Teacher Background for Unit ”Why Do Falling Stars Fall?“

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In this unit students are introduced to some basic ideas about stars, the solar system and gravity.

The theme of the unit is that one can develop models that can be used to explain patterns in data. A model that explains lots of different data in a simple fashion becomes part of the accepted theory of why that phenomenon happens. Thus the flow of ideas is:

• Notice the patterns →
• Develop a model that would predict those patterns if it were true, using science that you know →
• Ask what else (what other patterns or observations) the model predicts about the system using known science →
• Test whether that prediction matches your observations, and if not, refine your model until it does →
• Once you have a model that “works” well, use it as the basis of your explanation of how the system functions.

Thus in science we argue from evidence as we refine and test models, and once we have a good match between the model, the science that we know, and the phenomena in the system, we can use the model as the evidence-based explanation. Then we say we understand what happens because the system functions as described by the model.

In this unit we introduce multiple phenomena, some familiar to students, some less so. Many of the models are introduced. Only in a few cases do we ask students to develop their own because they would need much more data and information than they are ready for to be able to develop these classic scientific models themselves. The unit uses video and software simulation of the night sky to have students experience phenomena that they may not have noticed and that cannot be observed in the classroom. Part of your goal as a teacher should be to encourage them to share the wonder and awe that these experiences can stimulate and to have students ponder how small our home planet is as part of the enormous system we call the Solar system and the even larger one we call the Universe.

Using models is not the only science practice we meet in this unit. Students also engage in all the other practices in the course of this unit. Whenever they complete a segment of the unit it is worthwhile to have them reflect on which of the science and engineering practices they have used and how this has been helpful to their developing understanding.

Introducing the Phenomenon of Falling Stars

The video introduces them to the phenomenon popularly called “falling stars” which are in fact meteors or meteorites (smaller meteors that burn up before reaching the ground). Eventually they will establish models of how the Earth moves through space and the fact that from time to time it passes through regions where flows of objects, such as meteors, intersect our atmosphere. They will learn what happens to those objects as they pass through our atmosphere and, sometimes, fall to Earth’s surface. They also have the opportunity to examine rocks that come from meteors and compare them to other rocks. They will find out that some rocks found on Earth’s surface can be identified as coming from meteors because they have characteristics that are different from the usual Earth materials.

The meteor shower in the video is identified as coming from a particular part of the sky where we see a constellation known as the Bootes constellation. This is not particularly relevant to the initial lessons where students observe and are introduced to the phenomenon of falling stars, but is used later in the unit to connect to the more general idea of constellations, fixed patterns of stars that seem to move together across the sky—and to learn to recognize a few of the most visible constellations using the simulation software, Stellarium. You could encourage students to go out on a clear night, when the moon is not close to full, and look for a couple of constellations that are high in the sky. Constellations are artificially defined groupings of fairly bright stars, not actual physical objects that hold together. The main point of the idea of a constellation is that they are (near to) unchanging patterns. Stars do move relative to one another, but so very slowly compared to the motions of the Earth around the sun and the speed of its spin around its axis, so that on human time scales we can say that they almost don’t move.

Using Models to Explain Patterns

In many science activities we should ask students to suggest their own models. However, in this unit we guide them towards models that are the standard models for the system but that are not obvious to the observer. We actually invert the usual process of model development because what we stress is what patterns in our observation the model would reproduce in its predictions. Thus we ask them to use a model and to interpret what it predicts, but not to develop their own model. However, the goal of instruction is not that students simply learn to say that the Earth orbits around the sun each year and rotates (spins) around its axis each day. The goal is that students come to see that this model can explain multiple observed phenomena and take ownership of it (that is incorporate it into their own thinking) so that they can and will call on it again and again as the basis of their understanding of these phenomena.

In this unit the model of the annual orbit of Earth around the sun provides an explanation for why we see some stars at one time of the year and different stars at another. We also introduce the additional detail of this model that Earth spins once a day around the axis from its north pole to its south pole as it moves around its orbit. The unit introduces only those observations that can be explained by the model without the detail that the Earth’s axis is tilted, not perpendicular, relative to the plane of its orbit around the sun. This detail is needed to explain seasonal variation in weather patterns, length of day, and position of sunrise and sunset on the horizon, so those topics are not discussed in this unit. Studies have shown that the three dimensional (in space) model needed to explain these phenomena is too difficult to visualize to introduce at this stage. It works better for students to get familiar with the idea of the orbit around the sun and the fact that day and night has nothing to do with that orbit, but is explained by the spin, and then later to elaborate the model further. We also do not stress that the orbit is an ellipse rather than a circle. In fact, it is pretty close to circular, and nothing in this unit needs the extra complication of talking about ellipses.

Students may raise questions about the other annual patterns that the unit does not discuss—you can reassure them that they will learn more later about how this model needs to be refined to be good enough to explain those patterns as well, but they already have enough evidence to suggest that the model they are learning now is a useful one for explaining phenomena.

One set of observations that are not introduced in this unit, except peripherally, is the complicated pattern of motion of the planets in the sky relative to that of the stars. Historically it was this data that led to the model that the Earth and other planets orbit the sun. You may want to mention this history as you talk about how the stars stay in fixed relationships to one another even as they move across the sky. Planets look pretty much like stars to the naked eye, but they break this rule—that is one way we know they are not stars. The unit does mention that other planets have moons—in fact the invention of the telescope and Galileo’s use of it to observe the moons of Jupiter confirmed, and thus helped establish, the idea that the planets are more like Earth than they are like stars and that they, like Earth, orbit the sun. This is a famous bit of history of science. You as a teacher can choose what part of this history would be interesting to your students.

Gravity

We also come close to history that involves Galileo again when students investigate how objects fall—that everything near the surface of the Earth is pulled towards it by a force we call gravity. Galileo studied gravity by dropping various sized objects from the Leaning Tower of Pisa (which is leaning even more today than it was then, because, once a tower is crooked, gravity keeps pulling it more crooked). He was asking a more detailed question than the one the students investigate here—namely not just that objects fall but that there is a pattern of how fast they fall. However, there is enough of a parallel that you might wish to mention this glimpse from the history of science. It can help students see science as a process in which what they are doing echoes how scientists learned to understand the way the world works, and continue to learn more today.

What we expect the students to learn about gravity in this unit is simply that, near the surface of the Earth, anything that is not held up will fall towards that surface (or more precisely toward the center of the Earth), and that is the direction we call down. It requires some kind of force to make any object change its motion, for example, to fall. Students intuitively know something about this. Further they should have learned that a force is any push or pull in earlier grades. They have met invisible forces between objects that do not touch each other by playing with magnets (and possibly also electrostatic forces). They intuitively know that things fall down and experience that something pulls them down when they try to jump or climb. The point of the activity of observing a variety of falling objects in this unit is help them make this implicit knowledge more explicit, to name the force of gravity, and to develop a more robust concept of gravity that can answer questions such as “why don’t people on the other half of the globe fall off?” For most children the observation that things fall down is not something they think needs explaining—it is so much part of their experience that they take it for granted. The goal of making it a topic for learning is, in part, to help them see that we can learn something when we ask why, and make careful observations, even when the phenomenon we are asking about is one that we see all the time. (If time is tight in this unit you could skip this experiment and make it a thought experiment and a discussion, many students may see the actual observation of falling objects here as superfluous busy work, because they already know what will happen if they let something go.)

Patterns

The crosscutting concept of patterns is a theme throughout this unit. The idea is any time you start to wonder about the way a system works, it is useful to look for patterns in its form or function. Useful here means it provides a perspective, or if you like a lens, that leads to productive questions. If you ask questions about observed patterns you are focused in on features of the system that need some explanation. Thus looking for and asking questions about patterns is a productive way to approach a system about which you know little, and helps you in developing some understanding of it. For any model that is supposed to represent a system one important question is “Can my model help me explain the patterns that I see?”

In science an explanation is not simply a description, it answers a how or why question. Why do these observed patterns occur, day after day and year after year? The answer is given by the model—because if the Earth moves as the model describes then the observed patterns would be expected.

Questions Students May Raise

There are a couple of other science ideas that are implicit in this unit, and that perhaps will need some discussion if students ask about them or it becomes clear they need something clarified in order to understand an investigation they are engaged in:

• How light travels. Light travels in straight lines and shadows are made when an object blocks the path of the light (absorbs the light and stops it from reaching the surface where you see the shadow). The fact that light travels with a specific speed also needs some discussion. The idea of making a length unit out of a definite speed multiplied by a definite time can make sense to students with the right discussion, but it is not intuitive to them immediately, so probably needs a little more discussion than is given in the unit. We describe light-years with both kilometers and miles, as some students have no experience of kilometers, but using kilometers would also allow some newcomer students to be experts on something most U.S.-born kids do not know.
• Earth’s motion. Students may wonder why we don’t feel Earth rotating or orbiting. The motion of the Earth through space is not perceptible to us on the Earth’s surface because the atmosphere close to Earth’s surface moves as a whole along with the planet

As is always the case in science, the model even in its more elaborated form, raises new questions. While the models students use explain why we only see falling stars at night and at certain times of year, students may raise new questions: why do the Earth and the planets move as the model says? Why do they orbit the sun and the moons orbit their respective planets? Answers to these questions require further science to be understood—in this case, that gravity is not just a phenomenon on Earth but, in fact, there are gravitational forces between any two objects that have mass (or indeed even just energy, as gravity pulls on light too).

To understand the orbits you also have to recognize that motion in a circle (or an ellipse) is constantly changing motion, it changes direction at every moment, and thus needs a force that always pulls the object towards the center of the circle (or the foci of the ellipse). This story goes well beyond the scope of this unit, but students may raise some of the questions that the model raises for them. You need to be prepared to acknowledge how important these questions are, and that they will meet this model again in more detail as they learn more science and mathematics, because answering these very questions both reinforced scientists’ confidence in the model and led to new scientific discoveries.