Tilting Around the Sun
How do Earth’s movements in space cause the seasons?
Background It is the middle of December. What are your plans for the weekend? Will you go
sledding? Or perhaps you’ll go to the beach—if you live in Australia. The Northern and Southern
hemispheres experience different seasons at different times of year. What patterns related to
Earth’s motion in space lead to the seasons?
Materials (per pair)
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acetate sheet with thick grid lines
books
flashlight
masking tape
metric ruler
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plastic foam ball with North Pole, South
Pole, and equator labeled
plastic foam block
protractor
wooden dowel or chopstick
Safety
□Note the lab safety icons that appear as warnings within certain procedure steps. For an
explanation of the meaning of each icon, go to the Lab Safety information section of this
online course. Be sure to follow all safety procedures established by your teacher and your
school.
Pre-Lab Questions
1. Develop Models What do you think the flashlight and foam ball will represent in your
model?
2. Develop Models Which part of your model will represent Earth’s Northern and Southern
Hemispheres?
Procedure
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1.
On a table, 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 dowel into the South Pole and through the center of the
ball until about 1 inch emerges at the North Pole. Leave several inches of the dowel
extending below the South Pole. CAUTION: When pushing the dowel into the foam
ball, direct the sharp point away from yourself and others.
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4.
Use the protractor to measure a 23.5° angle in the tilt of Earth’s axis. Tilt the
North Pole away from the flashlight. This represents Earth on December 21.
5.
Push the bottom of the dowel into the foam block so Earth remains steady at the
23.5° angle (not shown). Hold the Earth model about 15 cm from the flashlight head.
6.
Turn on the flashlight. Dim the room lights. CAUTION: Do not shine the
flashlight directly into anyone’s eyes.
7.
The grid lines on the acetate sheet should show up on your model Earth. If the
lines are not clear, move the ball closer to the flashlight or dim the room lights more.
Observe the size of the squares in the Northern Hemisphere and the Southern
Hemisphere. Draw these shapes as you see them in the boxes below.
Sketch A: December 21
Squares in Northern Hemisphere
Squares in Southern Hemisphere
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8.
Use Models Use your model to simulate Earth’s revolution around the sun. One
student should carry the Earth model around the flashlight. Keep the tilt pointing in the
same direction with respect to the room. For example, if the tilt pointed toward the door
in Step 4, keep it pointed toward the door. At the same time, another student should
spin the flashlight to follow the globe. Continue the movement until the Northern
Hemisphere is tipped toward the flashlight. This represents Earth on June 21.
9.
Use your model to observe and draw the shapes formed by the grid lines in the
Northern Hemisphere and the Southern Hemisphere.
Sketch B: June 21
Squares in Northern Hemisphere
Squares in Southern Hemisphere
Analyze and Interpret Data
1. Make Observations Look at your sketch for December 21. Describe the different ways the
squares appeared as they were projected on the Northern Hemisphere and the Southern
Hemisphere.
2. Infer Each square represents the same total amount of light from the flashlight. What can
you infer about the concentration of light shining on Earth where the grid marks are spread
out, and when the grid marks are closer together?
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3. Distinguish Relationships In December, which hemisphere of Earth receives more direct
sunlight? What season does this hemisphere experience? What season does the opposite
hemisphere experience on the same date? Use details from your sketch to explain your
answers.
4. Determine Differences In June, which hemisphere of Earth receives more direct sunlight?
What season does each hemisphere experience on this date? Use details from your sketch
to explain your answer.
5. Use Models As your model Earth moves along its orbit from December to June, how does
its tilt change in relation to the sun? How did you adjust your model for the investigation?
6. Use Patterns Use what you have learned to explain the cause and effect relationship
between Earth’s titled axis and the occurrence of the seasons in different hemispheres.
Describe when winter and summer occur in each hemisphere.
7. Develop a Model How could you use your model to show how sunlight hits Earth’s
hemispheres on March 21 and September 21? In the space below, draw your model setup.
Then, predict how the squares of light would appear on the Northern and Southern
Hemispheres. What would the seasons be in each hemisphere during these times?
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Extend Your Inquiry
Develop a Model Earth’s tilted axis doesn’t affect only the concentration of sunlight hitting
different hemispheres. It also affects the number of total hours of daylight. Earth continuously
rotates on its axis as it orbits, causing day and night. At different parts of the year, the two
hemispheres experience more or fewer hours of daylight because of Earth’s tilted axis. Revise
your model setup using diagrams or computer art software to complete the following tasks: (a)
find a way to model how the Earth rotates on its axis, causing day and night, and (b) find a way
to model how many hours of daylight the Northern and Southern Hemispheres receive during
different parts of the year. Demonstrate your models and computer animations in front of the
class. Design your demonstration for an audience in the Southern Hemisphere.
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Teacher Support
Focus on NGSS Students will apply the following practices, ideas, and concepts in this
activity.
Science and Engineering Practices
Connect to the Core Idea
Crosscutting Concept
Develop and use a model to describe
phenomena.
This model of the solar system can
explain eclipses of the sun and the
moon. Earth’s spin axis is fixed in
direction over the short-term but tilted
relative to its orbit around the sun. The
seasons are a result of that tilt and are
caused by the differential intensity of
sunlight on different areas of Earth
across the year.
Patterns can be used to identify cause
and effect relationships.
-----------------------------------------------Connections to Nature of Science
Science assumes that objects and
events in natural systems occur in
consistent patterns that are
understandable through measurement
and observation.
Group Size Pairs
Class Time 40 minutes
Safety
1. Remind students that shining a flashlight into someone else’s face can cause blind spots,
which could lead to accidents.
2. Since this activity works better in dim light, remind students to be careful of objects in the
classroom if they walk around the model to view it from different angles.
3. Remind students to be careful when pushing the pencil point into the foam ball.
Advance Preparation (10 minutes)
1. For safety and effectiveness, LED flashlights produce a bright light without generating much
heat. Make sure that the flashlights are working properly. Have extra batteries on hand.
2. The room must be dim enough for the grid pattern from the acetate to show up on the ball.
3. Prepare the acetate sheets by copying a piece of graph paper onto the sheets. One acetate
sheet can be cut into six grids.
Procedure Tips
Step 4 If necessary, demonstrate how to use the protractor to set up the angle of Earth’s tilt. It is
difficult to measure a 23.5° angle with an ordinary protractor, but high precision is not
necessary for this activity.
Step 5 Remind students to maintain the orientation of Earth’s axis. If students have trouble
keeping this orientation constant, have a student hold a pencil parallel to the model’s axis at
the beginning of the “orbit.” That pencil must be kept stationary as the model revolves
around the flashlight, and the axis of the model should remain pointing in the same direction
as the pencil.
TE Annotations
Pre-Lab Questions
1. The flashlight represents the sun, and the ball represents Earth.
2. Earth’s Northern Hemisphere is the half of the ball between the equator and the North
Pole, and its Southern Hemisphere is the half of the ball between the equator and the
South Pole.
Procedure
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Step 7: Expected Outcome: The grid squares will be smaller and less distorted in the Southern
Hemisphere, where the light from the flashlight (sunlight) hits directly. The grid squares will be
larger and more distorted in the Northern Hemisphere, where the light hits at an angle.
Step 9: Expected Outcome: The grid squares will be smaller and less distorted in the Northern
Hemisphere, where the light from the flashlight (sunlight) hits directly. The grid squares will be
larger and more distorted in the Southern Hemisphere, where the light hits at an angle.
Analyze and Interpret Data
1. In the sketch for December 21, the squares in the Northern Hemisphere were large and
stretched out, while the squares in the Southern Hemisphere were smaller and less
distorted.
2. Where the grid marks are spread out, the light reaching Earth is spread out, so each spot
gets less sunlight. Where the grid marks are close together, the light is more concentrated.
3. In December, the Southern Hemisphere receives more direct sunlight. It is summer there.
In the opposite (Northern) hemisphere, it is winter. In the Northern Hemisphere, the light is
spread out, so places in the Northern Hemisphere get less light and heat.
4. In June, the Northern Hemisphere receives more direct sunlight. It is summer in the
Northern Hemisphere and winter in the Southern Hemisphere. My sketch shows that the
light in the Northern Hemisphere is concentrated, while the light in the Southern Hemisphere
is spread out.
5. In December, the north end of the axis was tilted away from the sun, but as Earth moved
around, the north end of the axis was tilted toward the sun. I had to make sure not to rotate
the model, so that the axis always pointed in the same direction.
6. Earth’s axis is tilted. As a result, one part of Earth tilts toward the sun, and one part tilts
away. In December, the axis is tilted such that the Northern Hemisphere points away from
the sun. The Northern Hemisphere gets less sunlight, so it is winter there. Meanwhile, the
Southern Hemisphere is tilted toward the sun. As a result, it gets more sunlight, so it is
summer there. At the opposite side of its orbit, in June, Earth’s axis is tilted such that Earth
“tips” in the opposite direction from the sun. The Northern Hemisphere has summer, and the
Southern Hemisphere has winter.
7. Drawings should show the axis tilted at the same angle and direction as in previous models,
but Earth should now be placed in its orbit so neither hemisphere tilts toward the sun.
Sample answer: The squares of light would be about the same in each hemisphere. In
March, the Northern Hemisphere has spring, while the Southern Hemisphere has fall. In
September, the Northern Hemisphere has fall, while the Southern Hemisphere has spring.
Extend Your Inquiry
Students’ models and illustrations should show the tilted Earth rotating on its axis, with distinct
“day” and “night” sides of Earth. During December, locations in the Southern Hemisphere
experience many hours of daylight, as the angle of Earth’s axis exposes more of the Southern
Hemisphere to sunlight. During June, locations in the Southern Hemisphere experience fewer
hours of daylight, because the tilted axis exposes less of the Southern Hemisphere to daylight.
Encourage students to focus on the Southern Hemisphere, as this will help reinforce that
seasons depend on one’s location on Earth.
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