Lesson Notes By Weeks and Term v5 - Grade 7

Lesson Video

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Performance objectives

• Explain the relative positions and movements of the Earth, Moon, and Sun.
• Describe the cause of the Earth's seasons.
• Describe the phases of the Moon.
• Differentiate between solar and lunar eclipses.
• Explain the role of gravity in orbits.

Lesson summary

This week, we'll delve into the fascinating relationships between the Earth, the Moon, and the Sun. This topic is fundamental to understanding our place in the universe and explains many phenomena we observe daily, like the changing seasons, the phases of the moon, and the occurrence of eclipses. Understanding these relationships helps us appreciate the precise movements of celestial bodies and how they affect life on Earth, from planting seasons to predicting tides that affect fishing communities along the South African coastline.

Furthermore, understanding these concepts empowers us to think critically about space exploration and scientific advancements.

Lesson notes

2.1 The Earth's Rotation and Revolution: The Earth has two main movements: rotation and revolution.

Rotation: The Earth spins on its axis (an imaginary line passing through the North and South Poles) once every 24 hours. This rotation is what causes day and night. As different parts of the Earth face the Sun, they experience daylight, while the parts facing away experience night. South Africa experiences day and night because of this rotation.

Revolution: The Earth orbits, or revolves, around the Sun in an elliptical (oval-shaped) path. One complete revolution takes approximately 365.25 days, which is one year. This revolution, combined with the Earth's axial tilt, is what causes the seasons. 2.2 The Earth's Axial 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 in determining the seasons. When the Northern Hemisphere (where South Africa is located, although we are in the Southern Hemisphere) is tilted towards the Sun, it receives more direct sunlight and experiences summer. At the same time, the Southern Hemisphere is tilted away from the Sun, receiving less direct sunlight and experiencing winter. Six months later, the Earth is on the opposite side of the Sun. The Southern Hemisphere is now tilted towards the Sun, experiencing summer, while the Northern Hemisphere experiences winter. During the equinoxes (around March 21st and September 23rd), neither hemisphere is tilted significantly towards the Sun. Both hemispheres receive roughly equal amounts of sunlight, resulting in spring and autumn (fall) in the respective hemispheres.

Example: Imagine you are holding a globe, and the light from a lamp represents the Sun. If you tilt the top (Northern Hemisphere) of the globe towards the lamp, that part will be brighter and warmer, representing summer. Tilting the bottom (Southern Hemisphere) towards the lamp does the opposite. 2.3 The Moon's Orbit and Phases: The Moon orbits the Earth in an elliptical path. One complete orbit takes approximately 27.3 days (sidereal month). Because the Earth is also moving around the Sun, the time it takes for the Moon to complete its cycle of phases (from new moon to new moon) is slightly longer – about 29.5 days (synodic month). The Moon itself does not produce light. We see the Moon because it reflects sunlight. As the Moon orbits the Earth, different amounts of its illuminated surface become visible to us, creating the different phases of the Moon.

Here's a breakdown of the main phases: New Moon: The Moon is between the Earth and the Sun. The side facing us is not illuminated, so we cannot see the Moon.

Waxing Crescent: A small sliver of the Moon becomes visible, growing larger each night.

First Quarter: Half of the Moon is illuminated.

Waxing Gibbous: More than half of the Moon is illuminated, growing larger each night.

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

Waning Gibbous: More than half of the Moon is illuminated, but the illuminated portion is decreasing.

Third Quarter: Half of the Moon is illuminated (the opposite half from the First Quarter).

Waning Crescent: A small sliver of the Moon is visible, decreasing in size each night. "Waxing" means growing, and "waning" means shrinking. 2.4 Eclipses: Eclipses occur when one celestial body blocks the light from another.

There are two main types of eclipses: solar and lunar.

Solar Eclipse: A solar eclipse occurs when the Moon passes between the Sun and the Earth, blocking the Sun's light and casting a shadow on Earth. For a solar eclipse to occur, the Moon must be in the new moon phase. Solar eclipses are relatively rare and only visible from a small area on Earth. Never look directly at the sun during a solar eclipse without proper eye protection, as it 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. For a lunar eclipse to occur, the Moon must be in the full moon phase. Lunar eclipses are more common than solar eclipses and are visible from a larger area on Earth. During a total lunar eclipse, the Moon often appears reddish in color, sometimes referred to as a "blood moon." 2.5 Gravity: Gravity is the force of attraction between objects with mass. The more massive an object, the stronger its gravitational pull. The Earth's gravity keeps the Moon in orbit around it, and the Sun's gravity keeps the Earth (and all other planets) in orbit around it. Sir Isaac Newton's Law of Universal Gravitation describes this force mathematically. The strength of the gravitational force depends on the masses of the two objects and the distance between them: the greater the masses, the stronger the force, and the greater the distance, the weaker the force.