6: The Spinning Earth: What's Up, Earth
Summary: Through discussion and work with models, the students will explore the implications of the fact that the Earth is round. By working with the models they will reconcile the intuitive perception of a flat Earth with the formal statement that it is in fact a sphere. The concepts of geography: poles, equator, rotation of the Earth, direction of the axis of rotation will be clarified by working with models that allow students to observe Earth "from space." This will be the students' first exposure to this model, and it will begin to teach them to relate the experiences of a person living on the Earth to their observations from space. This relation will be a central tool in the following units.
Science:
- Objective 3.01 Observe that light travels in a straight line until it strikes and object and is reflected and or/absorbed.
Math:
- Objective 3.01 Use the appropriate vocabulary to compare, describe, classify two and three-dimensional figures.
Pre-Req Knowledge
This activity is designed to follow up on the previous activities which will have introduced students to the concepts of light propagation, shadows, and the daily alternation of light and dark. Most importantly, they will have begun in the previous activity (Day and Night on the Spinning Plate) to construct the relation between observations from a stationary point of view in outer space and the experiences of a person on Earth. We will expand and build on this idea here.
Pre-Activity Assessment
Have students draw the Earth in their Science Notebook. Have them draw a stick-figure person on the Earth, and draw two stars in space (the rest of the page), one of which the person can see, and one the person cannot see.
After this activity, students should be able to:
- Understand that relative to the fixed stars, "up" is a different direction at different points on Earth.
- Explain why this direction changes as the Earth rotates, except at the poles.
- Understand why as the Earth rotates towards the East, celestial objects appear to move across the sky from East to West.
Activity Assessment
Positioning your toy figure on the globe, ask students to point in the direction that is "up" for this person. Using several locations on the globe, verify that they understand the dependence of "up" on location.
Using objects in the classroom, and positioning the figure as above, ask students whether the person is able to see the objects or not from the location on the globe. Rotate the globe to change the answers. Students should find that for most locations on Earth (anywhere except the equator or the poles) there is a part of the room (space) that is always visible, a part of the room that is never visible, and parts that are visible for part of the rotation.
Ask students to orient their model Earth so that the "person" is just able to see the teacher. Have them rotate the Earth so the teacher appears nearly overhead, and then "sets" for the person on their model.
Post-Activity Assessment
Ask students to repeat the pre-activity assessment, correcting the figure they produced to reflect what they have understood.
Ask students to consider where in the sky an object will appear immediately after it first becomes visible as the Earth rotates. The answer is that the object will appear to be rising just above the horizon, more or less to the East). Just before an object disappears, it will appear to set below the horizon, more or less in the West.
Background
That the Earth is round is a fact many of the students may be familiar with, but it is in puzzling contrast to our intuitive sense that the ground around us seems flat. The puzzle is, of course, resolved by the issue of size. We rarely observe enough of the Earth to note the curvature.
Historically, the Earth's shape was discovered by studying the way ships disappear below the horizon as well as the shape of Earth's shadow on the Moon during an eclipse. Both of these methods rely on rather intricate reasoning. Fortunately, there is no need to pursue the historical route. We know the Earth's shape because we have been able to observe it from space; showing a picture taken from space can be useful during this lesson.
At any point on Earth, one-half of the universe is hidden by the Earth itself (looking down you can see no stars). This would of course be true on a flat Earth as well. Because the Earth is round, which half of the universe is visible depends on location on Earth. Moreover, as the Earth rotates, the part of the universe visible from a given location changes, except at the poles. Thus the rotation of a round planet is responsible for the cycles of day and night (absent at the poles) as well as for the perceived motion of objects in the "sky" (in space).
The idea that the Earth - and us with it - is in constant rotational motion, is counterintuitive and confusing. The relation between this fact and the perceived motion of celestial objects across our sky is an exercise in spatial reasoning that challenges many students. The strategy here is to provide the students, through their model, with the view from space. The implications of Earth's shape and rotation are far easier to see from this point of view. The challenge, therefore, is to help them imagine that the classroom really is outer space, while their styrofoam balls are the Earth, and then to integrate this perspective with their everyday observations from ground level. To help students with this, a kinetic experience is useful. Have students stand and spin in place (slowly) with their hand held in front of them. They will notice that their hand appears fixed in their field of vision but the rest of the classroom appears to be spinning in the opposite direction to their motion. By analogy, objects on the spinning Earth appear stationary to us, while objects off the Earth (Moon, Sun, Stars) appear to be fixed on a "celestial sphere" which rotates from East to West.
Our perception of the rotation depends upon where we are on Earth, or more specifically on our latitude. To see this, imagine first a person standing precisely on the North pole. The way we hold our globes in class, this person appears to stand "on top" of the Earth - but remember that "top" and "bottom" are concepts that are meaningless in space! As the Earth spins, the person at the North pole remains in the same position. The effect of the rotating Earth is that they are slowly (once a day) rotating to their left. The one-half of the classroom-universe the person can see is always the same - it is the part of the classroom-universe that is above (again that meaningless term, "farther from the floor" would be better) the model Earth. Thus a person on Earth's North pole always sees the same stars. The Earth's rotation causes the stars to appear to rotate slowly (once a day) to his or her right. This is what one sees at the poles - stars neither rise nor set (indeed, when all directions are South, rising in the East is hardly an option!) but appear to rotate to the right once a day. If a person at the North pole looks directly overhead, they are always looking in the same direction in space (straight "up" in the classroom). Indeed, there is a rather bright star they would see directly overhead - always. Appropriately, it is named the North, or Pole Star. As the sky appears to rotate, the North star appears not to move.What of a person at the South pole? They too see the same half of the universe at all times, and they too see the same star directly overhead at all times (unfortunately, there is not a bright star in this direction so we do not have a South Star). But to them, the Earth's rotation appears to be to the right, so they see the stars circling to the left.Let us next consider a person on the Earth's equator. Things here are very different. As the Earth rotates, what this person sees when they look straight up changes. Indeed, the half of the universe they can see changes as the Earth turns. The North star always appears, just on the horizon, to their North, and the South star, if it existed, would always appear just on the horizon to the South. As the Earth spins, objects in the sky would to them appear to rotate on an axis connecting the North star to the South star. But this axis is horizontal unlike the case at the poles where the (same!) axis appears vertical. This causes stars to rise and set - as the visible half of the universe comes to include a star it rises, and as the visible half of the universe sweeps on to exclude it, it appears to set. If an object in the classroom (a "star") is at the "height" at which we hold the globe's equator, it will appear for the person at the equator to rise directly to the East. As the Earth spins, it will rise until it is directly overhead, and then descend to set directly to the West. Objects "higher" (farther from the floor) than the globe's equator will always appear in the Northern sky. They will rise in the East, move higher in the sky until at their highest point they appear due North, and then set to the West, appearing thus to move to the left. This is the way stars in the Northern sky behave at the equator. On the other hand, objects "lower" than the equator will appear to rise in the Southeast, rise until they are due South, and then set in the Southwest, appearing to move to the right. The pole "stars" do not appear to move at all.
At intermediate latitudes, we find some combination of the phenomena observed at the equator and at the poles. In the northern hemisphere, the North star and some of its immediate vicinity in the sky will always be visible. These stars will appear to rotate around the North star. The South "star" and some of its vicinity will never be visible. Stars in between these regions will be visible part of the time (will rise in the East and set in the West). They will appear to move to the right if they appear in the Southern sky, and to the left if they appear in the Northern sky. We have been talking about stars, because these are objects fixed (in direction) in space and so correspond to objects fixed in the classroom, and because we can find stars in any direction to use when talking about that part of the sky. In the next activity we will talk about a particularly important star, the Sun.
60 minutes
Group Size: Whole class and small group experiment teams
Teacher needs:
- Globe and a small toy figure
Each pair needs:
- Styrofoam ball
- Marker
- Pencil
- Small toy figure, golf tee, or some other object to represent a person on styrofoam "Earth."
Preparation: Getting Ready
Have one styrofoam ball for each two students. This will model the globe. Insert a pencil into the ball so it can be held, like a lollipop.
Part 1: Science Notebook Intro
- Introduce the activity with the following Motivation / Challenge: It is said that the Earth we live on is a round ball, spinning in space. What does that mean? What do you know or think about the Earth's shape? Do you think it spins? How would you find out what its shape is or whether it spins?" Allow the class to brainstorm and record their ideas in their science notebook for 5-7 minutes and then list their responses on chart paper to be referred to during the activity.
- Hold up the globe and initiate a discussion of a globe as a model of Earth. Explain the idea of scale - note that on the scale of the model they are giants, much larger than Earth! Note that in the model, their location, around the classroom and off the globe, corresponds to outer space. Point out objects along walls, floor, or ceiling of the classroom that can serve as fixed locations "in space" representing, for example, the location of particular stars or star groupings.
- Show the location on the globe of the North and South poles, and have students mark the locations of the poles on their "globes." Show students the location of the equator on the globe, and have them mark the equator on their models.
- Through demonstrations with the globe and experiments with their models, lead students to an understanding that at different points on the globe, looking "up" means looking in different directions, toward different objects in the classroom, which can represent different directions in space and different star groupings. It is helpful to have the toy figure representing a person perform exaggerated jumps "up" from various points on a model Earth and then fall back "down" to the surface. This phase of the discussion usually leads to many questions about the nature and properties of the force of gravity, which is what pulls us back to Earth when we jump. If you wish to pursue these, prepare a discussion using the resources available.
- Demonstrate the rotation of the globe about its axis, have students reproduce the rotation with their models. Discuss the fact that the Earth itself rotates. Be sure to keep the rotation in the correct sense - from West to East. The rotation is too smooth, slow, and regular to be noticed; a complete revolution takes about 24 hours. (Despite this, the size of the Earth means that in the US, we are moving at a speed of some 800 mph, but since everything around us moves in perfect concert with us, we do not notice the motion at all. Importantly, this includes the atmosphere, the layer of air surrounding Earth, which is dragged along with the Earth as it rotates. Were this not true, we would be subject to 800-mph winds!) Have students stand and spin (slowly) in place, toward their left, with their hand extended in front of them. They will observe that their hand appears stationary while objects fixed in the class appear to be spinning about them toward the right. A person at a fixed point on Earth will observe, as the Earth rotates West to East, that objects not on the Earth will appear to be rotating in the opposite direction - from East to West. Ask students to note what they see when they spin - slowly! - while looking straight up. They should note that while the ceiling appears to rotate, they are in fact looking at a fixed spot on the ceiling as they spin.
- The students' experience when spinning to their left in the classroom is most accurately representative of a person standing at the Earth's North pole. At this point the rotation is about an axis that appears vertical, and is directed to the left. In complete analogy with the spinning student, if a person standing at the North pole looks straight up, they will be looking at a fixed object as the Earth rotates and spins them with it. In fact, very near to their line of sight - along the extension of the Earth's axis - lies a star that is near enough to Earth to appear rather bright. Since it is always directly above the North pole, it is rather appropriately named the North star or the Pole star. What part of the classroom (space) can a person at the North pole see? They will be unable to see anything that lies below the model Earth (closer to the classroom floor), simply because the "Earth" will be in their way. But they can see anything "above" the globe.
- Ask students to try to figure out how this same motion of the Earth would be perceived by someone at the South pole. Because this person is "upside-down" relative to the person in the North, the rotation would appear to be to their right. Celestial objects would appear to be moving to the left. Like the person at the North pole, a person at the South pole, looking straight up, will always be looking at the same celestial object. Unfortunately, there is not a nearby star that lies right above the South pole.
- What about a person at the equator? If they look straight up, as the Earth spins, their gaze will sweep in a circle about the class, parallel to the floor. From the point of view of a person at the equator, celestial objects appear to be moving as if the sky were rotating about an axis that is horizontal. If you are feeling adventurous, you can challenge students to come up with a way they can spin in the class that would represent this. (The answer would be to lie flat on the floor while spinning.) A person at the equator can see one-half of the classroom at any time, just like the people at the poles, because the "Earth" obscures the other half. But as the "Earth" rotates, the half of the class (space) visible to the person at the equator changes, so that over the course of a complete rotation they have an opportunity to see in every direction at one or another point of the rotation.
- What about a person standing on the Earth at intermediate Northern latitudes, such as somewhere in the US, neither at the poles or the equator? As the Earth rotates, this person can always see a part of the classroom, near the ceiling (near the North star). They can never see points directly below the "Earth". In directions near the horizontal (in the classroom) they can see different parts of the classroom at different times during the rotation. What is the apparent motion of celestial objects as seen by a person in the Northern hemisphere? We will consider this in more detail in the next activity, but the answer will be that objects in the sky appear to be revolving about an axis that is neither horizontal (as at the equator) nor vertical (as at the Poles) but tilted relative to our local perception of up. Thus celestial objects appear to us to move along paths that are tilted. This is the path we observed for the Sun when we tracked it earlier, and the next activity will be devoted to understanding this connection in detail.
Safety Issues
- Warn students against aggressively spinning. Dizzy students may fall and injure themselves. For our purposes a slow, steady rotation in place is what is required.
Sphere: the three dimensional shape comprising all points at a fixed distance from a center. The approximate shape of the Earth (and many other celestial objects).
Up: At any location on Earth, "up" is the direction away from the Earth. Because Earth is a sphere, this is not the same direction at different points on the Earth.
Down: The opposite direction to "up," towards the center of the Earth.
Axis:the imaginary line through the Earth's center about which the Earth rotates once a day. The Earth's axis "points" in a fixed direction in space.
Poles:the two points on the surface of the Earth that lie on the axis. As the Earth rotates, the poles do not move at all, so looking straight up at one of the poles one is looking in a fixed direction in space.
Equator:the imaginary circle on the Earth's surface any point on which is an equal distance from the Earth's two poles. The equator divides the Earth into two hemispheres, northern and southern.
Address: Campus Box 7006. Raleigh, NC 27695 Telephone: 919.515.5118 Fax: 919.515.5831 E-Mail: KenanFellows@NCSU.edu