Time: 45 minutes
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by Lori Lambertson
model the seasons with their own earth globes.
For the whole group, you will need:
• a 150 – 200 watt light bulb (not frosted) or a flashlight
• a lamp or socket for the bulb
• an extension cord
• a room that can be made dark
For each student or pair of students, you will need:
• a styrofoam ball
• a large straw
• a rubber band
• a flexible plastic cup (5.5 oz)
• a thumbtack or sticky dot
• a protractor
• a ruler with centimeters
Making a Model of the Earth
Push the straw through the center of the styrofoam ball. This represents
the axis about which the earth rotates. One end is north and the
other end is south. Place a rubber band around the the center of
the ball (the earth’s equator). Looking at a globe or a map,
find your approximate latitude, and place the tack or dot there.
(For example, San Francisco is at 37.75¾ north, so placing the tack
not quite halfway between the equator and the North Pole is an acceptable
approximation.) Use the scissors to make a hole in the bottom of
the plastic cup, near the side, as shown. It should be just large
enough to accommodate the diameter of the straw.
Take an 8-cm piece of tape and stick a 2-cm piece to the center of
the 8-cm piece (sticky sides together). Place the straw into the hole
in the cup, and use the modified tape to hold the straw against the
side of the cup, yet still allowing the straw to rotate in the hole.
model should look something like the picture above. Use a protractor
to check the angle of the earth’s tilt. It should be 23.5 degrees.
up the Room
Use one bright lamp for the whole group, or use flashlights for small
groups. Designate some visual reference as Polaris, the North Star.
All straws should point to Polaris throughout the activity.
Set up the light in the center of the group, or flashlights in the
center of the group, if using small groups. Before darkening the room,
make sure all earth models are oriented correctly. Darken the room.
To Do and Notice: Model a “Day” on Earth
Each student should turn the straw so that the earth spins counterclockwise
(when viewed from the north) for one rotation. They should notice
that the dot is in light (day) for about half of the rotation and
is in shadow (night) for about half of the rotation.
Modeling the Seasons
Divide the class into four groups. Have each group move to one of
the four seasonal positions around the lamp (or one student at each
position around a flashlight if using small groups): December 21,
March 21, June 21, and September 21. It should look like this if it
were seen from above:
each group model a day at each position. At each position, they should
• For what fraction of the day is the dot in the light? More
than half? Less than half? About half?
• For what fraction of the day is the North Pole in the light?
• How is the light from the sun striking the dot? Is it direct
or at an angle?
For example, on December 21:
Angle at or above equator, direct below equator
Modeling a Year
After students have been to each of the four dates: December 21, March
21, June 21, and September 21, they will have modeled a year: one
earth revolution around the sun.
What’s Going On?
Many people think the seasons are caused by variations in our distance
from the sun. While the earth’s orbit is slightly elliptical,
it is very close to circular, and the variation in distance between
the earth and sun is not enough to account for our seasons. The seasons
are caused by the tilt of the earth. The earth holds its rotation
axis (tilt) fixed in space as it moves around the sun. In the summer,
the Northern Hemisphere tilts toward the sun. It is warmer because
(1) there are more hours of daylight, providing us with more heat
energy, and (2) the midday sun shines more directly head on, increasing
the amount of solar energy the earth receives. In the winter, when
the Northern Hemisphere tilts away from the sun, the sun’s rays
strike the earth at a lower angle, and the energy from the sunlight
is spread out over a larger area, which reduces its effectiveness
at heating the ground. Combined with shorter daylight hours, the temperatures
are cooler in winter. The seasons in the Northern and Southern Hemispheres
Knowing North | Make
Your Own Petroglyph | Seasons