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Solar
Time changes four minutes for each degree of longitude, making one hour for
every fifteen degrees. This is because it takes four minutes for the Earth to
turn enough for the vertical rays of the sun to sweep over one degree of
longitude.
When its is noon on the Prime Meridian, it is four minutes past noon at 1
east of the Prime Meridian, and four times before noon at 1 west of the Prime
Meridian. We number the hours from noon. PM refers to the hours after midday to
twelve midnight, and AM refers to the hours from midnight to midday.
Standard Time
To avoid the confusion that would result if each place used its own local
time, the United States has been divided into four time zones corresponding
roughly with the 75th, 90th, 105th, and 120th West Meridians. Alaska and Hawaii
have two additional time zones. Each time zone differs by one hour. Commencing
with the east, they are known as Eastern, Central, Mountain, and Pacific time
zones.
A person traveling from one time zone to another changes his watch only when
leaving one zone and entering another. Going east the watch is set ahead one
hour when crossing a dividing line and set back one hour when crossing a
dividing line while traveling west. The lines between zones are irregular due to
local adjustments.
Canada has the same time zones as the United States, plus two additional
time zones for the extreme eastern part of the country. Standardization of time
has become almost universal.
Time Dial
Most globes have a Time Dial at the North Pole. This is a disk that can be
turned and is divided into 24 equal parts, corresponding to the Meridians shown
on the Globe, and marked to represent the hours AM and PM of the Solar Day. The
dial may be used to quickly show the comparative time at any two points on the
Globe.
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By turning the Time Dial to a position so that any
given hour is over the correct meridian, the corresponding time of any
other point on the Globe may be determined. For example: the 75th
Meridian West passes through New York. By turning the time dial so that
1PM is on the 75th Meridian West, it is seen that 6 PM on the time dial
is on the Prime Meridian and therefore it is 6 PM in London when it is 1
PM in New York.
International Date Line
This is often referred to as the Sunday-Monday line
and follows approximately the180th Meridian. The date line departs from
the 180th meridian sufficiently to pass through the Bering Strait, west
of the Aleutian Islands, and east of certain islands near New Zealand.
When it is Sunday on the American side of the
International Date Line, it is Monday on the Asiatic side. The 180th
Meridian is chosen as the one at which this change should be made
because it is farthest from the Prime Meridian. |
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Learn
about measuring distance on a globe.
Learn about the
Earth's motion.
Learn about the analemma.
Learn about the
influences of sun and heat on the earth, global warming.
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1-World Global Gifts
