Time zones

1. TIME ZONES

2. Why is it a different hour in each city?

3. TIME ZONES
• One of the consequences of
the Earth´s rotation is the
existence of the hours of the
day.
• The Sun does not strike on the
Earth´s surface in the same
way and at the same time.
Consequently, each part of the
Earth has a different hour in
relation of the illumination of
the Sun.
Why does each part of the Earth has a different hour?

4. TIME ZONES
• Until the 19th century
each city had a clock
with its own hour.
• Consequently, it was
difficult to organise the
means of transport
among cities and
countries.

5. TIME ZONES
• In October 1884, the International Meridian
Conference was held in Washington, D.C., in
the United States, to determine a prime meridian for
international use to establish the mean time in the
world.

6. TIME ZONES
• In the International Meridian
Conference, the subject to
discuss was the choice of "a
meridian to be employed as
a common zero of longitude
and standard of time
reckoning throughout the
world".
• It resulted in selection of the
Greenwich Meridian as an
international standard for
zero degrees longitude.

7. TIME ZONES
• The Royal Observatory,
Greenwich, is home of the
Prime Meridian of the
World, Longitude 000⁰ 00’
00’’.
It´s located toi the south-east
of London.

8. TIME ZONES
International Meridian Conference
1884
The meridian passing through the Observatory of
Greenwich (Prime Meridian of the World)
Was adopted for charting
(Longitude)
The universal day for the world
begins at the moment of mean
midnight on the Greenwich
meridian.
(Greenwich Mean Time or GMT)
When does the universal day for the world begin?

9. TIME ZONES
• In the International Meridian Conference it
was decided to use the Time Zones System.

10. TIME ZONES
• Scientific knowledge of the Earth´s rotation
allows us to divide the solar day into 24 equal
parts, known as hours.

11. TIME ZONES
• We can divide the Earth into 24 imaginary
zones that correspond to an hour of the
Earth´s rotation.
This are called
TIME ZONES and
we can use them
to measure time.

12. TIME ZONES
• The time zones are defined by dividing the 360⁰ of
the earthly sphere into the 24 hours of the day. As a
result, each time zone corresponds to 15⁰ of the
Earth´s circumference and is the equivalent of 1 hour.
360 ⁰ / 24 hrs = 15 ⁰
15 ⁰ = 1 hour

13. TIME ZONES
• To establish the time in any
one place, we use the time
zone located at the 0⁰
longitude as reference.
• This line of longitude is
called 0⁰ Meridian,
Greenwich Meridian or
Prime Meridian of the
World.
On foot to the
West
On foot to the
East

14. TIME ZONES
• Using the Greenwich Meridian as a reference, a clock
is set forward one hour for each time zone to the
east, and set back one hour for each time zone to the
west.
Greenwich Mean Time
(GMT)

15. If it is midnight in London:
• What time is it in New York?
• What time is it in Hong Kong?
• What time is it in Tokyo?
• What time is it in Sidney?
• What time is it in Paris?
• What time is it in Berlin?
• What time is it in Moscow?
• If Galicia is to the north of Portugal,
why don´t they use the same hour as
Portugal?
• If China has five time zones, why is it
the same hour in all the country?
• If Spain is in the same time zone as
the U.K., why does Spain has one
more hour than London (GMT+1)?