Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Describe the effect of Earth's rotation on day and night.
• Explain how Earth's tilt causes seasons in South Africa.
• Describe the phases of the Moon.
• Identify the relationship between the Moon and tides.
• Predict effects of these relationships on daily life.

Lesson summary

The Earth, Moon, and Sun are constantly interacting, creating phenomena that profoundly affect our daily lives. Understanding these relationships is crucial for comprehending seasons, tides, day and night, and even how we tell time. Here in South Africa, where agriculture and fishing are vital industries, understanding these cycles is especially important. For instance, farmers need to know when to plant based on the seasons, which are a direct result of the Earth's tilt and orbit around the Sun. Fishermen rely on tide predictions, driven by the Moon's gravitational pull, to ensure safe and productive fishing expeditions.

Lesson notes

2. 1.

Earth's Rotation and Day/Night: The Earth is constantly spinning on its axis, an imaginary line running through the North and South Poles. This spinning is called rotation. It takes approximately 24 hours for the Earth to complete one rotation, which gives us day and night. As South Africa rotates towards the Sun, we experience sunrise. When we rotate away from the Sun, we experience sunset. The part of the Earth facing the Sun experiences day, while the part facing away experiences night. This is a continuous cycle.

Example: Imagine holding a ball (representing the Earth) and shining a torch (representing the Sun) on it. Only one side of the ball is lit up at a time. If you slowly turn the ball, different parts get lit up, simulating day and night. 2.

2. Earth's Revolution and Seasons: The Earth also revolves (orbits) around the Sun in an elliptical (oval-shaped) path. This revolution takes approximately 365.25 days (one year). The Earth's axis is tilted at an angle of 23.5 degrees relative to its orbital plane. This tilt is the primary reason for the seasons.

How Seasons Work: Summer: When the Earth's North Pole is tilted towards the Sun (around June 21st – the summer solstice in the Northern Hemisphere), the Northern Hemisphere, including South Africa, receives more direct sunlight and longer days, resulting in warmer temperatures (summer in the Southern Hemisphere, winter in the Northern Hemisphere).

Winter: When the Earth's North Pole is tilted away from the Sun (around December 21st – the winter solstice in the Northern Hemisphere), the Southern Hemisphere receives less direct sunlight and shorter days, resulting in colder temperatures (winter in the Southern Hemisphere, summer in the Northern Hemisphere).

Spring and Autumn (Fall): During spring (around March 21st – the vernal equinox) and autumn (around September 22nd – the autumnal equinox), neither hemisphere is tilted significantly towards or away from the Sun. The days and nights are approximately equal in length.

Example: Consider Johannesburg. During the summer months (December-February), Johannesburg experiences long, hot days with the sun high in the sky. In contrast, during the winter months (June-August), the days are shorter, and the sun is lower in the sky, resulting in cooler temperatures. Durban's climate is affected similarly. The annual pattern of temperatures and rainfall determine the growing season for farmers. 2.

3. The Moon's Phases: The Moon orbits the Earth in approximately 27.3 days. The Moon doesn't produce its own light; it reflects sunlight. As the Moon orbits the Earth, we see different amounts of the sunlit portion of the Moon, which gives us the phases of the Moon.

Key Moon Phases: New Moon: The Moon is between the Earth and the Sun, and we cannot see it.

Waxing Crescent: A small sliver of the Moon becomes visible. "Waxing" means growing larger.

First Quarter: Half of the Moon appears lit.

Waxing Gibbous: More than half of the Moon appears lit. "Gibbous" means bulging.

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

Waning Gibbous: The lit portion of the Moon starts to decrease. "Waning" means shrinking.

Third Quarter: Half of the Moon appears lit (the opposite half of the First Quarter).

Waning Crescent: A small sliver of the Moon is visible again.

Example: Think of the Moon as a ball being lit by a light source (the Sun). As the Moon goes around Earth, we see different amounts of the lit portion. 2.

4. The Moon and Tides: Tides are the rise and fall of sea levels caused primarily by the Moon's gravitational pull. The Moon's gravity pulls on the Earth, and this pull is strongest on the side of the Earth closest to the Moon. This pull creates a bulge of water on that side of the Earth. Another bulge occurs on the opposite side of the Earth due to inertia. These bulges are high tides. As the Earth rotates, different locations pass through these bulges, experiencing high and low tides approximately twice a day. The Sun also has a gravitational pull, but its effect on tides is smaller than the Moon's.

Spring Tides and Neap Tides: Spring Tides: When the Sun, Earth, and Moon are aligned (during the new moon and full moon phases), their combined gravitational pull creates higher high tides and lower low tides. These are called Spring Tides (they have nothing to do with the season of Spring).

Neap Tides: When the Sun, Earth, and Moon form a right angle (during the first quarter and third quarter moon phases), the Sun's gravity partially cancels out the Moon's gravity, resulting in less extreme tides. These are called Neap Tides.

Example: In coastal areas of South Africa like Cape Town or Durban, the times of high and low tides change daily. Fishermen and surfers need to know the tide schedules to plan their activities. Guided Practice (With Solutions)

Question 1: Why is it daytime in Johannesburg while it is nighttime in New York?

Solution: This is because the Earth is a sphere, and it rotates on its axis.