Safe Sun Viewer
If you followed the qr code on the "Safe Sun Viewer" cards, follow these steps:
- Remove the card from its plastic sleeve.
- Attach the card to something larger and sturdy. (Clip it to a clipboard, tape it to a book or piece of cardboard, etc. For the rest of the steps, I'll call it a board.)
- Find something sturdy to lean the board against (a rock, cinderblock, wall, table, chair, etc.)
- Find a sunny spot North of a light-colored wall (or window).
- Angle the board so that the reflection from the sun hits the ground not far away. This should be easy to see. Adjust the board until that spot is in a straight line between the mirror and the wall you want to project the image of the sun onto.
- Gradually tilt the board up so that the image of the sun is at the height you want it on the wall.
- Walk up to the image, and enjoy the view.
- Remember: the sun is constantly progressing across the sky, so your reflected image will also move. You may need to adjust your mirror occasionally, unless you use a large enough wall.
The farther the image is from the mirror, the larger the image will be, but it will also be dimmer. Shining through a window into a darkened room works very well. You can even prop up the mirror inside in a sunbeam to project to another part of the same room - but it's harder to get a big image.
More information about how this works and what you can expect to see is shown below, in the section about viewing the sun using reflections.
People have been viewing the sun for as long as there have been people - some in good ways, some dangerous ways. Advances in optics in the early 1600's created new techniques; since then, the tools people use have mainly been variations on the things from back then. So in that sense, there's nothing new under the sun.
These different methods are illustrated beautifully in the following image from the Rosa Ursina, a book published in 1630 detailing many observations of the sun (including many wonderful sketches of sunspots). So before explaining my favorite Safe Sun Viewer, here are other methods to view the sun.
Direct Observation
First let's consider these guys, who seem to be staring at the setting sun.
As the sun sets, sunlight must pass through far more of the atmosphere to get to our eyes than when it is directly overhead. So, people often sit on the beach and watch the sun set, without many problems. But generally, that's not a great idea - it may not be painful, but still pack enough energy to damage your eyes.
And of course, if you're viewing an eclipse, you don't get to pick the time of day when you see it. So people today using this technique should use "eclipse glasses" with strong solar filters. Those give a nice experience of "living the moment," but you don't see much detail - the sun takes up a small fraction of the sky.
Telescopic Viewing
The man on the left (at A) appears to be doing something massively unwise: trusting his precious vision to a bit of cloud that is thin enough to let the sunlight through (at B), but still thick enough to protect his vision. Don't do this. If you really want to look through a telescope (or binoculars) use a solar filter designed for that purpose (that you're sure is fully intact).
The man on the right (at D) at first glance appears to be in the act of blinding himself (from the sun at E). But since this is described as a "Helioscopio," I suspect that he's actually looking at a projection screen, rather than letting the image from the telescope be formed directly on his retina. A larger projection screen works even more nicely, and allows several people to view the sun at once. You can search for "sun gun telescope" to get the idea. These are great, but require a telescope, a sturdy mount, careful alignment, and time to build it. Not much of an option for those not already into astronomy.
Pinhole and Single Lens Viewing
The man at the lower left (T) is using two different methods at two different times.
Single Lens Viewing
When the sun is at Q, light passes through a simple converging lens (R), to reach a screen (S). In principle, this is easy to do: every magnifying glass is a converging lens. But the image only comes into focus at one particular point - the focal point of the lens. And unfortunately, the image of the sun will be very small unless the lens has an extremely long focal length (as suggested by the figure). That's why you can burn things with a magnifying glass: the image of the sun is tiny, concentrating a lot of energy in a small space. Trying to stare at that is almost as bad as staring at the sun, and won't let you see any details anyway.
I have gotten decent images of the sun using weak reading glasses. But the images are uncomfortably bright unless you block off most of the area of each lens. And you have to bring the image in focus, and be careful about how you tilt the lenses. Every other common lens has too short a focal length.
People sell combinations of lenses for solar viewing that are simpler and less expensive than the "sun gun telescope" described above - but still requires building something. A decent option, but not easy or quick.
Pinhole Viewing
Imagine you had a bright light in the shape of an "A" above a large plate with a small hole in it. Light from the tip of the A has to pass through the hole to get to the other side of the plate. The same is true for every other part of the A. This forms an image on the other side.
If the screen is close to the pinhole, the image will be small; the farther the screen, the larger the image.
The image will only be pefectly sharp if the hole is incredibly tiny. But that means only a little light gets through, which makes the image dim. A larger hole lets more light through, brightening the image - but blurring the image by the size (and shape) of the hole.
When the sun is at F, light passes through a small hole at G, to reach a screen (H). This is the same principle that a "pinhole camera" uses. If you make a small enough hole, light from one point in the sky can only take one path through the hole, to a screen on the other side. Each point in the sky arrives at a different point, forming an image.
This means that the sun will be in focus no matter how near or far you are from the pinhole (unlike with a lens). This "infinite depth of field" of a pinhole is related to the way cameras with small lenses (like cell phones) have things at a wide range of distances all in focus together, while cameras with larger lenses can blur objects that are closer to and farther from your subject (which can be used artistically as "bokeh").
This is simple and works well, but has a few drawbacks:
• Unless you really shade your screen, the light from your pinhole may be too dim to see. (So, common advice is to put the pinhole at one end of a closed box, with the screen at the other end.)
• Because the sun only takes up a small part of the sky, the image of the sun will be very small unless there's a long distance (many yards/meters) between the pinhole and the screen. (But that's hard to do with a box.)
• If the image is small, you don't want it to be blurry. But the image will be blurred by the size of the pinhole.
• A small pinhole will make a sharp image, but may be too dim to see.
I think pinholes are great for long-exposure photogrpahy, but I've never been satisfied with them for viewing the sun.
Reflection
My favorite experiences sharing solar views have used mirrors.
In this image, a man has a small mirror (K) mounted to a board. He reflects that light, directing it to the shady portion of a (presumably white) pillar forming an image at L. Man M then gets a nice close-up view view of the sun, which he can sketch and measure (N).
Perhaps this man is having fun because this is so easy.
To do it yourself:
• Get a small mirror, and mount it to a card or a board. (Smaller mirrors make images that are sharper but dimmer than larger mirrors. Mirrors from 1/4" to 1" (10 to 25 mm) across should work well.)
• Find a wall that's in the shade, preferably with a light color. Ideally, you can reflect light through an open window into a dark room.
• Go far away from the wall, and angle your mirror so that sunlight strikes the wall. The farther the mirror is from the wall, the bigger and sharper your image will be, but it will also get dimmer.
Hint: first aim the reflected dot from the sun onto the ground near you, then guide the dot toward the wall. Otherwise, it can be tricky to aim.
• Mount the mirror board (using tape, clay, a camera tripod, etc.) so that you can walk close to the image and enjoy the view.
• Remember: the sun is constantly progressing across the sky, so your reflected image will also move. You may need to adjust your mirror occasionally, unless you use a large enough wall.
As with the pinhole: A small mirror can also create an image of a bright light in the shape of an "A". Light from the tip of the A has to reflect from the mirror to get to a shaded wall, and likewise for every other part of the A. The farther the wall, the larger the image.
The image is blurred by the size (and shape) of the mirror - but if the wall is far enough away, you still get a pretty sharp image - sharp enough to see sunspots.
This works on the same principle as a pinhole: as long as the mirror is small enough, light from one point in the sky can only reflect in one direction to one final ending point - and so on for each point in the sky.
With this method, it's easy to place a mirror many yards away from a wall or screen that you want to project it on. And you can usually find a wall that is shading itself from the sun - so you don't have to use a blackout box like you do for a pinhole (since the screen of a pinhole is always straight "down range" from the sun). So you have all the benefits of a pinhole, and few of the drawbacks.
In fact, astronomers have suggested using disco balls to observe the sun. (Interestingly, that research article attributes the "pinhead mirror" idea to 1934. But as you can see, this was well-established even by 1630 - and likely as early as people were able to make decent small mirrors.)
So, take out your Safe Sun Viewer, and enjoy the show!
More About Pinhole and Pinhead Mirror Images
Image size
The angular size of the sun is around 0.54°. So, the angular size of an image of the sun will be from a pinhole or a pinhead mirror will be 0.54° (plus some extra due to the blurring by the size of the hole or mirror itself).
So, if you want to see how big the image of the sun will be from a pinhole or a pinhead mirror, multiply the distance from the pinhole to the screen by the tangent of 0.54°, which is 0.0094 - or just under 0.01.
If you hold a pinhole above the ground, it's hard for it to be much farther away than a meter or two (about 1 to 2 yards). At that distance, the image will be around 1 to 2 centimeters in diameter (about 0.4 to 0.8 inches).
But if you're using a pinhead mirror, it's easy to have a screen 20 meters from the mirror. That produces an image of the sun about 20 cm in diameter.
How can a square mirror make a round image?
If you have a square mirror 0.5 cm across, every point on the image will be blurred by 0.5 cm.
If your screen is 10 cm away from the mirror, the image of the sun itself is only about 0.1 cm across. That means it will look like a 0.5 cm square, rounded off by 0.1 cm (a slightly rounded square). (The "blurring" is about four times the size of the "image" - so you can't even make out the fact that it's the image of something round.)
But if your screen is 20 m away from the mirror, the image of the sun is about 20 cm across. Now, the 0.5 cm blurring is only about 1/40th the diameter of the image. So, it will look like a slightly blurred image of the sun - one in which you can see prominent sunspots.