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How to measure the strength of the Sun

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A short video showing how to measure the strength of the Sun with a toilet roll.

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Slideshow on how to measure strength of sun.

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WHAT YOU NEED: A large pad of paper, a pen, a ruler, a toilet roll tube and a sunny day. Don't forget your sunhat!
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STEP 1 - SHAPE TUBE: Start in the morning. Make a note of where you are and what the date is. Now flatten the toilet roll into a square shape - the squarer the better.
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STEP 2 - OUTLINE TUBE: On the paper, trace around the end of the square tube.
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STEP 3 - TRACE SHADOW: Now - keeping the edge of the tube on the edge of the square you drew - angle the tube towards the sun so the shadow cast is the thinnest possible line. Turn the paper if you need to. Then trace around the shadow that is cast. Note the time.
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STEP 4 - MEASURE SQUARE AND SHADOW: Measure the length of the original square you traced and make a note of it...
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...then measure the length of the shadow you traced and make a note of that.
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STEP 5 - CALCULATE: Divide the length of the square by the length of the shadow. This is how much of the sunlight coming through the tube is falling on the flat square on the paper. Multiply by 100 to give a percentage. Direct overhead tropical sun = 100%.
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STEP 6 - REPEAT: Take more readings around midday and towards sunset. You can also use a spreadsheet to help. By comparing results you can see how the angle and strength of the sun depends on the time of day, the time of year and where you are on earth.

How to measure solar energy on Earth

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