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How bright is the Sun?

By David, 7 October 2014 Activity

Wiping oil off paper

Pour a small amount of vegetable oil onto paper, then wipe it off.

You will need

  • Lamp and extension cord
  • Paper
  • Vegetable oil
  • Rag or paper towel
  • Ruler or tape measure
  • Pen and paper to do some calculations on

What to do

    1. Pour a small amount of vegetable oil onto a sheet of paper, and then wipe it off. You will end up with a semi-transparent oil stain on your paper.
Holding paper over lamp

Put the lamp outside in the sunlight, and hold the paper over the lamp.

  1. Put the lamp outside in the sunlight, and plug it in using the extension cord.
  2. Turn the lamp on.
  3. Hold the paper over the lamp and look at the oil stain. If the oil stain is darker than the rest of the paper, move the paper closer to the lamp. If the stain is brighter, move the paper away from the lamp.
  4. When the stain and the rest of the paper are the same brightness, hold the paper still. Measure the distance from the paper to the light bulb on your lamp.

The calculation

Things become less bright when they are further away. Scientists say there is an inverse square relationship – if something is twice as far away, it is one quarter as bright. If it is three times further, it is one ninth as bright.

The oil stain is brighter than the rest of the paper

If the oil stain is brighter than the rest of the paper, move the paper away from the lamp. If the oil stain is darker, move the paper closer to the lamp.

The Sun is approximately 15 000 000 000 000 cm away. Use this formula to work out how many times further away the Sun is from the lamp, compared to the paper and the lamp:

Sun vs paper distance = 15 000 000 000 000 cm ÷ paper to light bulb distance in cm

To work out how many light bulbs you would need to be as bright as the Sun, use this formula and the number you calculated last step:

Number of light bulbs to make a Sun = Sun vs paper distance x Sun vs paper distance

Measuring distance between paper and light

When the stain and the rest of the paper are the same brightness, measure the distance from the paper to the light bulb.

What’s happening?

The number you get at the end of this activity is immense. This is hardly surprising – the Sun is a lot bigger and a lot brighter than a lamp. In some ways, it is surprising that you can compare them at all!

This activity measures the brightness of the Sun, and uses the distance of the Sun to the Earth. If you rearrange the equation, you can use the Sun’s brightness to measure distance instead. Astronomers use this technique to measure distant stars.

If you climb a mountain at night, look at the brightest stars and the most distant streetlights. As you compare the brightness of stars and streetlights, remember that many stars are as bright as our Sun, and some are much brighter. You might get some idea of the gigantic distances between the stars.

More information

Comparing the brightness of the Sun and the Moon
A longer brightness activity from NASA (PDF)

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