The Earth and Its Movements

Introduction

In Primary school, you learnt about the solar system and the position of the planet earth in relation to the sun. In this chapter, you are going to learn about the shape of the earth, the movements of the earth and how these movements affect time, climate and people’s ways of life in the different parts of the world.

What Shape is the Earth?

The earth is the planet on which we live. All other living things live on earth. It is the only planet with life. This is because it has enough supply of oxygen and water, which are needed for life. Also, since the earth is not very far from the sun, it has moderate temperatures, that is, it is neither too cold nor too hot. What do you know about the shape of the earth?

Activity 5.1
In pairs, look at Figure 5.1 above and do the following tasks:
1. Identify the positions of the poles, and the equator.
2. Describe what the earth looks like at the poles and at the equator.
3. Describe the shape of the earth.
Through discussion, share what you have written with the class.
You have probably described the earth as a round planet. However, when you look at Figure 5.1, you will realise that the earth is flattened at the North Pole and the South Pole. Again, you will see that it is enlarged at the position of the equator and becomes smaller as one moves towards the poles. So it is not perfectly round or spherical. This shape is called oblate. Therefore, the true shape of the earth is an oblate spheroid or a geoid. The bulging of the earth at the equator is caused by its rotation.

Movements of the Earth

Activity 5.2
1. Draw a diagram showing the shape of the earth and on it include the main lines of latitude and longitude.
2. Conduct internet or library research about the solar system and make notes with diagrams.
3. Find out why it is called so and how many planets make up the solar system.
All the nine planets in the solar system move around the sun. Each follows a particular path called an orbit. No planet enters the orbit of another planet. Imagine what would happen if two huge planets met.
Because you live on the surface of the moving earth, it is not easy to know that it is actually moving. In fact, the earth makes two types of movement.
Firstly, it spins or rotates on its axis. This movement is called the earth’s rotation. Secondly, it moves around the sun. This is called the earth’s revolution.

The rotation of the earth

What time of the day do you feel the most heat outside? Is it hottest during the middle of the night, during the morning, or during the afternoon? If we receive heat from the same sun, with the same temperature, then why
doesn’t temperature outside remain the same the whole day? All these happen because every day the earth rotates on its axis. Figure 5.2 shows the earth’s axis.

Activity 5.3
1. In pairs, get a globe or football, torch and marker or bold ink.
2. Using a marker or bold ink, put an X-mark on the ball, in case you do not have a globe.
3. Hold the globe or ball in your hands such that your friend standing at the opposite side flashes a torch upon it.
4. Spin the globe or ball and observe what happens.
5. Write a paragraph to explain what you have observed.
6. Explain how we get daytime and night-time on earth.

In a similar way, the earth normally moves on its axis from the east to the west once every 24 hours. That is one complete rotation. This is why we see the sun rising in the east and setting in the west every day. As the earth
rotates, it exposes half of itself to the sun while the rest of it is in darkness. The part of the earth facing the sun experiences daytime while the one hidden away is in night-time.

Why places on earth have different time
Apart from causing day and night, the rotation of the earth also results in time differences. For every 15 degrees of longitude we move from the Prime Meridian eastwards, we gain one hour. And as we move westwards, for every 15 degrees of longitude, we lose one hour.

The sun reaches the highest position in the sky at 12 noon. At that time, the Greenwich Meridian lies under the sun. This is called 12 noon local time along the Prime Meridian. Local time at the Greenwich is called Greenwich Mean Time or simply GMT. All meridians to the east of the Greenwich Meridian have sunrise before the Prime Meridian. Local time along these meridians is ahead of GMT. Meridians to the west have sunrise after the Greenwich, and the local time is behind GMT. See Figure 5.4.

Activity 5.4
1. If the time at place X is 1600 hours and GMT is 1400 hours, find out:
i) How many hours X is from GMT.
ii) The longitude at which place X lies.
2. If the time at Greenwich is 7.00 GMT, calculate the time at:
(i) 750E

(ii) 750 W

Revolution of the earth
The earth takes one year or 365¼ days to complete its journey around the sun. This journey is called a revolution. After every four years, the earth takes 366 days to complete this same journey. The fourth year is called a leap year. All other years have 365 days. The earth’s axis always points in the same direction in the sky. It is permanently tilted at an angle of 66 and a half degrees to the earth’s path.

The movement of the earth around the sun leads to changes in the position of the latitudes in relation to the sun. On 21st March, the sun is overhead at the equator. This is called the equinox. Another Equinox occurs on 23rd September. On 21st June, the sun is overhead at midday at the Tropic of Cancer. This is called the summer solstice. On 22nd December, the sun is overhead at midday along the Tropic of Capricorn. This is called the winter solstice.

Activity 5.5
Study Figure 5.4 above and do the following:
1. Find out the hottest months in places along the equator.
2. When is it summer in the Northern Hemisphere?
3. When is it spring in the Southern Hemisphere?
4. When is it summer in the Southern Hemisphere?

5. Conduct internet or library research about how the four seasons influence people’s activities and way of life.

Why is the Earth Divided into Zones with Different Temperatures?

Activity: 5.6
1. Conduct internet or library research about world climate zones and in your notebook:
i) draw the earth, mark and name the equator, the tropic lines, and the Arctic and Antarctic Circles.
ii) mark out the tropical, temperate and the polar zones.
iii) describe the characteristics of each of these zones.
iv) explain the other factors that cause temperature differences in an area.
v) share your report with the class through discussion.

Look at Figure 5.2 again and find out whether the earth is upright. If it is not upright, estimate the angle at which it is inclined. You have realised that the earth slants at an angle of 66.5 degrees from the sun. This makes different places on earth lie at different angles from the sun. The places are hit by the sun rays at different angles and so they receive different amounts of heat. At the equator, the sun rays strike the earth directly over a small area. The heat is concentrated over a small area, thus increasing the temperatures.

At higher latitudes, the rays are slanting and spread over a large area. The heat is spread over a large area, thus reducing the temperatures. The distance travelled by the sun rays to strike different places varies. The distance is short at the equator while at the poles it is very long. This partly brings about the temperature differences. The shorter the rays, the greater the heat and the longer the rays, the less the heat. This explains why the temperatures in the areas near the equator are constantly hot. The division of the earth into zones is based on the differences in temperature.

Chapter Summary
In this chapter, you have learnt:
1. that the earth is the only planet with living things because it has large amounts of water and oxygen, and moderate temperatures.
2. that the earth is spherical in shape.
3. that the sun lies at the centre of the solar system and all planets in this system move around the sun following particular orbits.
4. that the earth makes two movements: rotation and revolution.
5. that the rotation of the earth leads to day and night while the revolution causes seasons.
6. how the distance of a place farther north and south of the equator brings about differences in temperature and climate.