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Summary of Article
This article begins by dispelling a myth about seasons. Many people believe that
seasons occur because the distance between the Sun and Earth changes throughout
the year as the Earth orbits the Sun. This is simply not true. Seasons occur because
of two factors working together. These include the orbit of the Earth around the Sun
while the Earth remains in the plane of the ecliptic, and the tilt of the Earth's axis.
This article uses animation to explain these two components. A series of "what if…"
questions follows. These questions focus on an Earth with no seasons: What would
life be like? The article winds down by reviewing NASA's recent and emerging
investigations into the changing of seasons, and the rotation and orbit of the
Earth.
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Suggested
Age/Grade Levels
Age Level: 14 - 18
Grade Level: 9th grade - 12th grade
Related Topics
Earth Science
Students can learn about the existence of seasons. This makes an
excellent introduction to a lesson block about the biosphere and the biomes of the world.
Astronomy
Students can study the relationship between the Sun and the Earth.
Geography
Students investigate the relationship between the tilt of the Earth's axis and the
coordinates of the tropic of Cancer, tropic of Capricorn, the poles, and the equator.
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Objectives
After studying the article, students should be able to:
- Become aware of the most common myth about seasons.
- Analyze the importance of the Earth's orbit to the existence of seasons.
- Identify and describe the plane of the ecliptic.
- Explain how the tilt of the Earth's axis fits into the cause of seasons.
- Describe the importance of the tilt of the Earth's axis
and the coordinates of the Tropic
of Cancer, the equator, and the Tropic of Capricorn during changing positions
of the Earth along its orbit.
- Define summer solstice, autumnal equinox, winter
solstice, and vernal equinox.
- State the reasons for seasons.
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Classroom Activities
Make Your Own Models
- Fishing line
- One Styrofoam ball painted orange
- One Styrofoam ball painted blue
- One 15-inch piece of thin strong wire
- One 5-inch piece of thin strong wire
- One pair of pliers
- One paperclip
- One globe
- A small pocket flashlight
- Masking tape
Plane of the Ecliptic Model
This model will help students understand that the orbit of the Earth
remains in a fixed plane as it revolves around the Sun. The objective
of this investigation is to show students that the centers of the
Earth and Sun remain in the same plane during the orbit.
It is important that students understand how the combination of the
plane of the ecliptic and the tilt of the Earth's axis produces seasons.
Assemble the model in front of the class so they can get an
understanding of the components involved. Ask a student from the class
to help assemble the model. Have the student hold one end of the
fishing line. Tie the other end to the paperclip and place
the "hook" into the orange ball. Now you have your Sun. Next, use the
pliers and twist the wire around the string just above the orange ball. The
wire simulates the plane of the ecliptic. Twist
the other end of the wire into a small circle. Place the second wire
through the small circle and twist it 360 degrees. Insert the two ends of
the wire into the blue Styrofoam ball at an angle close to 23.5
degrees. Now you have your axis. Make sure that the center of the
Sun (orange ball) and the center of the Earth (blue ball) are in the
same plane.
Now it's time to use your model. Show the class how the centers of the
Earth and the Sun remain in the same plane during the orbit
(counterclockwise). You can have your helper make the Earth revolve
around the Sun as you simulate the rotation of the Earth's axis
(counterclockwise) during the orbit.
The Sun, the Earth, and the Axis
Using a globe and a small flashlight is a great way for students to learn
about the relationship between the Sun, the Earth, and the 23.5 degree
tilt of the Earth's axis. It's a good idea to use a
small flashlight so the light is more concentrated on the globe.
Have the students gather around a central table and outline an
orbit with the masking tape. Ask two students from the class to help
with this experiment and turn out the lights. Have one of the
students slowly trace the orbit with the globe as you keep the
flashlight shining on it from a fixed position. Have the other student
slowly rotate the globe about its axis (counterclockwise) as it
continues on its orbit.
From this model you should explain the difference between indirect and
direct rays, pinpoint the equinoxes and solstices, and observe how the
orbit impacts the North and South Poles.
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Questions for
Discussion
Q: What would happen to the Earth if its axis was not tilted?
A: Climatic conditions would be very different if there was no tilt to the
Earth's axis. The Sun would always shine on the equator and no seasons would
occur. A steady climate would exist across the planet. The region around the equator
would always be hot and the area around the poles would always remain cold.
Q: What would happen to climatic conditions on Earth if the
angle of its axis was exaggerated greater than 23.5 degrees?
A: The plane of the ecliptic (the perpendicular angle of the Sun to the Earth
during the course of a year) would increase beyond the 23.5 degrees north and south
latitudes. More of the Earth's surface would be too hot. Polar regions would melt,
causing ocean levels to rise.
Q: If 23.5 degrees north latitude is perpendicular to the Sun, what
season begins in Australia?
A: When the Sun is perpendicular to 23.5 degrees north latitude, the
perpendicular angle of the Sun to the Earth is at its farthest point
from Australia, which means this date is the first day of winter
(the winter solstice).
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Quiz
Click here for
a Seasons quiz.
*Please use your browser's back button to return to the
Seasons teacher's guide.
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Just the Facts
The distance between the Earth and the Sun does not account for the seasons.
The Earth's tilted axis (23.5 degrees) causes some parts of the Earth to receive
direct sunlight while other parts are receiving indirect sunlight.
Seasons occur due to the tilt of the Earth's axis and the orbit of the Earth around the Sun.
The seasons of the Northern and Southern Hemispheres are reversed:
when the Northern Hemisphere is experiencing winter, the Southern
Hemisphere is experiencing summer.
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Links
to Relevant Web Sites and Additional Resources
http://observe.ivv.nasa.gov/nasa/earth/seasons/seasons10.html
*Page will open in a new window.
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Vocabulary
plane of the ecliptic: the apparent path of the Sun across the sky, or the orbit of the
Earth around the Sun in the same plane.
vernal equinox: the first day of spring in the Northern Hemisphere
(March 21), when the Sun is perpendicular to the equator.
summer solstice: first day of summer in the Northern Hemisphere
(June 21), when the Sun is perpendicular to the tropic of Cancer.
autumnal equinox: first day of autumn in the Northern Hemisphere
(September 22), when the Sun is perpendicular to the equator.
winter solstice: first day of winter in the Northern Hemisphere
(December 21), when the Sun is perpendicular to the Tropic of Capricorn.
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For Fun
Just for fun, play our
Seasons Wordsearch game.
*Please use your browser's back button to return to the
Seasons teacher's guide.
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