To understand that it is the tilt of earth’s axis that causes the seasons.
From their earth studies in grades 3 through 5, students have learned about earth itself and earth in relation to the sun. Students should understand that earth is on an axis and rotates (resulting in night and day) and that earth takes about a year to orbit the sun. More specifically, students should “reasonably understand the relative size, motion, and distance of the sun, moon and the earth,” or they will face a great challenge in understanding the phenomena of seasons.(Benchmarks for Science Literacy, p. 335-336.)
There is a common misconception (not mentioned in Benchmarks) of people of all ages regarding the seasons, and that is the notion that earth heats up because it is nearer to the sun. Though earth does make an elliptical orbit around the sun, it is not the distance that causes the seasons, it is the tilt of earth’s axis that results in the seasons: sunlight hits different latitudes at different angles at different times of year. (For more information on this misconception see the Miconceptions about Why Seasons Occur teacher sheet.)
You will need materials to demonstrate earth orbiting the sun. We recommend that you use an apple for the earth and poke a pencil through it to show the axis. A large ball is needed for the sun, and the bigger it is, the better. You can also use other materials. For example, clay works to make an earth and you can use a basketball for the sun (just remind students that the sun in proportion is much, much bigger than the earth).
The following questions will help you ascertain what your students think about how the seasons occur. Ask students the following questions and write their answers on chart paper. Save the chart paper so that you can review these answers later and allow students an opportunity to revise them.
- What are our seasons like? (These answers will of course depend on where you live. Be sure students explore weather changes and the difference in length of day when answering this question.)
- What are seasons like in other parts of the world? (Students may know that some parts of the world are warmer or colder. They may also know that some places are in darkness for most of the winter.)
- What do you think causes the seasons? (Students may come up with all sorts of suggestions, some of which may be wrong. Do not confirm or criticize their ideas, but ask them to elaborate, or reason out their ideas. See the Misconceptions about Why Seasons Occur teacher sheet for more information about common misconceptions that students might hold.)
Begin the lesson with a brief reading and activity at A Reason for the Season. You may choose to have students work in pairs to do the reading and activity, or you may have two students do it in front of the class. Either way, all students should read the page down to "Family X Files."
Note: Tell students not to follow the "cosmic map" link at this time.
After the activity, ask this question:
- If you are earth, as you orbited the sun, were you always in the same relation to the sun? (Clearly, the answer is no, but if students did not recognize this, have one pair do the exercise again. Have the "earth" student stop at various places and ask if she is facing the sun the same way.)
This exercise should help students visualize that the axis is always pointed in the same direction. It is not exactly analogous to earth orbiting the sun, however, because it doesn’t directly demonstrate how earth’s axis, in conjunction with the annual orbit, changes earth’s relation to the sun.
For instance, it is important for students to understand that the Northern Hemisphere "leans" toward the sun during part of the year resulting in more intense sunlight in that area of earth. During another part of the year it leans away, resulting in less intense sunlight and cooler weather.
The next exercise will demonstrate visually the "tilt" of earth and how different areas receive more intense sunlight throughout the year.
Put a large ball on a desk. Hold up an apple, stem facing up, and draw the equator with a marker explaining as you go along. Now poke a pencil all the way through the apple from top to bottom and explain that the top (where the stem comes out) is in the area of the Northern Hemisphere.
- If the earth is tilted on its axis how should I tilt the earth (apple)?
Now, with your earth (apple) tilted properly, be sure to keep the pencil pointed in the same direction as you start to walk it (orbit) around the bigger ball (sun). It is easiest to keep the pencil facing toward the same wall. Do one full orbit, then do another orbit and stop at when the Northern Hemisphere is leaning toward the sun.
Ask these questions:
- Where is the Northern Hemisphere pointed? (It is pointed toward the sun.)
- Do you think the Northern Hemisphere is warmer during this time of the year? (Yes, because sunlight is hitting it directly. Be sure that students don’t think it is because that part of earth is closer to the sun.)
Now, keep orbiting until the Northern Hemisphere is pointed away from the sun and ask:
- When the Northern Hemisphere points away from the sun, then what? (Now, the Northern Hemisphere gets cooler.)
- Why does the Northern Hemisphere get cooler? (Again it is important for them to realize that it is the sunlight falling less intensely that causes it to be cooler. )
- What about the Southern Hemisphere? (The Southern Hemisphere receives direct rays and is now warmer than it usually is.)
Students at this point should understand that sunlight falls more intensely on different parts of the earth throughout the year. Now, give them a sense of time. They probably know that it takes earth a year to orbit the sun, but may not have thought about it in the context of seasonal changes.
Asking this question will help students realize a complete picture of the seasons:
- How many months do you think go by between when the Northern Hemisphere receives direct rays from the sun as opposed the Southern Hemisphere? (About six months. This should bring home the idea that as the earth orbits, the seasons on different parts of the planet change. You may need to demonstrate the orbit one more time, stopping at each quarter to show that earth is in a different position in relation to the sun throughout the year.)
Now have students go back to A Reason for the Season and click on "cosmic map." This activity is a fun way to reinforce what they’ve just learned. (You should mention that the scale of earth and sun in this map are not accurate.)
Review students’ original ideas about what causes the seasons (discussed in the Motivation). You may want to use different color markers to amend certain ideas on the chart paper, or to add new ideas.
Discuss how their thoughts might have changed:
- Do you have the same ideas about why we have seasons?
- If not, what is different? (Students should recognize that they might have had misconceptions.)
- Does this make you wonder if you have ideas about other things that might not be exactly right? (Invite students to feel comfortable with the fact that they may have wrong ideas about some things, and that they should never assume they are right.)
To assess student understandings of the seasons, ask them to demonstrate with either a drawing, poster, or model how and why the seasons change. Regardless of how they demonstrate this concept, students should show that:
- The earth is tilted on an axis
- Different parts of earth receive direct rays from the sun at different times of the year
- The relation of earth’s hemispheres in relation to the sun are what result in different seasons
Consider developing a rubric for assessment of this activity. There are several resources on the Internet that describe the use of rubrics in the K-12 classroom, a few of which are highlighted here.
To learn more about rubrics in general, see Make Room for Rubrics on the Scholastic site.
For specific examples of rubrics, more information, and links to other resources, check out the following sites:
Finally, you can go to Teacher Rubric Makers on the Teach-nology.com website to create your own rubrics. At this site you can fill out forms to create rubrics suitable for your particular students, and then print them instantly from your computer.
At Starchild, a NASA site, there is a Song for All Seasons. This site is a great follow up to the lesson. It portrays the earth at four different times of the year, and asks students to play Vivaldi’s “Four Seasons” and match the seasonal song with the placement of the earth.
Additional resources that may be helpful: