Low-Tech Eclipse Viewing

What to do if you're caught without optics on eclipse day.

A simple pinhole projector made from wrapping-paper tubes serves the purpose of projection well. Use a round toothpick to poke a hole in the aluminum foil at the end.
Jerry Oltion

In preparation for this month’s total solar eclipse, my last two columns were about making solar filters and solar finders so you could observe the partial phases with a telescope. But what if you don’t have a telescope or didn’t get your filter material in time? Are you totally hosed?

Not at all! Observing the eclipse safely can be as easy as poking a hole in a piece of aluminum foil. No, don’t look through the foil at the Sun! That would still damage your eyes. Use the hole to project an image of the Sun on a screen a few feet away.

A pinhole acts like a tiny lens, creating a focused image. It doesn’t let much light through, so it only works well for bright objects like the Sun, but it works great for that. With a pinhole projection you can watch the Moon slide in front of the Sun on eclipse day, taking a bigger and bigger bite until mid-eclipse, then slide away again.

The biggest problem with a pinhole projector is that the image is small and dim. Moving the screen farther away from the pinhole will make the image larger, but because the light is spread out more, it will get dimmer. Making the hole bigger will let more light through, but the image will get blurrier.

How small is it? Pretty small. The Sun is only half a degree across, so the projected image will only be half a degree across, too, from the perspective of the pinhole. You’ll get about 1/10 of an inch of image diameter per foot of distance. So if your screen is three feet away, your image will only be about a third of an inch across. If you want the image to be an inch across, you would need to put your screen 10 feet away. That’s just not practical.

Fortunately there’s another way: Use a mirror to reflect an image of the Sun onto a shaded wall. If you mask down the mirror to half an inch or so, it’ll act like a pinhole and create a fairly good image. It’ll be blurrier than an actual pinhole but much brighter, so you can cast its light a lot farther and get a bigger image. For viewing the crescent shape of partial eclipse phases, it will work just fine.

You can cast a fairly bright image of the Sun on a wall with a small mirror. The smaller the reflective spot, the sharper (but dimmer) the image will be.
Jerry Oltion

If you want a sharper, brighter, bigger image yet, there’s one more good trick: You can use a pair of binoculars to project an image of the Sun onto a screen. The binoculars act as a complete optical system, using the entire front aperture to gather light and focusing it into a collimated beam that comes out the eyepieces. You can put a screen anywhere behind the eyepieces and project a crisp, clear image onto that screen, and the image will be much larger and brighter than with a pinhole or small mirror. You might have to adjust the focus of the binoculars, but you can get it crisp enough to see sunspots if there are any big ones.

You can use cheap binoculars to project an image of the Sun onto a screen. Don’t use binoculars you care about for this, and don’t let anyone look through the binoculars at the Sun.
Jerry Oltion

You need to be careful about a couple of things if you use binoculars. First of all, don’t ever look through them at the Sun without a proper filter, not even for a second. And make sure nobody else can, either. That means setting them up in such a way that no curious children can get their heads between the screen and the eyepieces.

Second, don’t use a good pair of binoculars. While you’re aiming them at the Sun, the intense beam of focused sunlight will be dancing around inside the eyepiece housing, heating up the field stop and melting any plastic parts it stays in contact with for too long. This is a project for a pair of Goodwill binoculars, not that fancy pair of Celestron 15×70s.

If you have none of the above methods at hand, nature gives you one for free: Just stand under a tree and look down at the dappled pattern of light filtering through the leafy canopy onto the ground. The gaps between leaves act as tiny pinholes, casting hundreds of crescent-shaped spots of light during the eclipse’s partial phases.

And during totality? Look up! It’s safe to look at totality (but only totality) directly. Remember to close your mouth or flies will buzz in!


This article originally appeared in print in Sky & Telescope's August 2017 issue.

2 thoughts on “Low-Tech Eclipse Viewing

  1. Cook-Anthony

    As an inexpensive improvement on a pin hole, I projected the transit of Venus using a 1-diopter $1 reading glass (which, by definition, has a focal length of 1 meter). To use it, I punched a business card with a 1 cm wide hole (a paper punch works fine), and taped it over one of the eyeglass lenses. I let the sun shine through the masked lens to a card, 1 meter away. The image is very sharp and bright, and shows nearly as much detail as filtered 7X binoculars, with the advantages of low price and the ability of several people to view at once. Of course, the components can be mounted on a stick to hold the focus. Other reading glasses can work, but the larger the diopter number, the shorter the focal length and therefore smaller the image. Focal length is 1m/diopter number.

    Also, as the caretaker for astronomical equipment at Griffith Observatory, I no longer permit our binoculars to be used for solar projection for the reasons that you mention– because many of them (even by well known astronomical brands) now have plastic components near the eyepieces. These heat, melt, and outgas on the interior optical surfaces, coating them with residue. This is also true of many telescopes.

  2. Mike NicholsonMike Nicholson

    You can also use a small plane mirror to project an image of the sun over distance. Such mirrors can be found in women’s makeup compacts and whilst front surfaced mirrors are not necessary, I am sure there would be some improvement in image quality. My understanding of the theory is the mirror acts as an infinite series of reflective pinholes. Over distances of 50′ – 100′, the partial phases are easily visible. I believe an English schoolboy discovered this in the 1960’s, (The Jacobs Effect?). I have been using this technique since the 1970’s and am surprised that it has remained completely unknown.

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