science paper
The Reasons for the Seasons
Ask a fifth-grader why he or she believes Earth
has seasons, and the answer usually involves a
mistaken notion about Earth’s distance from
the Sun. Not only are elementary students often
stumped by the seasons, but adults also commonly
misunderstand this concept—even Harvard University
graduates (Schneps, Sadler, and Woll 1988).
Children understand that temperature usually fluctuates
depending upon one’s nearness to a heat source, which gives
rise to the false analogy of the Sun’s heat and its presumed
effect on Earth’s seasonal temperature fluctuations. Another
explanation for this widespread erroneous impression
may lie in the two-dimensional drawings that often depict
Earth’s orbit around the Sun. Most diagrams emphasize the
elliptical nature of Earth’s orbit. Although it is technically
elliptical, Earth’s orbit is a nearly perfect circle, with only a
2% difference between its apogee (the point in Earth’s orbit at
which it is farthest from the Sun’s center) during the month
of June and its perigee (the point in Earth’s orbit at which it
is closest to the Sun’s center) during the month of January.
Perigee occurs in January, corresponding with the Northern
Hemisphere’s winter, and the apogee occurs in June, corresponding
with the Northern Hemisphere’s summer. The construction of a three-dimensional model of the
changing seasons using simple materials has been successful
in correcting students’ misinterpretation of the cause
of the seasons (Lambert and Ariza 2008).
Like the other planets, the Earth rotates on its axis as
it revolves around the Sun. Earth is currently tilted 23.5º
on its axis and remains in the same alignment with respect
to the background stars throughout its orbit around the
Sun, which takes 365.2 days. The
North Pole always points toward
Polaris or the North Star. We know
that Earth is tilted 23.5º because of
the geometric relationship between
Earth and the Sun. The difference
between the angle of the midday Sun
on an equinox (September or March) and a solstice (December or June) is equal to 23.5º.
As Earth revolves around the Sun, its axis remains
tilted 23.5º in the same direction. However, the direction
of Earth’s tilt with respect to the Sun does change, causing
the seasons. When the Northern Hemisphere is tilted
toward the Sun, that half of the Earth receives more direct
sunlight and has summer. At the same time, the Southern
Hemisphere is tilted away from the Sun and has winter.
In this lesson, students employ a simple model to
learn how Earth’s tilt and revolution around the Sun
causes our seasons.
Julie Lee Lambert ([email protected]) is an associate
professor at Florida Atlantic University in Boca Raton,
Florida. Suzanne Smith Sundburg (pro.wordsmith@
verizon.net) is a freelance science writer and editor in
Arlington, Virginia.
References
Lambert, J. and E.N. Ariza. 2008. Improving achievement
for linguistically and culturally diverse learners
through an inquiry-based Earth systems curriculum.
Journal of Elementary Science Education 20 (4):
61–79.
Schneps, M.H., P.M. Sadler, and S. Woll. 1988. A private
universe: Misconceptions that block learning [Videorecording].
Santa Monica, CA: Pyramid Films.
Explaining Seasons With
Tilting Toothpicks
What causes the seasons?
Grade Level: Grades 4–6
Process Skills: Observing, modeling, inferring, and
communicating
Engage
To assess students’ prior knowledge, first each student
answered a brief preassessment (see NSTA Connection).
The preassessment helped determine whether
students thought Earth’s changing distance to the Sun
causes seasons or whether students thought that the tilt
physically changes during different seasons. Additionally,
it helped teachers determine if students knew that
the Northern and Southern Hemispheres are experiencing
opposite seasons when shown a diagram of the
Sun’s rays and a tilted Earth.
Teams of students were then asked to make a model of
the seasons using a small craft light, four Earth models
made of Styrofoam, four toothpicks, and a protractor.
Students were told that the toothpick represented Earth’s
axis and to push the toothpick into the ball through the
North Pole so that the end would go out at the South
Pole. They also were told that each Earth model should
represent one season.
Teams were asked to sketch their physical model and
to answer a series of questions (the summary of embedded
assessments is available online; see NSTA Connection).
Each team then presented its model. The initial models
revealed students’ naïve or alternative conceptions. Most
of the teams initially explained the seasons as being the
result of Earth’s distance to the Sun. Most teams had the
tilt of the summer and winter Earth models correct, but
they were not sure what to do with the tilt in the spring
and fall Earth models. Figure 1 shows a typical model in
which the students placed Earth closer to the Sun during
the summer season and farther in the winter with the correct
tilt, but then made the tilt more vertical for the spring
and fall season. Occasionally, a model did not match the
verbal explanation. For example, a team may have said
that it kept the tilt the same, but the model showed a
change in the direction of the tilt (Figure 2).
Explore and Explain
Teams were then asked to read a narrative describing
Earth’s orbit and its proximity to the Sun throughout
the seasons, its tilt on its axis in relation to the Sun,
and the amount and angle of direct rays of sunlight
that each hemisphere receives during a particular
season (see NSTA Connection). Based on the information contained in the story, the teams were asked
to revise their models accordingly. Each team’s revised
model was then checked, and the previous
explanation was expanded on during a whole-class
discussion. Assessment was again embedded (see
NSTA Connection).
Each team eventually constructed a correct model of
the seasons. One student helped his team understand the
changing seasons by using a protractor to place each of the
four toothpicks (without the Styrofoam Earth spheres) on
the base, each at a 23.5º angle and all pointing the same
direction. Immediately, students on his team seemed to
understand the cause of the seasons. This simple explanation
seems to help most students construct a correct
model of the seasons.
The lesson highlighted one of the more difficult
concepts underlying the cause of the seasons—the idea
of direct and indirect light. Students sometimes asked
why the Arctic is not warmer when it receives almost 24
hours of daylight during the summer. To help students
understand why regions near the equator are warmer,
a teacher can hold a flashlight perpendicular to a line
drawn on a board. Using a marker, the bright area can
be circled. Then the light should be moved so that it
shines over the line at an angle, and the marker should
again be traced around the bright area. Students will
observe that the area was smaller when the light was
shone at a perpendicular angle, and therefore the Sun’s
rays would be spread over less surface area and the
area would be much warmer. When Sun’s rays strike
Earth’s surface nearer the equator, the Sun’s radiation
is spread over a smaller area than at higher latitudes.
See the “What Causes the Seasons?” Science 101 column
(Robertson 2007) for a detailed explanation of
this phenomenon.
Extend
Students next applied their understanding of the real
world by constructing a working sundial to measure the
time of day (find directions online; see NSTA Connection).
As the Sun shines on the sundial, the shadow of
the gnomon’s point will cover the current time on the
time dial (Figure 3).
Next, we made an astrolabe, an instrument used to
measure the angle of an object in the sky, such as the Sun
or Moon, above the horizon (see NSTA Connection). In
Greek, the word astro means “star,” and labe means “to
find.” Both the sundial and the astrolabe can be used to
track the Sun’s path across the sky throughout the day
or year.
Finally, students compared the number of daylighthours and the path of the Sun for each season in cities
at different latitudes. Sunrise and sunset times
for most cities can be found on the U.S. Naval Observatory’s
Astronomical Applications website (see
Internet Resource).
Reference
Robertson, W. 2007. What Causes the Seasons? (Science
101) Science and Children 44(5): 54–57.
Internet Resource
U.S. Naval Observatory’s Astronomical Applications
http://aa.usno.navy.mil/data