Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Describe the relative sizes and distances of the Earth, Moon, and Sun.
• Explain the causes of day, night, and seasons.
• Explain the phases of the Moon.
• Describe solar and lunar eclipses.
• Explain how the Moon causes tides.

Lesson summary

This week, we delve into the fascinating relationships between the Earth, the Moon, and the Sun. Understanding these relationships is crucial, not just for science class, but also for understanding everyday phenomena we observe right here in South Africa. From predicting tides along our coastlines to understanding the changing seasons that affect agriculture, this knowledge is vital. Imagine knowing exactly when to plant crops based on the sun's position or understanding why the sea retreats further during spring tides – it all comes down to understanding these celestial relationships.

Lesson notes

2.1 Relative Sizes and Distances: The Sun is a star, vastly larger than both the Earth and the Moon. Imagine the Sun as a gigantic soccer ball. If the Sun were that size, the Earth would be approximately the size of a peppercorn about 25 meters away, and the Moon would be even smaller (about the size of a grain of sand) orbiting the Earth about 6 centimeters away.

Sun: Huge, a star, provides light and heat.

Earth: Relatively small compared to the Sun, our planet, rotates and revolves.

Moon: Smaller than Earth, a natural satellite, orbits Earth. It's important to realize that these distances are vast. Light from the Sun takes about 8 minutes to reach Earth. 2.2 Day and Night and the Seasons: Day and Night: The Earth rotates on its axis, an imaginary line running through the North and South Poles. This rotation takes approximately 24 hours, creating day and night. As South Africa rotates towards the sun, we experience sunrise, and as we rotate away, we experience sunset.

Seasons: The Earth's axis is tilted at an angle of approximately 23.5 degrees. This tilt causes different parts of the Earth to receive more direct sunlight at different times of the year as the Earth revolves around the Sun. When the Southern Hemisphere (where South Africa is located) is tilted towards the Sun, we experience summer (more direct sunlight, longer days). When the Southern Hemisphere is tilted away from the Sun, we experience winter (less direct sunlight, shorter days). Spring and autumn occur when neither hemisphere is tilted significantly towards or away from the Sun.

Note: The distance between the Earth and the Sun does NOT primarily cause seasons.

Example: It's December, and school holidays are starting in South Africa! This is summer because the Southern Hemisphere is tilted towards the Sun, resulting in long, hot days. Six months later, in June, we're bundled up in winter clothes because the Southern Hemisphere is tilted away from the Sun, and we experience shorter, cooler days. 2.3 Phases of the Moon: 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 the illuminated portion, resulting in the phases of the Moon.

The main phases are: New Moon: The Moon is between the Earth and the Sun; we cannot see it.

Waxing Crescent: A small sliver of the Moon is visible, increasing in size. ("Waxing" means growing.)

First Quarter: Half of the Moon is visible.

Waxing Gibbous: More than half of the Moon is visible, increasing in size.

Full Moon: The entire face of the Moon is illuminated.

Waning Gibbous: More than half of the Moon is visible, decreasing in size. ("Waning" means shrinking.)

Third Quarter: Half of the Moon is visible.

Waning Crescent: A small sliver of the Moon is visible, decreasing in size. The cycle from New Moon to New Moon takes approximately 29.5 days.

Example: A farmer in KwaZulu-Natal observes that the moon is almost fully illuminated. He knows it will be a Full Moon in the next few days. Traditional farmers often use the phases of the moon to predict rainfall or influence planting and harvesting. 2.4 Eclipses: Solar Eclipse: Occurs when the Moon passes between the Sun and the Earth, blocking the Sun's light. The Moon's shadow falls on a portion of the Earth. Solar eclipses are rare because the Moon's orbit is tilted relative to the Earth's orbit around the Sun. They can only happen during a New Moon.

Warning: Never look directly at the sun during a solar eclipse without proper eye protection.

Lunar Eclipse: Occurs when the Earth passes between the Sun and the Moon, casting a shadow on the Moon. Lunar eclipses can only happen during a Full Moon. The Moon often appears reddish during a lunar eclipse because some sunlight is bent around the Earth and scattered onto the Moon. 2.5 Tides: Tides are the rise and fall of sea levels. They are primarily caused by the Moon's gravitational pull on the Earth. The Moon's gravity pulls strongest on the side of the Earth closest to it, creating a bulge of water. Another bulge occurs on the opposite side of the Earth due to inertia.

High Tide: Occurs when a location is in one of the bulges.

Low Tide: Occurs when a location is between the bulges. The Sun also contributes to tides, but its effect is smaller than the Moon's.

Spring Tides: Occur during New and Full Moons when the Sun, Earth, and Moon are aligned. The combined gravitational pull of the Sun and Moon results in higher high tides and lower low tides.

Neap Tides: Occur during First and Third Quarter Moons when the Sun, Earth, and Moon are at right angles. The Sun's gravity partially cancels out the Moon's, resulting in lower high tides and higher low tides.

Example: Fishermen in Cape Town need to understand the timing of tides to know when to safely launch and retrieve their boats and where fish might be concentrated during high and low tide. Guided Practice (With Solutions)

Question 1: What causes day and night on Earth?