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How do you measure time?
Since the start of time, human beings have looked to the sky to
tell time.
Think of Earth as a rounded (“but not perfectly round”) sphere.
Looking down on a sphere, you see a circle that contains 360 degrees
(360°). Scientists also know that the Sun appears
to move 15° 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° in one hour, how long would it take you
to go through all 360° on the Earth? You must travel a distance
of 15° 24 times to move through the entire 360° (to travel
around the Earth). We have 24 hours in a day that are matched
with 24 time zones around the Earth. At the end of a day, the
Sun has appeared in all 24 time zones. An “additional” 25th
time zone is created in the Pacific Ocean by the international
date line.
Earth’s longitude lines separate time zones,
which are 15° wide. Sometimes, a longitude line will run
through a state or a city and could place the two parts of the
state or city within two different time zones. Usually, the longitude
line at the edge of the time zone goes “around” a
state along its border. This way, an entire state or city would
stay in the same time zone. These time zones extend from the
North Pole all the way to the South Pole, and that is why Virginia
residents are in the same time zone as people near the Panama
Canal or in Miami Beach.
Time zones begin at 0° longitude. This line is called the Prime
Meridian. Time in this zone is called Greenwich
Mean Time (GMT), which is named for the city of
Greenwich, England where you find the Prime Meridian). This
time zone is about 75° from the time zone for Virginia;
75° divided by 15° equals 5 hours (75/15 = 5), which
means that time in England is 5 hours earlier than Virginia
time. In other words, because England is east of Virginia,
you would add five hours to the clock to know the correct
time in England.
If it is 6:00 PM in Virginia, it is 11:00 PM in England (add 5
hours to 6:00 PM). For places west of Virginia, subtract hours
from your time.
The United States is so large it has several time
zones: Atlantic, Eastern, Central, Mountain, Pacific, Alaska,
and Hawaii-Aleutian. As an example, if it is 6:00 PM in the Eastern
time zone, 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” Virginia).
NASA will take a new clock—an atomic
clock—into the sky and put it on the Space Station.
The orbit of the Space Station will help make keeping track
of time easier.
Clocks work because of pendulums that measure
beats of time. We call the beats seconds. The atomic clock works
with lasers. A laser shines on atoms to cool
them. The atoms move slower and the beats are easier to measure.
Many things happen on Earth that change the way atoms work. Atoms
fall quickly on Earth. In space, atoms float around in their
containers, just like astronauts; therefore it is easier to measure
time in space than on Earth.
We already use atomic clocks on Earth. They help drivers find
their way. Sailors, truck drivers, soldiers, hikers, and pilots
use atomic clocks. They are even used in some cars. You might
have seen one. The car has a map display on the dashboard. This
map is drawn by using atomic time signals. Farmers use
atomic clocks, too. The clocks tell the farmer’s tractor
when to water crops and when to put insecticide on the plants. Atomic
clocks will make it easier for people to schedule tasks that
have to be done on a regular basis.
KSNN™ thanks NASAexplores as a source of information.For
more information about this topic and additional teaching resources
go to http://www.nasaexplores.com
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