Lesson Notes By Weeks and Term v5 - Grade 4

Lesson Video

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Performance objectives

• Describe the shape of the Earth and give evidence.
• Explain that the Earth spins on its axis (rotation).
• Explain that the Earth orbits the Sun (revolution).
• Relate the rotation of the Earth to day and night.

Lesson summary

This week, we delve deeper into our amazing planet, Earth, and start exploring the vast universe beyond! We'll be focusing on the characteristics of the Earth, particularly its shape and how it moves. Understanding the shape and movements of the Earth is crucial because it directly affects our daily lives here in South Africa. The seasons we experience, the length of the day, and even how farmers plan their planting schedules are all linked to these characteristics. For example, knowing why we have summer and winter is essential for understanding which crops grow best at different times of the year and how to conserve water effectively during drier seasons.

Lesson notes

The Shape of the Earth: For a long time, people believed that the Earth was flat.

However, scientists and explorers have proven that the Earth is a sphere – more accurately, an oblate spheroid. This means it's shaped like a ball that's slightly flattened at the top and bottom. Why isn't the Earth perfectly round? The spinning motion of the Earth causes it to bulge slightly at the equator.

Evidence of Earth's Shape: Ships disappearing hull first over the horizon: When a ship sails away from the shore, you see the bottom of the ship disappear first, followed by the masts. This wouldn't happen if the Earth was flat. Instead, the whole ship would gradually get smaller. Different constellations in different hemispheres: In South Africa (Southern Hemisphere), we see different stars and constellations than people in Europe (Northern Hemisphere). This is because we are looking at different parts of the sky from a curved surface. Imagine shining a torch on a ball – only part of the ball gets lit up at any one time.

Photos from space: Astronauts have taken many photos of the Earth from space, clearly showing its spherical shape.

Circumnavigation: People have sailed and flown around the world, always travelling in the same direction and eventually returning to their starting point. This wouldn't be possible on a flat Earth.

Earth's Rotation: The Earth is constantly spinning, or rotating, on an imaginary line called its axis. The axis runs through the North and South Poles. One complete rotation takes about 24 hours, which we call a day. What causes day and night? As the Earth rotates, different parts of the Earth face the Sun. The part of the Earth facing the Sun experiences day, while the part facing away from the Sun experiences night. As the Earth continues to rotate, the day and night cycle continues.

Example: When it's daytime in Johannesburg, South Africa, it's nighttime in New York, USA. As Johannesburg rotates away from the sun, it will become night there too, and as New York rotates towards the sun, it will become day.

Direction of Rotation: The Earth rotates from west to east. This is why the sun appears to rise in the east and set in the west.

Earth's Revolution: The Earth also moves around the Sun in a path called an orbit. This movement is called revolution. The Earth's orbit is not a perfect circle but an ellipse (an oval shape). One complete revolution around the Sun takes approximately 365.25 days, which we call a year. What causes the seasons? The Earth's axis is tilted at an angle of 23.5 degrees. As the Earth revolves around the Sun, different parts of the Earth are tilted towards the Sun at different times of the year. The hemisphere tilted towards the Sun receives more direct sunlight and experiences summer, while the hemisphere tilted away from the Sun receives less direct sunlight and experiences winter.

Example: When the Southern Hemisphere (including South Africa) is tilted towards the Sun, we experience summer. At the same time, the Northern Hemisphere experiences winter. Six months later, the Northern Hemisphere is tilted towards the Sun, and they experience summer, while we experience winter.

Leap Year: Because it takes 365.25 days for the Earth to revolve around the Sun, we have a leap year every four years. In a leap year, we add an extra day (February 29th) to account for the extra quarter of a day each year. Guided Practice (With Solutions)

Question 1: Imagine you are on a ship sailing away from Durban harbor. Describe what you would see happening to the ship as it travels further away, and explain how this supports the idea that the Earth is round.

Solution: As the ship sails further away, you would first see the hull (the bottom of the ship) disappear over the horizon. Then, gradually, the masts (the tall poles) would disappear last. This happens because the Earth is curved. The curve of the Earth blocks our view of the bottom of the ship first. If the Earth were flat, the entire ship would simply get smaller and smaller until it was too far away to see.

Question 2: Explain how the rotation of the Earth causes day and night in Cape Town.

Solution: The Earth spins on its axis. As Cape Town rotates towards the Sun, it experiences daylight. The sunlight shines on Cape Town, making it daytime. As the Earth continues to rotate, Cape Town moves away from the Sun, and it becomes night. The part of the Earth that Cape Town is on faces away from the Sun, so it is dark.

Question 3: Why do we have different seasons in South Africa? Explain this using the words "revolution," "tilt," and "Sun." Solution: The Earth revolves around the Sun. During its revolution, because the Earth's axis is tilted, different parts of the Earth are tilted towards the Sun at different times of the year. When the Southern Hemisphere (where South Africa is located) is tilted towards the Sun, we receive more direct sunlight and experience summer.