May
12
2009

How can I safely view a solar eclipse?

Crescent sun through solar eclipse glasses (photo by Sailor Coruscant - CC-BY)

Crescent sun through solar eclipse glasses (photo by Sailor Coruscant - CC-BY)

Whenever a solar eclipse approaches, we are admonished to watch the eclipse safely by watching it on television. That nannying advice deprives people of the chance to experience one of nature’s most beautiful and dramatic spectacles—the few seconds of totality in the middle of the eclipse.

It is possible to harm your sight by viewing a solar eclipse unsafely, but let’s focus on spreading information rather than fear.

It’s completely safe to view the total phase of an eclipse with your eyes. As NASA eclipse expert Fred Espenak says:

The total phase of an eclipse can and should be viewed without any filters whatsoever. The naked eye view of totality is completely safe and is overwhelmingly awe-inspiring!

The total phase is the part where the moon completely covers the bright part of the sun. It lasts from a few seconds to a few minutes, and is the highlight of the experience. If you can trust yourself not to look at the sun during other parts of the eclipse, you don’t need any special protection.

But there’s a lot of “hanging around” waiting for totality, and it’s interesting to watch the moon slowly covering up the sun. To do this safely you must watch through a suitable filter or view only a projected image.

During the partial phases of an eclipse, or during an annular eclipse (where the moon doesn’t quite cover the whole sun) do not look at the sun directly.

Don’t look at the sun through binoculars or a telescope, because they concentrate the light even further. It’s possible to set up binoculars or a telescope to project an image of the sun onto a sheet of white cardboard, but that’s beyond the scope of this article.

You can make a simple projector out of a Pringles can with a pinhole in one end and wax paper across the other (to form a screen). This will depict the crescent shape of the sun as the moon advances, but it’s not particularly dramatic. You may see similar crescents on the ground, due to “natural pinholes” made by overlapping leaves.

For the most satisfying viewing though, you can’t beat using a safe filter so that you can watch the sun.

In days gone by people would use ad-hoc solutions such as looking through smoked glass. No doubt some people “got away with it”, but there are simply too many variables and some people lost their sight. Even if a device blocks enough of the visible light to make viewing comfortable, it may be letting through so much infra-red or ultra-violet light that it burns your eyes—and you won’t feel any kind of sensation as it’s happening because the retina can’t feel pain.

There is one home-made solution that is sound. You can use two layers of photo film, provided it meets all of the following requirements: it must be black-and-white film, it must be a kind that uses a silver emulsion, it must have been maximally exposed to light, it must have been developed to maximum density, and it must be unscratched. I viewed the 1976 Bombala solar eclipse using a home-made filter of that kind, but I wouldn’t do it again. Technology has moved on and provided better options.

You can use number 14 welder’s glass, but much more convenient are metallized films such as aluminized mylar. You can buy these in sheets, or inserted into cardboard frames to make viewing glasses. Any such product from a reputable astronomy supplier is likely to be suitable, but make sure it’s unscratched.

As a bonus, you can use these solar eclipse viewing filters to look at the sun even when there’s not an eclipse. In times of extreme solar activity you may be able to make out groups of sunspots, but the most interesting thing I saw through a solar viewing filter was a transit of Venus. Usually we see Venus as a star-like dot of light, but when it passes in front of the sun you can see it as a tiny disc. It looks as tiny as a flyspeck, but it gives a real sense of a planet orbiting in space.

So don’t be put off by the fear-mongering. You can watch a solar eclipse safely without resorting to television, and you will be glad that you did.

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