Seasons and Why the Equator is Warmer than the Poles
and is replicated here as part of the SERC Pedagogic Service.
The guided-discovery activity described here helps students confront and overcome both of these common misconceptions as it guides students toward an understanding of how and why the angle of incident sunlight determines the intensity of the solar energy that strikes the ground and hence how the angle of incident sunlight can be used to explain both seasonal and latitudinal differences in temperature. Along the way, this activity helps students visualize the true dimensions of the solar system and the various objects within it. This seemingly unrelated topic is included in this activity because an accurate perception of the scale of the solar system helps students understand that (1) Earth's equator is not significantly closer to the sun than are its poles, and (2) all sunrays intercepted by Earth are essentially parallel to each other, whether they strike the equatorial or polar regions – a concept that is essential for understanding how and why the angle of incident sunlight varies systematically with latitude and season.
- Visualize the true-scale proportions of the solar system – the sizes of objects and distances between them.
- Clearly and fully explain why it is warmer at the equator than it is at the poles.
- Show how the tilt of Earth's axis and Earth's revolution around the sun cause seasonal variations in temperature by causing seasonal variations in day length and maximum daily solar altitude.
Context for Use
Description and Teaching Materials
- Student Handout (Microsoft Word 522kB Sep5 09) for the Activity on the Seasons and Why it's Warmer at the Equator than the Poles
- Grid (Acrobat (PDF) 240kB Sep5 09) to be printed on an overhead transparency.
- Other materials needed (in chronological order):
- Collection of spherical objects to represent celestial bodies as follows:
- Light fixture with a single unshaded frosted light bulb.
- Pencil inserted into one 3 inch diameter white Polystyrene ball. An ordinary Styrofoam ball will not do; the ball must be opaque. Suggested source: Molecular Model Enterprises, 116 Swift St., P.O. Box 250, Edgerton, WI 53334, (608)884-9877. Prices are under $1 each.
- Per classroom
- Yellow or white exercise ball – 65 cm (25.5 inch) to 75 cm (29.5 inch) diameter, to represent the sun, which has a diameter of 1.4 million km. (The ideal scale model size would be 70 cm or 27.5 inches.)
- Exercise ball stand
- 10 cm Pilates toning ball to represent Proxima Centauri, the nearest star
- Outdoor space at least 120 m long
- Tape measure (optional; students can also measure distances by counting paces)
- 3 ring stands
- Overhead projector
- Large piece of white stiff (foam backed) poster board
- Large globe on a stand; 30" - 36" diameter is best.
Teaching Notes and Tips
General Comments: This activity can be completed in two hours if students give it a superficial treatment, but three to four hours are required for an in-depth exploration, including the confrontation of any misconceptions and the construction of a full understanding of the concepts.
As students work through these activities, encourage them to engage in lively conversations, continually brainstorming, questioning, and checking for consistency. In my experience, students are often inconsistent – gently call them on this. For example, one student adamantly insisted – despite the protestations of her team members – that the equator was not significantly closer to the sun than were the poles. Yet, she then attributed the temperature differences between the equator and the poles to differences in their distances to the sun. I pointed out that she was contradicting herself, which caused her to engage in some deep thinking which culminated in an "Aha!" moment that she absolutely delighted in.
Notes on Lab Activity #1: Scale Model of the Solar System
I recommend doing the outdoor activity as a whole class with student volunteers to represent each planet; choose the tallest person in the class to pace off the distances. When you get to "Earth," have the students place the moon at the appropriate distance from Earth and point out that, coincidentally, the moon and sun look the same size as seen from Earth.
In the table at the bottom of page 2, fill in appropriate landmarks in your local area that are located at the model distances.
Questions 1 and 2 on page 3 typically generate a great deal of discussion. The students may need a fair amount of prodding and questioning to get to the correct answers.
Notes on Lab Activity #2: Why is it Warmer at the Equator Than it is at the Poles?
Place the globe on a counter so that it is tilted to the right or left (i.e. Spring or Fall position) with the Pacific Ocean facing the overhead projector – having a relatively featureless part of the globe face the projector helps students see the grid projected on it more clearly, without being distracted by the details of complex shorelines or borders.
Notes on Lab Activity on the Causes of the Seasons
Before students begin this activity, explain to them that each group will be asked to formulate an initial hypothesis to explain the causes of the seasons. The students will then try to use this hypothesis to explain a series of facts. If their hypothesis is not up to the task, they must discard and reformulate, modify, or add to their initial hypothesis until it can provide a satisfactory explanation of the facts.
At the end of this activity, I typically assess student learning by having student groups present their answers to the rest of the class. I divide the different parts of this activity among the student groups, assigning each group to prepare illustrations and orally present their part to the rest of the class. Each presentation is then followed by a whole-class discussion.
I assign this Homework Assignment on the Causes of the Seasons (Microsoft Word 58kB Sep5 09) as assessment, but also as a way for students to consolidate the understandings that they have built during the guided-discovery activity. Students often benefit from a formal traditional presentation of concepts that they have previously struggled to grasp while working through a hands-on guided-discovery activity. When the traditional presentation follows guided discovery, it can be very meaningful, building students' confidence in their freshly hatched ideas and organizing their discoveries into a logical and elegant construct. Without some kind of traditional presentation of the concepts, students are often so unsure of themselves that they feel lost. Yet, in my classes, students rarely complete (let alone comprehend the material presented in) a "Read Chapter 3" type of assignment, even when it's followed by a quiz. But the vast majority of students will complete an assignment like the one presented here, which requires students to extract and record specific information from the textbook. Higher-order thinking it's not, but it is a helpful incentive to encourage students to open their textbooks and actually comprehend what is written there.
I give students practice answering questions in low-stakes ConcepTests (using clickers) or on-line practice quizzes before asking such questions on high-stakes exams. Here are some sample questions:
Why is it hotter at the equator than it is at the poles?
a. Because the equator is closer to the sun.
b. Because the sun's rays travel through more atmosphere at the equator.
c. Because the sun's energy is more spread out at the equator.
d. Because the sun's rays hit the earth's surface at a higher angle at the equator.
e. Because the sun is always directly overhead at the equator.
If the Earth's axis only had a 5° tilt, how would the seasons in Chico be different from how they are now?
a. The seasons would be shorter.
b. The transitions between seasons would be more abrupt.
c. The contrast in temperature between summer and winter wouldn't be as great.
d. Summer days would be longer than they are now and winter days would be shorter than they are now.
e. All of the above.
On exams, I ask students open-ended essay questions (Microsoft Word 4.3MB Sep5 09) that require them to synthesize the concepts gained from this lab activity.
References and Resources
Gould, A., Willard, C. and Pompea, S., 2004, The Real Reasons for Seasons: Sun-Earth Connections: GEMS (Great Explorations in Math and Science), Lawrence Hall of Science, University of California, Berkeley.
This teacher's guide features many engaging hands-on activities. Although it was written for teachers of grades 6-8, it is also useful for high school teachers and college professors.Schneps, Matthew H., 1989, A Private Universe.
This video was produced by the folks at the Private Universe Project who studied common misconceptions about the seasons and the phases of the moon. To view this video, scroll to the bottom of the page and click on the "VoD" icon. You will have to first register with the Annenburg Media Center (learner.org) to see the video, but registration is free and it grants you access to many other videos for educators.An extension of this activity that helps students deepen their understanding of the concepts is to model the path of the sun across the sky at different latitudes and at different times of the year. An excellent inexpensive solar motion demonstrator kit is available from the Astronomical Society of the Pacific (www.astrosociety.org). I have written a worksheet for students to complete (Microsoft Word 63kB Sep5 09) as they work with these solar motion demonstrators.