Reasons for the Seasons

Transcription

1 Reviewing Content There are two main reasons for the seasons. The first is that Earth is tilted on its axis. This tilt causes sunlight to strike different parts of Earth in different ways. Sunlight hits Earth s surface most directly near the equator. Near the poles, the sunlight strikes at a very shallow angle. The second reason for the seasons is Earth s orbit around the sun. As Earth moves around the sun, the north end of its imaginary axis is tilted away from the sun for part of the year and toward the sun for part of the year. As a result, most places on Earth have four distinct seasons: winter, spring, summer, and autumn. Reviewing Inquiry Focus Models are often used in science to visualize processes or events that cannot be directly observed. Scientists use such models to make inferences about these processes or events. An inference is a possible explanation that is based on fact or observation. Because we currently cannot actually observe Earth s orbit around the sun from elsewhere, scientists have made models to study this movement. In this, you will make a model that will allow you to see how the sun shines on Earth as Earth makes its orbit around the sun. You will be able to make conclusions based on your observations about the seasons that most of Earth experiences. In this investigation, how will you model Earth and the tilt on its axis? How will you model the sun in this investigation? What types of inferences will you be able to make about seasons on different parts of Earth? 18

2 INQUIRY FOCUS Make Models, Observe, Infer Materials books flashlight masking tape paper pencil protractor toothpick acetate sheet with thick grid lines drawn on it plastic foam ball marked with poles and equator metric ruler Problem How does the tilt of Earth s axis affect the light received by Earth as it revolves around the sun? Procedure 1. Make a pile of books about 15 cm high. 2. Tape the acetate sheet to the head of the flashlight. Place the flashlight on the pile of books. 3. Carefully push the pencil into the South Pole of the plastic foam ball, which represents Earth. 4. Use the protractor to measure a 23.5º angle to represent the tilt of the axis of your Earth away from your flashlight sun, as shown in the top diagram. This position represents winter in the Northern Hemisphere. 5. Hold the pencil so that Earth is steady at this 23.5º angle and about 15 cm from the flashlight head. Turn the flashlight on. Dim the room lights. 6. The squares on the acetate should show up on your model Earth. If the squares are not clear, move the ball closer to the flashlight or dim the room lights more. Observe and draw the shape of the squares at the equator and at the poles. 19

3 REASONS FOR THE SEASONS continued 7. Carefully stick the toothpick straight into your model Earth about halfway between the equator and the North Pole. Observe and measure the length of the shadow of the toothpick. Record your measurement on the line below. cm 8. Without changing the tilt, turn the pencil about 90 (a quarter turn) to rotate the model Earth on its axis. Observe the length of the toothpick s shadow and record it on the first line below. Rotate the model Earth 90 two more times. With each turn, measure and record the length of the toothpick s shadow below. cm cm cm 9. Now tilt your model Earth 23.5 toward the flashlight, as shown in the diagram below. This position models summer in the Northern Hemisphere. 10. Observe and draw the shape of the squares at the equator and at the poles. 11. Repeat Step 8 to observe how the toothpick s shadow changes in summer. Continue to rotate the model Earth and note how the toothpick s shadow changes. With each turn, measure and record below the length of the shadow. cm cm cm cm 20

4 REASONS FOR THE SEASONS continued Analyze and Conclude Make Models When it is winter in the Northern Hemisphere, which areas on Earth get the most direct sunlight? Which areas get the most direct sunlight when it is summer in the Northern Hemisphere? Observe Compare your observations of how the light hits the area halfway between the equator and the North Pole during winter (Step 7) and during summer (Step 11). Infer If the squares projected on the ball from the acetate sheet become larger, what can you infer about the amount of heat distributed in each square? Predict At what time of year will the toothpick s shadow be longest? When will the shadow be the shortest? 21

5 Draw Conclusions How are the amounts of heat and light received in a square related to the angle of the sun s rays? Interpret Data Which areas on Earth are consistently coolest? Which areas are consistently warmest? Why? Summarize Describe what you learned in this lab about what causes seasons and what questions you still have. What I learned What I still want to know Relate Evidence and Explanation Use your observations and results to write an original paragraph that describes the orientation of Earth relative to the sun when it is winter and summer in the Southern Hemisphere. 22

6 INQUIRY FOCUS Make Models, Observe, Infer Materials books flashlight grid transparency pencil protractor toothpick foam ball marked with poles and equator marker metric ruler Problem How does the tilt of Earth s axis affect the light received by Earth as it revolves around the sun? Design an Experiment 1. You have probably noticed that during winter, the sun is much lower in the sky and average temperatures are considerably lower than during summer. What is the connection between the angle at which sunlight strikes Earth s surface and the temperature on Earth s surface? Imagine that you have to explain this connection to someone who is convinced that winter in the Northern Hemisphere is colder only because the days are shorter and there s less sunlight in general. 2. Your teacher will provide you with a flashlight and a special transparency with a grid on it. The grid on the transparency will help you visualize what happens to sunlight when it shines on a surface at an angle. You will also be given a foam ball and pencil to make a model of Earth on its axis. The toothpick can be used as an object to cast shadows on the Earth model. You will use the protractor to tilt the Northern Hemisphere both 23.5º away from the sun and 23.5º toward the sun to model sunlight during the different seasons. 3. Gather your materials and design an experiment to determine how the tilt of Earth s axis affects the way sunlight strikes different parts of the Northern Hemisphere during summer and winter. Consider the following questions as you design your experiment: a. When the North Pole is tilted away from the sun, what season is it in the Northern Hemisphere? What season is it when the North Pole is tilted toward the sun? b. Using the flashlight, how can the projected pattern of the gridlines on Earth demonstrate what happens when light hits a curved surface at an angle? c. How do shadows cast by objects on Earth s surface help demonstrate the connection between the tilt of the axis, seasons, and higher temperatures on Earth s surface? 23

7 REASONS FOR THE SEASONS continued 4. Use the notepad below to describe your procedure and record your observations. Be sure to include sketches that show how the modified flashlight projects the gridlines onto the Earth model when the model s axis is tilted away from the source of light and when it is tilted toward it. Have your teacher review your procedure before you begin. CAUTION: Do not shine the flashlight into your eyes or anyone else s. Procedure 24

8 REASONS FOR THE SEASONS continued Analyze and Conclude Make Models Use your model to show that when the North Pole is not tilted toward or away from the sun, sunlight hits both hemispheres equally. How does the distribution of the sun s rays near the equator differ from the distribution of the rays near the poles? How does this relate to the temperature in these areas? How did your model capture this difference? Interpret Data When both hemispheres are receiving the same amount of sunlight, what seasons is Earth experiencing? What is the term that describes this event? Draw Conclusions When the Northern Hemisphere is tilted away from the sun, describe what happens to sunlight that strikes the high latitudes in the Northern Hemisphere, and explain how this relates to the arctic climate during winter. Infer The lengths of shadows can give some clue about what season it is and how far the object is from the equator. Where on Earth would shadow lengths change the least from season to season? Explain. 25

9 Design an Experiment What did you find difficult about this activity given the materials you had to work with? How would you improve upon your Earth/sun model? Infer What would be the effect on the seasons and the hemispheres if Earth were not tilted on its axis? Explain. Summarize Describe what you learned in this lab about what causes the seasons and what questions you still have. What I learned What I still want to know Redesign Revise your model set-up, or use diagrams or computer art software, to show how the tilted axis of Earth and Earth s orbit around the sun produce the seasons as we know them. Design your demonstration for an audience in the Northern Hemisphere. Be sure to mark or discuss the following: winter and summer solstices autumnal (fall) and vernal (spring) equinoxes 26