In the old days each town would keep their own times, with a town clock being
set to noon when the Sun reached its highest point in the sky. However by the
late 19th century rail had begun to move people across great distances. Schedules
became very confusing as each stop was based on a different towns time. Thus a
need emerged to standardize time so rail could operate more efficiently. In 1884
delegates from 26 countries met in Washington DC to agree on a Prime
Meridian as a common zero for longitude. The result was Greenwich, England being
selected as the international standard for zero degrees longitude and
established the 24 time zones. Today most countries follow the hourly deviations,
though they do not always align with longitudes for reasons such as keeping
the entire country in one time zone, or altering times at certain times of the
year, for example daylight savings time. Time zones are based on the fact that
the earth moves 15 degrees longitude each hour. Since there are 24 hours in a
day, there are 24 standard time zones. Time
zones are counted from the prime meridian which is zero degrees longitude.
Each time zone is counted at 15 degree intervals and extends 7 and a half
degrees either side of a central meridian. For example Sydney, Australia
lies in the 150 degrees east central meridian, and the time zone includes all
locations between 142 and 1/2 degrees east and 157 and 1/2 degrees east now. I
will show you how to calculate time zones. First determine whether the time
zone is in the eastern or western hemisphere. If the time zone is east of
the Prime Meridian, such as Australia, it will be at an earlier time than Greenwich
Mean Time or GMT. If it is west of the Prime Meridian such as Canada, it will be
a later time. Second count how many time zones away from Greenwich the location
is. For example Greenland is one time zone to the west of Greenwich so I will
subtract one hour from GMT to determine of a time there.
Oslo in Norway is one time zone to the east the Greenwich so I will add just
one hour to the current Greenwich Mean Time. When counting the number of time
zones away a location is, count the time zone your location is in but do not
count the time zone that greenwich is in. For example, I will count for
Buenos Aires, Argentina. One, two, three, four. So if the time is 4:34 for GMT I
will subtract four hours to find out the time for Buenos Aires. The time will be
zero 34. The International Dateline is an imaginary line, lying 180 degrees
in the line of longitude on the opposite side of the earth to the prime meridian.
This line separates one calendar day from another. The International Dateline
deviates in places to avoid crossing any land. In 1994 Kiribati shifted the
dateline 2,000 miles east to keep the entire country in the eastern hemisphere.
In 2011 Samoa and Tokelau skipped the day and moved westward to be in the eastern
hemisphere so they could have the same date as Australia in New Zealand,
their main trading partners.