Longitude and latitude

We are learning to:

• describe the characteristics and purpose of lines of longitude and latitude

• identify and name the significant lines of latitude and longitude

• explain the purpose of latitude and longitude lines

• locate places and give locations of places using lines of latitude and longitude.

Characteristics of lines of latitude

Lines of latitude are imaginary lines that run horizontally from east to west. Latitude is measured in degrees (º) north or south of the Equator. The North Pole is at 90º north and the South Pole is at 90º south. There are 180 lines of latitude. The main lines of latitude you will notice on maps are:

• the Tropic of Cancer (23.5ºN)

• the Tropic of Capricorn (23.5ºS)

• the Arctic Circle (66.5ºN)

• the Antarctic Circle (66.5ºS)

• the North Pole (90ºN)

• the South Pole (90ºS).

The Equator is at 0º – this line divides the Earth into northern and southern hemispheres.

Characteristics of lines of longitude

Lines of longitude are imaginary lines that run vertically from north to south. They are measured in degrees.

The most important line of longitude is the prime meridian (0º). This is also known as the Greenwich Meridian, because it runs through the Greenwich Observatory in Britain. Exactly halfway round the Earth from the Greenwich Meridian is the International Date Line, at 180º. The Eastern and Western hemispheres are situated to the east and west of the prime meridian.

On the map to the right, you can see that Cayo Coco’s latitude is halfway between 22ºN and 23ºN. Its longitude is halfway between 78ºW and 79ºW. We can write its position as 22.5°N, 78.5°W.

In pairs discuss the purpose of lines of latitude and longitude, and identify two characteristics of both.

Purpose of longitude and latitude

Remember, lines of longitude and latitude do not exist in real life. They are tools that help people locate places on maps and describe positions. Look up Trinidad and Tobago in an atlas. Next to the name of Trinidad and Tobago , you will see the page number, followed by a grid reference and then the coordinates (10º37’N 61º30’W). The coordinates for Trinidad and Tobago tell you that it is situated 10 degrees and 37 minutes north of the Equator and 61 degrees 30 minutes west of the Greenwich Meridian. The table shows you the longitude and latitude of cities in Trinidad and Tobago, as well as some of the major cities of the world.

The International Date Line

In 1884, delegates attended the International Meridian Conference in the USA to work out an international prime meridian. They agreed on the Greenwich Meridian. Fortunately, this meant that the 180º meridian (exactly opposite the 0º meridian) mostly passed over water. This line is known as the International Date Line (IDL). When it is Monday to the left of the IDL, it is Sunday to the right of the IDL. People who cross the line from east to west skip forward by one day. People who cross the line from west to east repeat the day.

Exercise

1. Name the line of latitude that is: a) furthest to the north and south of the Equator b) at 23ºS and at 23ºN.

2. Lines of longitude are given in degrees east or west of the Greenwich Meridian. Is the International Date Line 180ºW or 180ºE of the Greenwich Meridian? Explain your answer.

3. Complete the following sentences: a) St Lucia is situated ____________ of the Equator and _________ of the Greenwich Meridian. b) Belize is situated ____________ of the Equator and _________ of the Greenwich Meridian.

4. Look at the cities in the table above. Which city/cities: a) are in the southern hemisphere? b) is furthest north? c) is furthest east? d) is closest to the Equator?

CityCoordinates

Activity

On a globe of the world, find the Equator, the Tropic of Cancer and the Tropic of Capricorn. Also identify the Greenwich Meridian and the International Date Line. For each line, identify two countries that have that line passing through them.

Key vocabulary line of latitude hemisphere line of longitude prime meridian

The longitude problem

Up until the 18th century, there was no internationally accepted system for calculating longitude. This was a problem for sailors because as soon as they set sail they were ‘at sea’, an expression that came to mean lost.

Scientists had problems working out how far they had travelled from east to west. It was easier to calculate latitude because they could work out the length of the day, the height of the sun or the distance of the stars above the horizon.

Early mariners used a ‘dead reckoning’. They threw a log overboard and timed how long it took for the log to pass the boat. They would use this to work out how fast they were travelling, and from that calculation work out how far they had travelled. However, this was very inaccurate because the boat’s speed could easily increase or decrease.

In 1707, five British naval ships got lost in heavy fog. Because there was no system of longitude, they miscalculated where they were and 2,000 sailors died as a result. In response to this, in 1714, the English parliament offered a prize of £20,000 to anyone who could solve the longitude problem. One solution was to measure the time at sea and to measure the time on a clock at another place and compare the difference between the two clocks.

However, this was not simple. Clocks at this time operated using metal pendulums. A pendulum swings around wildly during storms at sea and metal expands and contracts with changes in temperature, which affected how they worked.

A clockmaker called John Harrison eventually won the prize, after many years of work.

Today, Global Positioning Systems (GPS) have made it possible to identify coordinates from anywhere on Earth.

Exercise

1. Name three ancient civilisations that learned to navigate using the Sun, Moon and stars.

2. Name two ancient Greeks whose discoveries helped navigators to find their latitude position.

3. Why was it easier to work out latitude than longitude?

4. Create a jingle about the history of latitude and longitude

5. Create a timeline about the history of longitude and latitude. Add photos from the internet.

Research

Work in groups. Each group chooses and using the internet researches one of the following navigation instruments:

• back staff

• kamal

• astrolabe

• octant

• sextant

• quadrant

• bearing compass

• magnetic compass

• ring dial.

As a class, make a timeline with pictures of the different navigational instruments at the correct places, showing when they were invented or used.

Activity

In groups, create a jingle of two stanzas using the names of the people who created longitude and latitude.

(Tip: Eratosthenes and John Harrison.)

Key vocabulary pendulum

Global Positioning System (GPS)