In the summer of 2017, GBH’s All Things Considered host Arun Rath traveled with his family to the town of Rexburg, Idaho, to witness the total solar eclipse. “It was, without question, the most extraordinary, sublime, awe-inducing natural phenomenon I have ever experienced,” he recalled. 

It was quite a trek getting to the remote location, Rath says, so he was thrilled to hear the 2024 eclipse totality would be a less than four-hour drive from the Greater Boston area. 

The event this Monday, April 8, will be the last time the lower 48 states will be able to see a total solar eclipse until 2044. Grant Tremblay with the Harvard and Smithsonian Center for Astrophysics and vice president of the American Astronomical Society joined Rath to help digest this especially unique celestial phenomenon. What follows is a lightly edited transcript of their conversation.

Arun Rath: Let’s start with some eclipse terminology because we’re talking about a totality. So, everybody in the country will be able to see at least a partial solar eclipse on Monday, right?

Grant Tremblay: Yeah, that’s right. If you look at any shadow — literally, any shadow — there are two components to it: an “umbra” and a “penumbra.” Basically, if you’re standing in an umbra, the thing that’s casting the shadow is completely blocking out the source of light that’s behind it. Then, the penumbra, the thing that’s casting the shadow, is only partly blocking out that source of light.

The moon casts a shadow at all times, as you know. It’s orbiting the Earth roughly every 27 days. Every once in a while, that shadow crosses the Earth, and in this very rare event — once every 375 years in any given spot on Earth — you will see a total solar eclipse, which means the moon moves in front of the sun and completely blocks out that disk.

It’s a geometric effect; it’s a luck of alignment. We will be sitting under the umbra of the moon, that darkest part of the shadow, in which case the disk of the sun is completely blocked by the moon.

Rath: At that moment, when you’re in the totality, it’s this incredible ring of fire. It’s really hard to put into words.

Tremblay: It’s really hard to overstate. Your listeners might think, “Oh, the eclipse is just a shadow. Who cares?” It’s also true that if you’re not in that total path of totality, the eclipse is very cool — like, very cool — but it’s not bonkers, mind-blowing spectacular until you really are in that path of totality.

What happens is — what you don’t really realize is that the sun is a star. Now that’s a silly statement; of course, the sun is a star. But the sun to us looks like a giant bright blob in the sky, this thing that you can barely look at because it hurts your eyes.

What you don’t really realize is that the sun is the lord of our solar system. When you can see this majestic corona — this multimillion-degree Fahrenheit crown of plasma, these ribbons of plasma that extend eight million miles above the surface of the sun itself — until you can really see that with your own eyes, you can’t really appreciate how majestic and awe-inspiring our star really is, our home star that gives life to everything on Earth.

It’s a cosmic moment. It’s the most cosmically spectacular shadow you will ever see because [we’ll] finally, finally be able to see the corona. It makes the sun look so enormous and so magical and so powerful. In contrast, it makes us feel smaller and closer together, and therefore, more unified. It really is a magical moment.

A flare of the sun's light shoots out from the edge of the moon's shadow as it reaches the moment of totality during a solar eclipse.
FILE - Total solar eclipse on July 2, 2019 in Paiguano, Chile.
Marcelo Hernandez Getty Images

Rath: I could just pile on here, and at the risk of sounding flaky — I would call it like a spiritual, nonreligious experience. I felt changed by it. I felt like one of the apes at the beginning of “2001.” It was just, like you said, mind-blowing.

Tremblay: Yeah. Arun, one of the saddest eras of time in life — I have three kids, and the saddest thing of [my] life is to watch them as they’re born, everything in the world is completely magical, right? From their first rainstorm to their first encounter with a dog.

As you get older, that magic of nature is sucked away from you. It’s stolen from you in so many ways, such that by the time you’re in high school, you’re staring into your phone, you’re disillusioned with life. Dreams are harder to achieve than we advertised.

One of the amazing things about astronomy — why we invest in giant, multibillion-dollar telescopes, why we put mirrors to the sky, why we get in the shadow of the moon to observe the sun — is because the cosmos is a constant reminder that nature is as magical as we once believed as kids.

It really is as magical as we believed, and that is a unifying and, yes, borderline spiritual moment to realize that we are just star stuff, right? The material that’s in your body — dead serious, from the bottom of my heart, I swear to you — was once in the heart of a star. That star that you’re going to be able to see if you’re in the path of totality.

That’s the sort of cosmic connection, the sort of [astronomer Carl] Sagan-like, “We are star stuff” thing. We are the cosmos trying to understand itself, and it’s really, really powerful.

“If you’re not in that total path of totality, the eclipse is very cool — like, very cool — but it’s not bonkers, mind-blowing spectacular until you really are in that path of totality.”
Grant Tremblay

Rath: To go from truly the sublime to the more material: We should talk about safe eclipse viewing. Run through that with us.

Tremblay: Yeah. I’m the VP of the American Astronomical Society, and I’m contractually obligated, legally, to tell you to please not look at the sun with your naked eyes. The reason we have to keep hammering this point is because, technically, if you try to look at the sun, it only kind of hurts. It’s a bearable pain. The problem is that, at that moment, the lens of your eye is focusing these incredibly powerful light rays from the sun onto your retina. And as you are staring at it, you are doing lifelong, permanent damage to your retina.

I swear to you, it’s not worth it. You will not see anything by looking at the sun with your naked eyes, so don’t even bother. You need to view the eclipse properly, especially if you’re not in totality. You must have eclipse glasses, or you can search online to figure out how to make a pinhole camera. It’s not that hard; it’s not that spectacular, to be totally frank. If you can get your hands on proper glasses, that’s great, but please do not stare at the sun.

Now, one caveat to that. If you’re in the path of totality, you have anywhere between one minute and, maximally, four minutes of total coverage of the moon: Please do take off your glasses. It will be very obvious when you can — very obvious. Please do take them off so that you can see with your own naked eye the corona of the sun, which is the whole majestic point of a total solar eclipse. Just in that brief moment of totality, if you’re in the path.

And then, it’ll be obvious again as the moon starts to move away and that sliver of the sun reappears on the other side of the moon. It will be obvious that you need to put your glasses back on again. Please, do not damage your eyes for this. It is not worth it.

“Why we get in the shadow of the moon to observe the sun — is because the cosmos is a constant reminder that nature is as magical as we once believed as kids.”
Grant Tremblay

Rath: Finally, let’s talk about: an eclipse is always special, but the fact that this is just a relatively short drive from where we are, just how rare that is.

Tremblay: Yeah, that’s right. Look, the moon is always casting a shadow. And by an accident of geometry — and I encourage them to look at a little movie online of the sort of geometric dance that creates an eclipse — the moon going around the Earth and the Earth going around the sun. Because the orbit of the moon is inclined, relative to the so-called “ecliptic” — the plane in which the Earth orbits around the sun — total eclipses are rarer than you think.

Somewhere on Earth, a total solar eclipse will happen, on average, roughly every 18 months. The problem is, your listeners need to know, that we live on an ocean world, right? Seventy percent of the Earth is covered in oceans, so plenty of total solar eclipses are viewable only from the middle of the Atlantic or the South Pacific. And so, in any given location on Earth, if you were to stand still, a total solar eclipse, on average, will occur from your viewpoint once every 375 years.

It’s very likely that if your listeners are within a drive to the path of totality, the only time in their lifetimes they will be a short drive from totality. Because once every 375 years is the average, with the caveat that there are huge error bars because it very much is dependent on where you are.

But this particular path is really, really accidentally perfectly aligned with a ton of people.