Activity 1: Our position on Earth: latitude
Students find their city on the globe. Use the app which shows you the view of the night sky on an iPad or mobile phone based on your location. The app can do this based on your location or you can type in your location (or the location of your nearest city).
If there is one device for the class, ask for a volunteer to locate the North Star if you are in the Northern hemisphere or the Coalsack if you are in the Southern hemisphere.
The student rotates and moves the device around until they find the North Star or Coalsack. Use the compass included on the app or another student can use a compass to find North or South as appropriate. Ways to locate these objects are included in the background information.
Once they have located the object ask them to keep pointing the device in the same direction.
Ask for a second student to measure the angle of the North Star or Coalsack above the floor/ horizon with a protractor. Alternatively if you have a quadrant, the night sky view can be presented on a vertical screen and students look through a quadrant to measure the angle of the object above the horizon. This activity could also use a quadrant to look at the real night sky on a clear night.
Students note the angle they found. (This angle is the latitude of the place you are in).
Ask students if everyone sees the North star (or Coalsack) at the same angle? Why/why not? Try inputting different locations around the world in both hemispheres and measuring the angles of the north star (or coalsack if you saw this in your location) above the horizon or noting if you cannot see it. Point out where they are on the globe to compare. Come to the conclusion that people in the Northern hemisphere can see the north star (and people in the southern hemisphere cannot) but see it at different angles depending on their location (or equivalent with the Southern hemisphere and the coalsack). Explain that this angle tells us how North or South we are. This is called our latitude. Write this word on the board.
Show students the lines of latitude on the globe. Have they heard of the equator and do they know where it is? Show them and explain that the equator goes around the middle of the Earth. We label the equator 0°. If we travel north, the angle increases until we reach the North Pole which is at 90°N, and travelling south from the equator, the angle increases to the South Pole at 90°S.
Ask students how we could use this information. Come to the conclusion that we could measure the angle of the North star/ Coalsack to find our latitude and help find our location on Earth, as explorers travelling by sea did.
Activity 2: Our position on Earth: longitude
Ask students if knowing how North or South we are is enough to pinpoint our exact position on Earth. The answer is no. Even knowing how North or South we are, we could still be anywhere along the same parallel line, e.g. Nice in France and Toronto in Canada are both at a latitude of 43.7° but are very far away from each other.
Ask students to say/ point out the location of the two different cities on the globe. Ask students if they can think of another piece of information they could use to distinguish the two cities: how are they different?
Come to the conclusion the horizontal position on the globe, i.e. the East-West location are what separates them. Explain we divide the globe using lines from the North to the South pole, like the segments of a satsuma. Ask students to point out the lines on the globe. Explain we call these lines of longitude and write this word on the board.
Ask students to imagine getting on a plane and flying in a plane travelling East. The plane would travel in a circle. Ask students how many degrees there are in a circle. Answer: 360°.
Ask students if there is anywhere obvious to start counting the lines of longitude from? No, there isn’t as the Earth is a sphere. Explain that in 1884 it was agreed that the line passing through London would be the Prime Meridian with a longitude of 0°. Moving East, the angle increases up to 90°E and then 180°. Moving west from the prime meridian, the angle increases up to 90°W and then up to 180°.
Students use the globe or a world map to find the longitude of their location. They note the result. Students can now give the full coordinates, longitude and latitude, of their location.
Ask students if they know of a way to find your location more precisely than using a globe. Can we use technology? How can you navigate in a car?
Explain that today, we are able to give much more precise coordinates for a location using the Global Positioning System (GPS). This is a network of satellites orbiting the Earth in space used for navigation. If a GPS receiver can ‘see’, i.e. communicate, with four or more of these satellites, its location can be found by using a triangulation technique.
Activity 3: Direction of the Sun
Cape Verde is just off the west coast of northern Africa with a latitude of 15°N, placing it between the equator (0°) and the Tropic of Cancer (23.5°N). Using this knowledge, we investigate why the inhabitants of Cape Verde sometimes see the midday sun in the north and at other times in the south.
Make sure students can all see the globe. Ask students to find their location, the Equator and the north-south axis on the globe. Could wrap a piece of string around globe to show location of the equator.
Light up the globe with a lamp and tilt the axis slightly towards the light so that the area around the North Pole, outlined by the Arctic Circle, is lit up. Place a pencil against the globe and move it vertically between the North and South poles on the globe. Show that if you move the pencil to the north, its shadow will point north and the Sun will be to the south. If you move the pencil towards the south, its shadow will point south because the midday sun is to the north.
Ask students what they observe. Moving northwards, ask which direction is the shadow pointing in if the pencil is above the Sun? (To the south). Moving southwards, ask which direction is the shadow pointing in if the pencil is below the Sun? (To the North). They should also be able to see that there is a point where the pencil casts no shadow.
Tell students there is a circle of latitude, like the equator but it does not go around the middle of the Earth. It is called the ‘Tropic of Cancer’ and is at 23.5º north of the equator. Can they use what they learnt earlier to work out where this is? Place the pencil against it at this latitude.
Tilt the axis so that there is no shadow when the pencil is on the Tropic of Cancer. The axis of the globe is now inclined 23.5º relative to the vertical. For anyone standing on the surface of the Earth at the point where the pencil meets the globe and who can see the Sun in the position of the lamp, it is midday on June 21st.
Tell students the latitude of Cape Verde is 15°N. Can they work out where this is on the globe? Is it north or south of the Tropic of Cancer? You may wish to place a sticker to indicate its location.
Keeping the same tilt as above (so the Sun is directly above the Tropic of Cancer), move the pencil to the location of Cape Verde. Ask students in which direction the shadow is pointing (should be to the south).
Now move the Earth/ globe so that is on the opposite side of the Sun/ lamp to represent six months later. The north axis should now be pointing away from the Sun at the same angle so the South pole is lit. Ensure that Cape Verde is pointing towards the Sun. Ask students over which part of the Earth is the Sun directly over and demonstrate as before with the shadow of the pencil. (The Sun is directly over the Tropic of Capricorn which is 23.5ºS).
Place the pencil on Cape Verde. Ask students what they notice about the direction of the shadow. (It is now pointing to the North).
So people in Cape Verde sometimes have midday shadows to the North or the South depending on the time of year because the direction of the midday sun changes during the year. In fact, all people who can see the midday sun to the north or south at different times of the year live between the Tropic of Cancer and the Tropic of Capricorn.
The answer to our question is simple. As the north-south axis of the Earth is inclined relative to the plane of the Earth’s orbit, the midday sun on the first day of summer south of the Tropic of Cancer will be in the north. But in the autumn, between the Equator and the Tropic of Cancer, the midday sun will be in the south.