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Discover Your Changing World with NOAA

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When we talk about Earth’s climate, we are basically talking about the effects of energy from the Sun, and how much 

of that energy is received at various places on Earth. This energy 

heats Earth’s land, ocean, and atmosphere. Strong winds and 

large ocean currents are the results of heat moving from warm 

areas—like the Equator—to colder areas of our planet—like 

the North and South Poles. So, some of the Sun’s heat energy 

is changed to motion energy. A Solar Heat Engine is a tool we 

can build that also changes heat energy from the Sun to motion 

energy. 

How It Works

Many plastics shrink when they are heated. This engine uses 

strips of plastic attached to a flywheel that is mounted on a 

drum that can rotate on an axle. When one of the plastic strips is 

exposed to sunlight, it shrinks and pulls the flywheel off-center. 

This movement causes the drum to rotate. As the drum rotates, 

another plastic strip is exposed to the sun, and the motion 

continues. As the strips move into the shadow of the drum, they 

cool and lengthen again.

How to Do It

[NOTE: This activity should be done with adult supervision.]

1. Lay a garbage bag flat on a large surface (the floor is fine) and 

cut the bag into strips that are about three inches wide and 

ten inches long. You need eight strips to make your engine, 

but you should cut some extra ones for practice. 

2. Stretch each of the plastic strips by holding one end in each 

hand, gripping the plastic tightly between the ends of your 

fingers and the base of your palm. Slowly stretch the plastic 

strip, until it is a little more than twice its original length and 

is about 1” wide. (Photo 1). Some of the strips will probably break 

because garbage bags aren’t perfect, but be patient and work 

slowly. When you have six stretched plastic strips, cut about 

two inches off of each end (the parts that you held onto and 

didn’t get stretched).

3. Measure the diameter of the large and small ends of a 

Styrofoam cup, and use your compass to draw two circles with 

the small diameter and one circle with the large diameter 

on the Styrofoam freezer tray. Use a compass instead of just 

tracing the circles around the cup so that you will know where 

the exact centers of the circles are. Cut the circles out with a 

sharp knife. (Be careful and wear gloves!)

What You Will Need

Adult partner

1 Black plastic trash bag

2 Styrofoam cups, 16 oz capacity

1 Wood dowel, about 1/4-inch diameter

1 Styrofoam freezer tray

2 Straight sewing pins 

1 Plastic lid (from yogurt, margarine or 

similar container), about 4-inches 

diameter

2 Metal food cans, tops completely 

removed

Masking tape

Scissors

Hot glue gun (low temperature)

Metal file

Ruler

Drawing compass

Unsharpened pencil

Sharp knife

Gloves for protection while using the 

knife

* (adapted from Strahl, 2007;

http://www.instructables.

com/id/Solar-Thermal-Motor/)

Photo 1. Stretching the plastic strips. Photo 2. Photo 3.

Big Idea

The Sun is the primary source of 

energy for Earth’s climate system.

(Climate Science Principle 1)

Activity 1: The Great, Glowing Orb

What You Will Do: Make a Solar Heat Engine*
3

1. The Great, Glowing Orb

Climate Science Principle 1: The Sun is the primary source of energy for Earth’s climate system.

4. Prepare the Fixed Cup Assembly: Make a hole in the 

bottom of one Styrofoam cup that is the same 

diameter as your wood dowel. Make a similar 

hole in the center of the large Styrofoam circle 

and in the center of one of the small circles that 

you cut out in Step 3. Use hot glue to fasten these 

circles to the large and small ends of the cup. Put 

the wood dowel through both holes in the cup 

assembly, and adjust the dowel so that about one 

inch extends past the large end of the cup (Photo 

2). Use hot glue on both ends of the assembly 

to hold the dowel in place. Add extra hot glue 

around the dowel at the small end of the cup 

assembly to form a smooth rounded blob that will 

provide a pivot point around which the Wobble 

cup can wobble (Photo 3).

5. Prepare the Wobble Cup Assembly: Make a hole in the 

bottom of the remaining Styrofoam cup that is 

slightly larger than the diameter of your wood 

dowel. Be sure the hole is large enough so that 

the cup can wobble freely on the dowel. Make a 

similar hole in the center of the small Styrofoam 

circle that you cut out in Step 3. Use hot glue to 

fasten the small circle to the small end of the cup 

(Photo 4). 

6. Apply a one-inch line of hot glue along the inside 

lip of the Wobble Cup Assembly (Photo 5), and 

place the end of one of the plastic strips on top 

of the glue. Use an unsharpened pencil to hold 

the plastic in place until the glue sets. Glue seven 

more strips around the rim of the cup in the same 

way. The strips should be evenly spaced around 

the cup with a gap of about 1/4-inch between the 

strips (Photo 6). 

7. Slide the Wobble Cup Assembly onto the dowel 

so that the small ends of the Styrofoam cups 

are close together. Use pieces of masking tape 

to temporarily hold the Wobble Cup so that it 

is centered on the dowel (Photo 7). Glue the free 

ends of the plastic 

strips onto the 

Styrofoam circle 

on the Fixed Cup 

Assembly. Be sure 

the strips are taut 

when you glue them, 

but should not be 

stretched so tight that 

they pull the Wobble 

Cup off center.

8. Cut a one-inch hole 

in the center of the plastic lid. (Be careful and 

wear gloves!)Remove the masking tape, and glue 

the plastic lid onto the large end of the Wobble 

Cup Assembly. Be sure the hole in the plastic lid 

is centered on the wood dowel. Stick a sewing 

pin into each end of the dowel. Your engine is 

finished! 

9. Use a metal file to make a small notch in the rim 

of each metal can. Space the cans so that you 

can rest the sewing pin at each end of the dowel 

into the notch on one of the cans. (Photo 8).Test 

the balance of your engine by slowly spinning it. 

If one side seems heavy, stick sewing pins into 

the Styrofoam circle on the opposite side until it 

seems balanced.

10. Put your 

assembled engine in 

the sun, and watch it 

spin!

Sewing Pin

Stretched Plastic Strip

(only 2 shown)

Metal Food Can

Cup

Glue Blob Plastic Lid

(Flywheel)

Dowel

Styrofoam Circle Styrofoam Circle

Cup

Sewing Pin

Metal Food Can

Photo 4. Photo 5. Photo 6. Photo 7.

Figure 1. Exploded view of Solar Heat Engine. Red arrows show direction of assembly.
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Discover Your Changing World with NOAA

CHALLENGE: 

How could you increase the power of this type of 

solar heat engine?

THINK ABOUT IT: 

Would strips made from a white plastic bag work 

as well as strips made from a black plastic bag? 

Is it important to consider the principles of heat 

reflection and heat absorption when choosing the 

color of plastic strips for this type of heat engine?

Your Solar Heat Engine is really just a model, so 

it isn’t powerful enough to do much work; but 

can you think of any other ways that it could do 

something useful? Here’s one idea: Use a permanent 

marker to draw a line at one point on the edge of 

the flywheel (plastic lid). Now you can count the 

number of revolutions as the Engine spins. You 

can find out how clouds affect the amount of heat 

energy that reaches your Engine by counting the 

number of revolutions in one minute when it is 

cloudy, and comparing the result to the number of 

revolutions when there are no clouds. In the same 

way, you could measure the number of revolutions 

at different times throughout the day to find the 

times when the most solar heat energy is received. 

You could also compare the number of revolutions 

at different times of the year, but be sure to make 

your measurements at the same time each day! If 

you could operate your Solar Heat Engine in two 

locations at the same time, do you think it would 

spin faster in Northern Canada or Southern Florida? 

Hint: Differences in the amount of solar energy 

that arrives at different latitudes is one of the major 

controls on climate.

What’s Controls Earth’s Climate?

Two things happen when sunlight reaches the Earth: 

1. Some of the energy is reflected back into space; 

and 

2. Earth’s land, ocean, and atmosphere absorb 

some of the energy and release it as heat.

Earth’s overall temperature depends on how much 

energy is absorbed and how much is released back 

into space. If the land, ocean, and atmosphere 

together lose the same amount of energy as they 

absorb, Earth’s energy budget is in balance and its 

overall average temperature doesn’t change. If Earth 

doesn’t release as much energy as it absorbs, its 

energy budget is not balanced, and our planet will 

get warmer.

Climate is not the same as weather. Weather 

describes the temperature, precipitation, and other 

conditions of Earth’s atmosphere at a specific place. 

Climate lis the result of the long-term average 

of conditions in the atmosphere, ocean, and ice 

sheets and sea ice. These conditions are described 

by statistics, such as means and extremes. Here are 

some easy ways to remember the difference: 

• “Climate is what you expect; weather is what you 

get.” (Robert Heinlein) 

• “Climate lasts all the time, weather lasts only a 

few days.” (Mark Twain) 

• Weather tells you what to wear on any given day; 

climate tells you what wardrobe to have.

We know that different places on Earth have 

different climates (think about Antarctica and the 

deserts of the Middle East, for example). Since 

Earth’s overall climate is caused by energy from the 

Sun, places with warmer climates must receive more 

Photo 8. Completed Solar Heat Engine
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1. The Great, Glowing Orb

Climate Science Principle 1: The Sun is the primary source of energy for Earth’s climate system.

of the Sun’s energy than places with colder climates. 

The amount of energy that a particular place on 

Earth receives from the Sun depends upon several 

things.

We also know that Earth moves! It moves through 

space, following a path around the Sun that is called 

an orbit. If you tie a rubber ball onto a piece of string 

and whirl the ball in a circle, the path of the ball’s 

motion is an orbit. The time needed for Earth to 

travel one complete orbit around the Sun is how we 

define the length of a year.

First, Earth is shaped like a ball. The most direct 

sunlight is received at the equator (the equator is an 

imaginary line that goes around the Earth halfway 

between the North and South Poles). The area 

around the equator is very warm. Places located 

at high latitudes (far from the equator) receive less 

direct sunlight than places at low latitudes (close to 

the equator), so high latitudes have colder climates. 

Second, as Earth moves around the Sun through 

the year, different parts of it receive more direct 

sunlight and have longer days and shorter nights 

than others. This is because Earth’s axis (an imaginary 

line that goes through the Earth from the North Pole 

to the South Pole) is tilted compared to the plane 

of Earth’s orbit. The changing amounts of sunlight 

cause Earth’s seasons. During June, Earth’s northern 

hemisphere is tilted toward the Sun. The day that 

the northern hemisphere points most directly 

toward the Sun (on or around June 21) is called the 

summer solstice. During December, Earth’s northern 

hemisphere is tilted away from the Sun. The day that 

the northern hemisphere points most directly away 

from the sun (on or about December 21) is called 

the winter solstice. When the apparent position of 

the Sun is halfway between the summer and winter 

solstices, every place on Earth receives equal periods 

of day and night (remember, the Sun does not 

actually move; it is Earth that moves around the Sun). 

This happens on March 21 and September 21, which 

are called the spring equinox and fall equinox.

Third, the amount of energy produced by the Sun 

also changes slightly over time,. Over the past 30 

years, the Sun’s energy output has changed very 

slightly; sometimes increasing a little, sometimes 

decreasing a little. There is no evidence that recent 

changes in the Sun’s energy output have caused 

much change in Earth’s average temperature.

Fourth, over thousands of years, the motions of 

Earth’s rotation on its axis and its orbit around the 

Sun change very slightly. These gradual changes 

cause the amount of sunlight received by different 

parts of Earth to change as well. When less sunlight 

is received in the northern hemisphere, snow and 

ice can accumulate there. Then, Earth’s average 

temperature drops and we have ice ages. When 

more sunlight is received, the average temperature 

increases and we have warmer periods between 

the ice ages. For the last one million 

years, changes in Earth’s rotation 

and orbit have happened in cycles 

that are about 100,000 years long. Earth’s orbit around the Sun. The illustration shows how Earth would look if you were looking at it from a location beyond Earth’s orbital plane. The red dots show 

the approximate location of the North Pole,which is off center because Earth’s axis is not perpendicular to its orbital plane. Note that this illustration is not to scale, 

because the Sun is actually MUCH larger than Earth! (Image credit: Mel Goodwin, LuAnn Dahlman)

December 21 (Winter Solstice)

The northern hemisphere

is tilted away from the Sun

June 21 (Summer Solstice)

The northern hemisphere

is tilted toward the Sun

March 21 (Spring Equinox)

Earth’s equator is in the

same plane as the Sun

September 21 (Fall Equinox)

Earth’s equator is in the

same plane as the Sun