How to View the Sun (and Eclipse)

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 this 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 method, here are other methods to view the sun.

For my favorite method (explained in Step 4), all you need are a small mirror (1/4" to 1" in diameter), a sunbeam, and a shaded, light-colored wall.

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.

(And, if you're reading this now, odds are you don't have a set of those eclipse glasses.)

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 (or those procrastinators who waited until now).

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 (see here, for example) - but still requires building something. A decent option, but you need to order something and get it delivered to you.

Pinhole Viewing

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.

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:

1. 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.)
2. 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. It also works very well (especially using a mirror on the smaller size) to keep the mirror inside, in a sunbeam coming through the window (and directing the reflected beam onto a shady wall).
3. 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.
4.     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.
5. Mount the mirror board (using tape, clay, a camera tripod, etc.) so that you can walk away from the mirror and close to the image, to enjoy the view.
6. 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.
7. This works on the same principle as a pinhole (Step 3): 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.)