The sun is a star, and it is very large and very far away. Its distance from Earth is about 149 million
kilometres. The light from the sun takes about 8 minutes to get here, travelling at 300,000 kilometres per
second. However, despite the fact that the sun is so far away, we're going to show you a simple method for
calculating the sun's diameter, so we can find out how big it really is.
To measure the sun's size, you will need a stiff piece of very light cardboard, a sheet of white paper, a pin,
a ruler, and a calculator. Before we start, though, we should caution you:
Never ever look directly at the sun, with your eyes, or through a telescope, camera, or
sunglasses. Staring at the sun for even a few seconds can permanently damage your
eyes.
Not only are we going to measure the sun, but you'll learn a safe way to look at an image of the sun
without damaging your eyes.
The first thing you need to do is use the pin to make a tiny pinhole in the centre of the cardboard. This
pinhole will act as a tiny camera aperture, letting the light from the sun through and making an image of the
sun on your piece of paper. In fact, pinhole cameras were the first sort of camera invented a century or two
ago. When you hold up the paper you will see a tiny perfect image of the sun, just like a photograph!
Try the experiment to see what kind of
results you get. Make a pinhole about
the size of the end of a paperclip in a stiff
piece of paper. Hold the pinhole up to
the sun, and position a piece of white
paper on the other side at a spot where
you can see an image of the sun on the
paper.
As you move the pinhole farther away
from the paper the image gets bigger;
chose a distance where the sun image is
big enough to measure its width
accurately. A second person is needed to
measure the distance of the paper from
the sheet with the pinhole.
Using geometry, a simple ratio, and the known distance to the sun how would you
determine the sun’s diameter. (Distance to sun ~ 149,000,000 km.)
This is a simple ratio question, since the triangles on the left and right are similar.
Content, graphics & design by Bill Willis
© 1996 -2004