Viewing the Sun
Build a solar viewer to safely watch an eclipse of the Sun.
Warning: Never look directly at the Sun directly with your unprotected eyes. The
Sun puts out about 400 million billion billion watts of power, in the form of both visible
light and invisible radiation. That’s a lot of power. To prevent permanent damage to your
vision, the visible light must be reduced to tolerable level, and the harmful radiation
filtered out. To safely view an eclipse of the Sun, you can use the indirect projection
Viewing the Sun indirectly involves projecting an image of the Sun onto some sort of
viewing screen and observing this projected image. This allows a group of people to
observe the projected image in complete safety, and so is the recommended method of
solar viewing for classes.
To find out when the next solar eclipse will be, visit the NASA Eclipse Home Page at
You can make a simple projection system out of two index cards. Punch a small (1 mm)
hole in one of the index cards. Pick a nice sunny day and stand with your back to the
Sun. Hold the card with the hole in it up in front of you so that sunlight passes through
the hole and shines on the ground in front of you. Now place the other card in the path of
this beam of sunlight (see diagram 1). The hole will act like the lens in a slide projector,
and focus a tiny image of the
Sun onto the second card.
You will have to move the
second card closer or father to
get a sharp focused image.
[This works on the principal
that light passing through an
aperture (the hole) will form
an image of the source (i.e.
the Sun) and not the
aperture.] A larger hole will
make a brighter image,
allowing you to move the
second card farther away to
get a bigger image, but this Diagram 1. Solar projection with 2 index cards
decreases the resolution of
the image. Note: Never look THROUGH the hole at the Sun.
This is not safe. Only use the card with the hole to
project an image onto the other card.
A sturdier version of this projection
system can be made out of a
cardboard box, as shown in Diagram
2. This gives the advantage of
shading the viewing screen, and gives
you something to decorate in art class
with space pictures, etc. (This is
similar to a pinhole camera, except
instead of recording the image on film,
we are viewing it with our eyes.) The
image is projected onto the back of
Diagram 2. Pinhole projection method
the box, and viewed through a hole in
the side of the box.
The longer the box is, the larger the
resulting image. However, if the image is
Safe Filters too large it will appear too dim to be useful.
To safely view the Sun directly, the light must be filtered before it reaches the eye.
• A piece of #14 welder’s glass is the ONLY safe filter for directly viewing the Sun with
the unaided eye. DO NOT USE exposed photographic film, sunglasses, or
photographic filters because they are NOT SAFE. You can hold the #14 filter
between your eyes and the Sun, and observe the Sun in safety. Only the #14 is
dense enough; do not use a lower number. Note that this is for unaided eye viewing
• If you are using binoculars or a telescope, you must use a special filter that fits over
the FRONT END of the telescope. NEVER use a filter that fits over the eyepiece of a
telescope. This sort of filter is extremely dangerous and can cause permanent
What to look for
The most obvious features of the solar disk are sunspots. These are regions of the sun’s
surface that are cooler than the surrounding area. Occasionally they are large enough to
be seen with a pinhole camera. The Sun’s activity changes over an 11-year cycle, and
the number of sunspots rises and falls during this cycle. The solar maximum of 1990
produced up to a hundred spots at once: during solar minimum in 1994-95 it was rare to
see any spots at all. A good science project would be to observe these spots every day,
to see the charges they undergo, and to observe the rotation rate of the Sun. More
advanced students may be able to determine the Sun’s rotation rate from these
Viewing the Sun 2