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.
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.
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.
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.
about measuring distance on a globe.
Learn about the
Learn about the analemma.
Learn about the
influences of sun and heat on the earth, global warming.