If you are just getting up to speed on the 2017 Total Solar Eclipse in the United States it may be too late to find safe, certified Eclipse Glasses in your area. Don’t despair, there are other ways to view the event. This is one of them.
A Pinhole Solar Viewer is a simple to make type of Indirect Viewer that will allow you to view the upcoming eclipse safely. Indirect Viewers, as their name states, allow you to view the Sun indirectly and thus safely. With this type of indirect viewer you are facing away from the Sun and can therefore view the image safely without the need for solar filters.
Many of you may have made a pinhole camera in the past, and a pinhole solar viewer works in exactly the same way. Because we are at such a distance, light from the Sun is traveling in rays that are close enough to parallel that focal distances and hole diameters are not highly critical. In fact, just about any hole in any opaque material will do. The size of the hole can (and should) be much bigger than a pin. The simplest form of this type of viewer can be constructed with just an index card! Simply poke a hole in the card, hold it above the ground (or another index card) while facing AWAY from the Sun and look at the shadow of the card. The light coming through the hole will form an image of the Sun. As the Moon starts to pass in front you will see its round shadow begin to cover the Sun. By constructing the viewer inside a cardboard box we can block some of the ambient light for a better view.
Materials needed are a medium sized cardboard box, a small piece of aluminum foil, some tape, a sharp knife or razor blade, some black paint, a white index card, and a nail or ice pick.
The dimensions of the hole are surprisingly un-critical. During the eclipse event you can even look around at the ground under a tree and you will see tens or hundreds of tiny eclipse images. Everywhere light filters through the small spaces between the leaves you will see an image.
As it turns out, the most important thing you can do to get a good image is to create a hole with clean, sharp edges. The size of the hole and its distance to the “screen” will affect the brightness and sharpness of the image.
Start by cutting a small square out of the side of the cardboard box.
Next take a piece of the aluminum foil, smooth it out, and tape it over the hole in the side of the box.
Now poke a hole in the center of the aluminum foil using a nail, ice pick, or other tool. For a box that is 12″-18″ across to the opposite side try a 1/8″ hole. For larger boxes you may want to start with a 3/16″ hole. The larger the hole, the larger and brighter the image will be, but you will sacrifice sharpness or small details in the image. If you go too large, simply install another piece of foil and you are good to start over.
At this point you could call it a day and you will get an image.
To improve the image we painted the inside of the box black to reduce glare and taped a white index card to the inside of the box across from the hole to serve as a screen.
You can see the image of the Sun is now much nicer.
Of course there is no eclipse happening during this demonstration so you are seeing the whole Sun. We cut off the box flaps to make it easier to take photos, but you can leave them on to provide more shading.
To use this type of solar viewer hold the side of the box with the hole at the top and face AWAY from the Sun. Look at the shadow of the box and tilt the box to minimize the size of the shadow. You should now have a nice solar image on your screen inside.
If you tape a piece of paper over your “screen” you can even trace over the image to create “snapshots” of the event to keep as a memento.