Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Explain Earth's rotation and revolution and their effects.
• Describe the Moon's phases and their cause.
• Explain solar and lunar eclipses.
• Explain how the Sun and Moon create tides.

Lesson summary

The relationships between the Earth, Moon, and Sun are fundamental to understanding our planet and its place in the solar system. This week, we will delve into the intricacies of how these celestial bodies interact and the effects of those interactions, such as day and night, seasons, and the phases of the Moon. Understanding these concepts is crucial because they directly affect our daily lives here in South Africa. For instance, knowing the phases of the Moon can be culturally significant for certain traditions and can also impact agricultural practices.

Lesson notes

a)

Earth's Rotation and Revolution: Rotation: The Earth rotates on its axis, an imaginary line passing through the North and South Poles. One complete rotation takes approximately 24 hours, which we experience as a day and night cycle. As South Africa rotates into the sunlight, we experience sunrise, and as we rotate away, we experience sunset. The speed of rotation at the equator is faster than near the poles. This difference in rotational speed contributes to global weather patterns.

Revolution: The Earth revolves around the Sun in an elliptical orbit. One complete revolution takes approximately 365.25 days, which we define as a year. The Earth's axis is tilted at an angle of approximately 23.5 degrees relative to its orbital plane (the plane of Earth's orbit around the Sun). This tilt is the reason for the seasons.

Seasons Explained: Because of the Earth's tilt, different parts of the Earth receive more direct sunlight at different times of the year. When the Northern Hemisphere is tilted towards the Sun, it experiences summer, while the Southern Hemisphere experiences winter. Conversely, when the Southern Hemisphere is tilted towards the Sun, it experiences summer, while the Northern Hemisphere experiences winter. In South Africa, our summer months are from December to February, and our winter months are from June to August. The equinoxes (spring and autumn) occur when neither hemisphere is tilted towards the Sun, resulting in approximately equal day and night lengths globally.

South African Perspective: The seasons have a direct impact on agriculture in South Africa. Farmers need to plan their planting and harvesting seasons based on the expected rainfall and temperature patterns associated with each season. For example, maize, a staple food crop, is typically planted during the summer months when rainfall is abundant in many parts of the country. b)

Moon's Phases: The Moon does not produce its own light; it reflects light from the Sun. As the Moon orbits the Earth, we see different amounts of its illuminated surface, resulting in the phases of the Moon. The phases of the Moon follow a cycle that takes approximately 29.5 days (a lunar month).

New Moon: The Moon is between the Earth and the Sun, and its illuminated side faces away from the Earth. We cannot see the New Moon.

Waxing Crescent: A thin sliver of the Moon becomes visible as it moves away from the Sun in our sky. "Waxing" means getting bigger.

First Quarter: Half of the Moon's face is illuminated.

Waxing Gibbous: More than half of the Moon is illuminated. "Gibbous" means bulging.

Full Moon: The entire face of the Moon is illuminated. The Earth is between the Sun and the Moon.

Waning Gibbous: The illuminated portion of the Moon decreases. "Waning" means getting smaller.

Third Quarter: Half of the Moon's face is illuminated again, but the opposite half compared to the First Quarter.

Waning Crescent: A thin sliver of the Moon is visible again, as it moves closer to the Sun in our sky.

Mnemonic: Remember the phases using the mnemonic device: NEWts WAXing GIBe FULLy WANted QUARTERs. c)

Eclipses: Eclipses occur when one celestial body blocks the light from another.

Solar Eclipse: A solar eclipse occurs when the Moon passes between the Sun and the Earth, blocking the Sun's light. Solar eclipses can be partial (part of the Sun is blocked), total (the entire Sun is blocked), or annular (the Moon appears as a dark disk surrounded by a ring of sunlight). Never look directly at the Sun during a solar eclipse without proper eye protection.* This can cause permanent eye damage.

Lunar Eclipse: A lunar eclipse occurs when the Earth passes between the Sun and the Moon, casting a shadow on the Moon. Lunar eclipses can be partial (part of the Moon is in shadow) or total (the entire Moon is in shadow). During a total lunar eclipse, the Moon may appear reddish due to the scattering of sunlight by the Earth's atmosphere.

South African Experience: Both solar and lunar eclipses can be observed from South Africa, although total solar eclipses are relatively rare. Local astronomy clubs and science centers often organize viewing events when eclipses occur, providing opportunities for the public to learn more about these phenomena. d)

Tides: Tides are the rise and fall of sea levels caused by the gravitational forces of the Moon and the Sun. The Moon's gravity has a greater effect on tides than the Sun's gravity because the Moon is much closer to the Earth.

High Tides: Occur when the Moon's gravity pulls the water towards it, creating a bulge on the side of the Earth facing the Moon. A corresponding bulge occurs on the opposite side of the Earth due to inertia.

Low Tides: Occur in the areas between the two bulges.

Spring Tides: Occur during the New Moon and Full Moon phases when the Sun, Earth, and Moon are aligned. The gravitational forces of the Sun and Moon combine to produce higher high tides and lower low tides.