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