--Earth and Space Science
Students consider the impact our moon has on earth and explore a theory of how the moon came to exist.
Moon, lunar, earth, satellite, space, gravity, axis, orbit, tides, phases, giant impact hypothesis
Assess prior knowledge by asking what students know about the moon. What is it made of? How did it get there? What impact does it have on earth? Why does it look bigger at some times and smaller at other times? What have you always wondered about the moon? If desired, have students record their answers on a class chart paper, or on their own sheets of paper or K-W-L charts.
Then, frame the discussion as follows:
Since humans first appeared on earth, we have been fascinated by the moon. Despite having sent teams of astronauts to the lunar surface over 40 years ago, scientists are only now starting to understand the origins of the moon. While no one’s 100% sure, scientists have some ideas about how the moon may have come to exist. Let’s explore those ideas, then find out what the moon is like now and why it’s so important to earth.
These vocabulary terms are highlighted in bold the first time each appears in the lesson content below. When delivering the lesson, it makes sense to pause each time you encounter one of the words. When possible, ask students to use context clues to guess what each one means, then as a group, define the word correctly.
You may also choose to have students do Internet research on these vocabulary terms prior to the lesson. If so, ask them to serve as experts, stepping in to share a definition or two with classmates at a relevant point during the lesson. By the end of the lesson, students should have written down all of the terms and their correct definitions.
Students also can do pre-lesson research to determine the size of the moon and its distance from earth. During the What Is the Moon Like? portion of the lesson below, have them share these facts with the class.
Theia: This rogue planet, roughly the size of Mars, is believed to have collided with earth, causing the release of debris that resulted in the formation of the moon.
Orbit: The path a celestial body takes as it travels around its parent celestial body. Orbits tend to be elliptical (oval), but also can be circular.
Giant Impact Hypothesis: A currently accepted scientific theory of the formation of the moon. It involves a rogue (out-of-control) planet crashing into a still-molten earth about 4.5 billion years ago. (Molten means made of hot, liquid rock.) The debris (broken stuff) from the collision eventually coalesced (grew together, or fused) and formed the moon, while the earth reformed as well.
Satellite: A small celestial body that orbits a planet. Satellites are always smaller than their parent planets, though their size in relation to that planet may vary greatly.
Gravity: When physical bodies attract each other with a force proportional to their masses (more mass equals more force). Gravity is what gives weight to objects and causes them to fall to the ground when dropped.
Silica: Rock made from a combination of silicon and oxygen atoms.
Craters: Cup-shaped “dents” in the surface of the moon, caused by asteroids and other objects hitting the moon’s surface.
Full Moon: Occurs when the earth, moon and sun “line up” so that the moon is on the opposite side of the earth. The entire sunlit part of the moon is facing earth, so it appears large, round and bright.
New Moon: Occurs when the earth, moon and sun “line up” so that the moon is between the earth and sun. The entire sunlit part of the moon is on its back side (the half we cannot see from earth), so the moon appears dark and barely visible.
Tides: The rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon, the sun and the rotation of the earth.
High tide: When the moon pulls the ocean water up further onto the beach, creating less sand space.
Low tide: When the moon pulls the ocean out further, creating more sand space.
Axis: The angle at which a planet or satellite rotates relative to its poles. The earth rotates on an axis that is tilted by approximately 23 degrees.
How Did the Moon Form?
While no one is exactly sure how the moon formed, scientists have proposed that 4.5 billion years ago, a collision (crash) occurred between a nearly formed earth (that was still in molten, or liquid, form) and a planet roughly the size of Mars (called “Theia”). It is believed that when Theia hit earth, Theia flung debris (broken stuff) from itself (and, some believe, also from earth) into orbit around earth. Scientists have shown that it’s possible the moon formed from that orbiting ring of hot debris. This idea is called the giant impact hypothesis. Evidence for this hypothesis comes from studies of the rock that makes up the moon.
(If desired, explore the meaning of the word hypothesis. How is a hypothesis different from a pure guess? How is it different from a fact? How does a hypothesis guide scientific research?)
Display for the class an artist’s depictions of the moon’s formation here.
What Is the Moon Like?
Earth’s moon is the fifth-largest moon in the Solar System and is the largest natural satellite of a planet (the moon is about one-quarter the size of earth).
If students did pre-lesson research to determine the size of the moon and its distance from earth, have them share those facts now. (The moon's circumference [measurement around] is about 6,700 miles, and its surface area is about 14 million square miles. The moon is about 225,000-250,000 miles from earth at any given time.)
When it first formed, the moon was probably much closer to earth, and spun around much faster than it does now. Earth’s gravity, pulling on the moon, has gradually slowed it down so that now the same side of the moon is always facing the earth.
The surface of the moon is like a rocky desert. Like earth’s rocks, most moon rocks are made of silica, a combination of silicon and oxygen atoms. The surface of the moon also contains many mountains (caused by volcanoes) and craters (caused by objects hitting the moon’s surface). The moon’s sky looks black, because there is no atmosphere to reflect the sunlight and make it seem blue.
Why Does the Moon’s Appearance Change?
The moon has no light of its own; instead, it reflects the light of the sun. Depending on where the moon is and where the sun is, and where a person is on earth, he may be able to see all of the side of the moon that’s lit up by the sun (called a full moon). Or he might only be able to see a tiny sliver of the part of the moon that’s lit up by the sun (called a new moon). It takes about a month for the moon to get all the way around the earth, which takes us through all the “phases” of the moon (from one full moon to the next).
Display for the class this image of the different phases of the moon.
The Moon’s Impact on Earth
The most noticeable effect the moon has on the earth is the rise and fall of the ocean tides. The curved shape of both the earth and moon, along with the moon’s orbit, make the moon’s gravitational pull stronger on one side of the earth than the other. This creates two tidal swells, known as high tide and low tide, which happen on opposite sides of the planet at the same time. Low tide (when the ocean is pulled the farthest backward from the sand) occurs when the Moon’s gravitational pull is strongest. High tide happens when the ocean is pulled the farthest forward onto the sand.
Display for the class these images of specific locations at both low and high tides.
A less noticeable, but almost certainly more important, effect of the moon’s presence is the steadying of earth’s axis. The earth rotates on a 23-degree axis, which means it’s tilted a little to the side. This axis is kept relatively stable by the moon’s gravitational pull. Without the moon’s influence, scientists believe that the earth’s axis would shift as much as 85 degrees. This would mean severe climate (weather) change and the end of most forms of life.
Extending the Lesson
Lesson Plan Source
Jason Tomaszewski, EducationWorld Associate Editor
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