Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Describe the movements of the Earth, Moon, and Sun.
• Explain the causes of day, night, seasons, and lunar phases.
• Describe and explain solar and lunar eclipses.
• Explain how the Moon affects the tides.
• Understand the duration of a year and a day.

Lesson summary

This week, we will explore the fascinating relationships between the Earth, the Moon, and the Sun. Understanding these relationships is fundamental to comprehending many natural phenomena we experience daily, from the changing seasons to the tides and the phases of the moon. This knowledge is essential not only for scientific understanding but also for practical applications like agriculture (understanding planting seasons) and even navigation and cultural traditions linked to lunar cycles. Imagine how understanding the angle of the sun can help rural communities optimize solar panel placement for electricity. Or how knowing about the moon phases is important for fishermen in coastal areas.

Lesson notes

2.1 Earth's Rotation and Revolution: Rotation: The Earth spins on its axis, an imaginary line passing through the North and South Poles. This rotation takes approximately 24 hours (more precisely, 23 hours, 56 minutes, and 4 seconds) and is what causes day and night. As South Africa rotates into the sunlight, we experience sunrise; as we rotate away, we experience sunset. Imagine holding a globe and shining a torch on it – only one side is lit at a time.

Revolution: The Earth also orbits, or revolves, around the Sun in an elliptical path. One complete revolution takes approximately 365.25 days (one year). This is why we have leap years - to account for the extra 0.25 days each year. 2.2 Earth's Tilt and the Seasons: 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 CRUCIAL for the seasons. As the Earth revolves around the Sun, different parts of the Earth are tilted towards the Sun at different times of the year. When the Northern Hemisphere (where South Africa is located) is tilted towards the Sun, we experience summer (longer days, more direct sunlight, warmer temperatures). When the Northern Hemisphere is tilted away from the Sun, we experience winter (shorter days, less direct sunlight, colder temperatures). The solstices (summer and winter) mark the points when the tilt is at its maximum. The equinoxes (spring and autumn) mark the points when neither hemisphere is tilted towards the Sun, resulting in roughly equal day and night lengths. Summer Solstice (around December 21st in the Southern Hemisphere): The Southern Hemisphere is tilted most directly towards the Sun, resulting in the longest day of the year. Winter Solstice (around June 21st in the Southern Hemisphere): The Southern Hemisphere is tilted most directly away from the Sun, resulting in the shortest day of the year.

Autumn Equinox (around March 21st): Day and night are approximately equal in length.

Spring Equinox (around September 23rd): Day and night are approximately equal in length. 2.3 The Moon's Orbit and Lunar Phases: The Moon orbits the Earth in an elliptical path, taking approximately 27.3 days to complete one orbit (sidereal month).

However, because the Earth is also moving around the Sun, it takes slightly longer (about 29.5 days – synodic month) for the Moon to go through all its phases. The phases of the Moon are caused by the changing angles at which we see the sunlit portion of the Moon as it orbits the Earth.

New Moon: The Moon is between the Earth and the Sun, and we cannot see it because the side facing us is not illuminated.

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

First Quarter: Half of the Moon is illuminated.

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

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

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

Third Quarter: Half of the Moon is illuminated again, but the opposite half from the First Quarter.

Waning Crescent: A sliver of the Moon remains visible as it approaches the Sun. 2.4 Eclipses: Solar Eclipse: A solar eclipse occurs when the Moon passes between the Sun and the Earth, blocking the Sun's light. This can only happen during a New Moon.

Total Solar Eclipse: The Moon completely covers the Sun, turning day into darkness (very rare).

Partial Solar Eclipse: The Moon only partially covers the Sun.

Lunar Eclipse: A lunar eclipse occurs when the Earth passes between the Sun and the Moon, casting a shadow on the Moon. This can only happen during a Full Moon.

Total Lunar Eclipse: The Moon passes completely into the Earth's umbra (the darkest part of the shadow), turning the Moon a reddish color (sometimes called a "blood moon").

Partial Lunar Eclipse: Only part of the Moon passes into the Earth's umbra. 2.5 Tides: Tides are the rise and fall of sea levels caused primarily by the gravitational forces exerted by the Moon and, to a lesser extent, the Sun. The Moon's gravity pulls on the Earth, causing the water on the side of the Earth closest to the Moon to bulge outwards (high tide). A similar bulge occurs on the opposite side of the Earth due to inertia. Areas between these bulges experience low tide.

Spring Tides: Occur during New and Full Moons when the Sun, Earth, and Moon are aligned, resulting in the strongest gravitational pull and the highest high tides and lowest low tides.

Neap Tides: Occur during First and Third Quarter Moons when the Sun, Earth, and Moon form a right angle, resulting in weaker gravitational pull and less extreme tides. Guided Practice (With Solutions)

Question 1: Explain why South Africa experiences different seasons throughout the year.

Solution: South Africa experiences different seasons because the Earth's axis is tilted at 23.5 degrees.