Students and their families of the Academy of Aerospace and Engineering are invited to an eclipse viewing outside of the academy facility on Monday, August 21st during the partial eclipse over our part of Connecticut from 2:00 to 3:30 local time (Eastern Daylight Time)–the peak is at 2:45. A couple weeks ago, I tweeted a link to a Space.com article that gives an excellent interactive map and explanation of the eclipse, so if you are somewhere else in the US, you can use this to plan your viewing. My recommendation is to make a projection viewer, not to use the eclipse “safety” glasses. I’ll explain how to make a projection viewer, then I’ll explain why the glasses are a bad idea.
A projection viewer puts the image of the eclipsing Sun on a screen where you can see the image magnified and dimmed so that it’s safely and easily visible with the naked eye. The simplest projection viewer is using a box where you poke a pinhole in one side to let the Sun’s image through, then tape white paper to make a screen on the facing inner side of the box. You also need another opening where you can look through and see the image on the screen. Here is a NASA video that shows how to make such a viewer using an empty cereal box. A smaller and cleaner pinhole makes a clearer image, and a longer box (between the pinhole and the screen) makes a bigger image. If you look through and aren’t happy with the image, experiment with the projector to improve it — you have about an hour where the Moon is clearly overlapping the Sun, so it’s plenty of time to make changes. One major safety tip: DON’T LOOK THROUGH THE PINHOLE AT THE SUN. To aim the Sun through the pinhole, you just need to hold the box so its shadow is as narrow as you can make it–this means the box’ top where the pinhole is placed is squarely facing the Sun.
Another type of projection viewer is using a telescope to project the Sun’s image onto a screen. The advantage of the telescope is that it will make a much bigger image in a shorter distance due to its magnification, and it allows you to focus the image. Our academy telescopes have 90-degree eyepiece mounts that make them ideal for this purpose since we can project the image sideways into an open topped box with a big screen, and the sides of the box shade the image so it’s very visible. Here are photos from a previous post where I show how we did this to view sunspots earlier this year.
If your telescope does not have a 90-degree eyepiece mount, then make a hole in one side of a box to fit around your telescope so that the eyepiece points into the box and projects the image on the opposite inner side of the box. Sky and Telescope gives a good description of several ways to project the image in this article. A couple safety tips using a telescope eclipse viewer:
- Never look through a telescope at the Sun, unless you have a solar filter on the telescope–and I don’t recommend using this either unless you are very sure about the filter and its safety qualifications. To point the telescope directly at the Sun, look down at the telescope’s shadow–make it as small as possible, then it will be facing the Sun.
- Don’t use a telescope with a diameter bigger than about four inches, or else it may magnify the Sun so strongly that it burns the interior of the telescope. Similarly, use the lowest magnification eyepiece for the eclipse viewer.
- Use a refracting telescope (one with a primary lens) vs. a reflecting telescope (one with a primary mirror) to also avoid damaging the telescope.
Finally, what about the eclipse “safety” glasses that everyone is scrambling to get? I think these are a bad idea for several reasons:
- While many of the glasses are certified to be safe, there have already been recalls and findings of counterfeit, unsafe glasses. Given the huge popularity of this eclipse, there is a likely to be a fast growing black market for the glasses, and at this point I would not trust any of them.
- Even with safe, certified glasses, people may not wear them properly–especially small children. The glasses are typically cardboard and ill fitting, so it’s easy for them to slip out of position. The glasses also might get torn or scratched, allowing sunlight through. Any unfiltered ultraviolet light will permanently damage your eyeball. The danger from eclipses is that the reduced sunlight is not as painful as full sunlight, so we tend not to blink and look away, but the sunlight that is still coming through can damage our eyes.
- To use the glasses, you have to look up at the Sun. This means you are pointing your face at the Sun for an hour or more on a summer day. Similar to eye damage, sunburn is possible during an eclipse where you may not feel the Sun’s strength as much.
- The angle you have to look up at the Sun is fairly high, as the Sun is being eclipsed around the middle of the day when the Sun is highest in the sky, especially so during the summer months. This makes viewing the eclipse uncomfortable, and many people will probably get out lawn chairs to lie on their backs to view it, making for full frontal Sun exposure.
- All of the previous reasons are primarily safety reasons, but the final reason the glasses are not very good is that the image is disappointing. The Sun’s diameter is only about 32 arc minutes (about one half of a degree) across. It seems bigger because of its brightness. This means you will see a very small light circle eclipsed by a very small dark circle during the eclipse. In contrast, if you use a projection viewer as I recommend above, then you can make the image much bigger. Our telescope projection viewers easily make an image that is about six inches or more in diameter. With a telescope projection viewer, you can also easily see any sunspots on the un-eclipsed portion of the Sun.
For all these reasons, I recommend enjoying this eclipse with a projection viewer.
NOTE: Featured image of eclipse map is from https://www.space.com/33797-total-solar-eclipse-2017-guide.html#sthash.GsaXvkjN.uxfs.