks
C. the position of the sun
D. train schedules
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2. How many time zones are there in the world?
A. 12
B. 24
C. 4
D. 6
3. Lines of longitude are also called ______.
A. lines of latitude
B. date lines
C. clock lines
D. meridians
4. Each time zone has ______.
A. fifteen degrees of longitude
B. one hour of time
C. both A and B
D. none of the above
5. Lines of longitude are ______.
A. not parallel to each other
B. running north-south
C. like the long sides of a ladder
D. all of the above
6. What does A.M. mean in time?
A. before midday
B. after morning
C. always moving
D. all of the above
7. What does P.M. mean?
A. after midday
B. after morning
C. added on
D. all of the above
8. The International Date Line is which line of longitude?
A. 360 degrees
B. 180 degrees
C. 90 degrees
D. 0 degrees
9. What happens when you cross the International Date Line
going west?
A. you lose one minute
B. you lose a day
C. you gain a day
D. you gain one minute
10. How many time zones does the fifty United States have?
A. 4
B. 6
C. 12
D. 24
Suppose you are taking an airplane flight from New York City to
Honolulu, Hawaii. You will travel through six time zones. Explain
what time you leave New York and what time you arrive in Hawaii.
How does this make you feel or affect you?
Write a summary of this lesson about time zones.
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