TIME ZONES

Why is it a different hour in each city?

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?

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.

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.

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.

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.

TIME ZONESInternational 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?

TIME ZONES• In the International Meridian Conference it

was decided to use the Time Zones System.

TIME ZONES

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

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.

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

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

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)

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)?