[Host] For months, Vermonters have been firming up plans for Monday’s total solar eclipse — it’s for many folks a once-in-a generation event that won’t happen again in North America until 2044.
Some, though, have been down this celestial road before. That includes John Thorstensen, a professor of physics and astronomy at Dartmouth College.
He spoke with Vermont Public’s Mitch Wertlieb about his experiences with eclipses and how to view them safely. Thorstensen starts by describing eclipse-related discoveries. This piece was produced for the ear. We highly recommend listening to the audio, if you're able. We’ve also provided a written version.
John Thorstensen: The element Helium was first found in solar eclipse spectrum, which is the second most abundant element in the universe. The other thing that I think towers above all others, is that one of the very first empirical tests of Einstein's theory of gravity — general relativity — was done in a 1919 solar eclipse expedition by the British by Sir Arthur Eddington and others. They saw that the light from stars near the sun was deflected slightly by the sun's gravity. This was a prediction of Einstein's general theory of relativity. And they were able to confirm that Einstein's relativity gives the answer that's closest to reality. I think there's been no single, convincing observation that shows it's wrong.
Mitch Wertlieb: That is kind of amazing.
I want to talk a little bit about your personal relationship with eclipses. Because as I understand it, you're kind of like a storm chaser for tornadoes, except with eclipses. You've seen three total solar eclipses already in your lifetime — in-person. I'm wondering if you can describe those experiences for us.
John Thorstensen: There's almost nothing that you really can say that describes it adequately. It's just existentially strange to have it get really dark in the middle of the day. Furthermore, as the light fades away, it does not get redder like it does at sunset. It stays the same color as the light does at midday, which is much bluer than sunset light. So it has an earthly look to it; it's really eerie. It's hard to explain. Then when the totality comes, the darkness comes rushing in. If you have a view towards where the eclipse is coming from, you can actually see the darkness coming at you. It is astonishing.
Mitch Wertlieb: I understand that you had quite the memorable experience back in 1979. You were trying to race from Berkeley to Montana to catch a total solar eclipse. And maybe you didn't have the most reliable van in the world — let's say — to make that distance.
John Thorstensen: I was just thinking about that. We had two vehicles: one rental car and one rental van. Then it was Feb. 26, the eclipse date. And we got into an accident because somebody, some English drivers, slid out on the ice going across the Rockies and we had a nasty fender bender. We all had to jam into this van to get the rest of the way. It kept happening, flat tires and bad alternators and you name it —but we finally got there. We finally pulled in Elena, the night before, and then drove out to find to see the eclipse, which happened in the morning, if I remember. Montana is the big sky country, and you can see miles and miles and miles. We could all see the eclipse shadow coming from far away, and we watch the mountains in the distance getting dimmer and dimmer— as if there was a gigantic cosmic dimmer switch just turning down as the shadow came to us. It was stunning.
Mitch Wertlieb: It's occurring to me now, thinking of these previous eclipses you've seen, I don't know if back in 1979 or previously, they had what they have now —these safety glasses that you can use to look. How did you view the event? What are some risks people should avoid when looking at this eclipse?
John Thorstensen: Very good point. I don't remember how we did that. I think we might have used eyepiece projection, which is a trick where you can use a white card to project light from a telescope — but do not look in the telescope at an eclipse. Never do that — at least not during the partial phases. I want to emphasize though that if you are in the path of totality — I'll be very clear on this — you can look right at it with no filtration while it is total just during that time. That is only a couple of minutes.
Mitch Wertlieb: Why is that?
John Thorstensen: It's because the whole bright disk of the sun is covered and the rest of it isn't all that bright. The hazard from looking at eclipses isn't something mystical. It's just that there is so much energy in solar light that it'll damage your retina if you look right at it. Whereas if once the sun, the visible disk of the sun — the photosphere — is completely gone, then it's perfectly safe. In fact, you can look right at it with binoculars.
Mitch Wertlieb: When you're talking about the danger then, I want to make sure that I've got this right because I think I'm in the path of totality where I live — when you see that shadow approaching, when you see the darkness coming, it's not over the sun yet, but it's coming — that's when you should not be looking.
John Thorstensen: In fact, actually, that's almost the most dangerous time because if there's any bit of the sun exposed, it's going to be too much. When you look at it, it doesn't look overwhelmingly bright like the normal sun, you will know when the sun is totally eclipsed. When it comes back, it tends to come back in little spots right on the edge of the moon. As the sun reappears, remarkably, it shines through the cracks or the valleys between the mountains on the moon on the edge of the moon. So there's a little arc of little bright spots — and the instant you see that stop looking at it. You can look right at the eclipse when it is total and if it is total. You can't look at it any other time except with the safety glasses.
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