The sun sits at the centre of our solar system and provides us with the light and heat that we need to survive. It is actually a medium sized star, which utilises the forces of nuclear fusion to create huge amounts of energy every second. The light that results from this reaction takes over eight minutes to reach the earth over the huge distance of deep space.

In this activity, you will use everyday materials to show how the size of the sun relates to the earth, and how far away it is from us.

Why not try wishing upon the star that's closest to our own planet Earth, the one we see almost every day, the one that provides the light and heat we need to survive?

Our sun is just one of the 100 billion or so stars in our galaxy, and there are billions of other galaxies in the universe. It may seem like the sun is close to us, but it's actually many millions of kilometres away. It's also so big that a million Earths could fit inside it!

The sun may be only one among billions of other stars in the universe, but it's the one that makes our life on Earth possible. How? By providing energy in many forms--solar power, fossil fuels, wave power, wind power. Without heat and light from the sun, Earth would be just another dark, cold place in space where life as we know it couldn't exist.

Where does the sun get all this energy? The sun's mass is approximately 300,000 times more than Earth's, and the greater an object's mass, the greater the pressure at its centre. Charles's law tells us that when you squeeze--or compress--a gas, it gets hot. Most of the sun's mass is composed of hydrogen gas atoms, and about 100 years ago, physicists came up with the hypothesis that the sun's tremendous mass squeezed the hydrogen atoms until they ignited, releasing heat and light energy that eventually made it through space to us. Based on calculations of the mass of the sun, they figured that the sun would burn itself out in 6,000 years.

Evolutionary biologists and geologists knew from their own studies that life on Earth had been around much longer than 6,000 years, so the research continued. Decades later, a new hypothesis arose. Think about the hottest oven you can imagine, then turn up the temperature to about 14,000,000^oC. That's how hot it gets in the centre of the sun. At that temperature the hydrogen nuclei are moving so fast that when they crash into each other they stick together to form helium .nuclei.

The "fallout" from this crash is a tremendous amount of energy, released mainly in the form of heat and light. This reaction at the nuclear level is called nuclear fusion. Scientists calculate that there is enough hydrogen in the sun to continue the fusion reaction and provide heat and energy for at least another five or six billion years.

Charles's law When you squeeze a gas, it heats up.

nuclear reaction reaction that takes place at the core of an atom. It converts mass into energy.

Introductory Activity

Just how much bigger is the sun than Earth? And how far away is it?

If you can't get a reservation on the next space shuttle flight, you'll have to go out to a field with a few friends and a few supplies to find the answers. You'll be amazed at what you observe when you compare the size of Earth with that of the sun and see how far you'd have to travel to get from one to the other.

• heavy butcher paper or 4 large sheets of poster board
• Scissors
• tape or glue
• metre stick
• long-distance measuring tape
• marble, about 2 cm in diameter, to represent Earth
• athletic field or large open area, at least 250 meters long
• stones or bricks to use as markers
• at least three people

1. Make a paper circle to represent the sun. It should be 2.3 meters in diameter.
2. Next, go outside and measure a length of 246 meters on an athletic field or other large area. Use bricks or stones to mark each end.
3. You'll need at least two people to stand next to one marker, holding the paper sun.
4. The third person holds the marble, representing Earth, and walks from the sun over to the other marker.

Discuss the difficulties involved in representing relative diameter and distance in the same model.

The earth is approximately 150,000,000 km from the sun. This distance varies slightly with the seasons because of Earth's elliptical (oval) orbit. We can make a simple instrument that will provide quite accurate data to measure the sun's diameter.

The relationship that we will use is:

diameter of sun (km) = diameter of sun's image (mm)

distance to sun (km) distance between cards (mm)

From this relationship we can derive a formula:

Dia. of sun (km) = dia. of sun's image X dist. to sun

dist. between cards

• 2 small cardboard boxes--size not critical but should be ridged enough to hold their shape well
• 2 pieces stiff cardboard 10 cm X 20 cm (perhaps from a shoebox)
• metre stick
• sharp knife or craft knife (careful!)
• masking tape
• small piece of aluminium foil

1. Tape the lids of the boxes shut securely.
2. Cut slits in opposite sides of each box, directly opposite each other.
3. Make each slit in the form of a capital "I" and of a size that will fit the metre stick snugly when the box is pushed on to the meter stick. If measurements and cuts are made carefully the face of the box will be perpendicular to the meter stick - this is important.
4. Tape one box securely near one end of the metre stick but leave the other box free to slide.
5. Cut a 5 cm X 5 cm hole near one end of one piece of cardboard and cover with the aluminium foil. Tape the foil in place.
6. Punch a very small hole through the foil near the centre of the foil with a sharpened pencil lead or a pin.
7. Tape this card to the face of the box that has been secured to the meter stick. Make sure that the foil-covered section sticks up above the top of the box.
8. Draw two parallel lines exactly 8.0 mm apart near the centre of the remaining cardboard.
9. Tape the card with the parallel lines to the face of the sliding box. Note: Be certain both cards are as nearly perpendicular to the meter stick as is possible. The lines should also be perpendicular to the meter stick.
10. Point the end of the meter stick that holds the foil-covered card toward the sun. CAUTION: Do not look at the sun! Move the meter stick around until the shadow of the foil-covered card falls on the other card. A bright image of the sun will appear on the sliding card.
11. Move the sliding card until the bright image of the sun exactly fills the distance between the parallel lines.
12. Measure the distance between the cards on the meter stick. Distance between the two cards = mm.
13. Use the formula from the theory section to calculate the diameter of the sun. Use 150,000,000 km as the distance from Earth to the sun.

List factors which could account for the difference between your measurement and the accepted diameter of the sun.

Calculation A was a test of measurement ACCURACY. What could you do to test the PRECISION of your meter stick instrument?

The actual distance between the earth and sun varies from a minimum of 147,097,000 km to a maximum of 152,086,000 km.

Using the formula, recalculate the diameter of the sun using the number from your measurement for the distance between the cards and the number for the minimum distance between the earth and sun.

Again, recalculate the diameter of the sun using your measurement for the distance between the cards and the calculation for the maximum distance between the earth and sun.

Does the accepted actual diameter of the sun fall between your calculations B and C? How do calculations B and C affect your estimation of the accuracy of your measurement as opposed to the percent difference you calculated in step B above?

When we talk about the sun, we must use extremely large numbers. When working with very large or very small numbers, we use scientific notation, which means re-writing a number as the product of a number between 1 and 10 and a power of 10. For example, the sun's diameter, approximately 1,391,000 km, would be written as 1.391 x 10⁶ km. Write some other large numbers in scientific notation.

Refer to the relationships described in the theory of this lab and derive a formula for calculation the distance from the earth to the sun. Use measurements from your meter stick instrument to calculate this distance.

Obtain an astronomy reference which gives the actual distance between the earth and sun on a given day or week to check the accuracy of your instrument.

What changes or refinements would you make in your meter stick instrument if you were to plan to chart the earth-sun distance through the remainder of the school year? How could you present the results of such a charting project in a meaningful way?