Activity 6 Earth Is Round

What's Up, Earth? Header Insert Image 1 here, right justified to wrap Image 1 ADA Description: ___? Caption: ___? Image file path: ___? Source/Rights: Copyright © ___? Grade Level 3rd Time Required: 60 minutes Group Size: Whole class and small group experiment teams 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. Keywords 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. Educational Standards · Science: o Objective 3.01 Observe that light travels in a straight line until it strikes and object and is reflected and or/absorbed. · Math: o 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. Learning Objectives 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. Materials List 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.” 6.2 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. 6.3 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.

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