JUDY WOODRUFF: People gathered across the country for a historic event today, a total eclipse of the sun in a 70 mile- wide band, crossing from the Pacific to the Atlantic coasts. With special eyeglasses or homemade boxes, tens of millions looked to the sky to witness a sight not seen in most people's lifetimes.

Our science correspondent, Miles O'Brien, was in Idaho to watch for us and in partnership with our colleagues from the PBS program, "Nova." Miles gets us started and then, he and William Brangham discuss the day's celestial and earthly events.

MILES O'BRIEN: It is the first coast to coast American eclipse in a century. Millions had front row seat for a celestial minuet of moon and sun.

JACK KRUMP, Tourist: We got to Charleston yesterday morning, came up because this was in our path and we could come and when you can, you should, so we came to see the eclipse because it's a once-in-a-lifetime deal.

STEPHEN ULMAN, Chicago Resident: I'd never seen an eclipse so I figured this was my chance since I was so close to Chicago.

MILES O'BRIEN: Beneath a 70-mile wide path from Salem, Oregon, to Charleston, South Carolina, day turned to night for two minutes or more.

It thrilled the public and the experts alike.

Williams College astronomer Jay Pasachoff was among them. He has traveled the world for years chasing eclipses. This is his 66th. No one has seen more.

Pasachoff is drawn by the beauty, and the scientific opportunity, when the moon appears to swallow the sun.

JAY PASACHOFF, Astronomer: And then this white corona appears all around you. It's dark and it's just a wonderful experience to have. And there's great science that you can do. Only on the days of eclipses do we see the corona appear, and so we want to take advantage of that as much as possible.

MILES O'BRIEN: Understanding the sun's corona is a priority for scientists. Among the mysteries: why is it hotter than the surface of the sun itself?

But there are practical reasons as well. Sometimes, the corona breaks free of the sun's magnetic field, causing a coronal mass ejection — billions of tons of hot plasma moving at 2,000 miles per second.

Normally, the earth's magnetic field deflects most of the highly charged particles. But every now and then, a large coronal mass ejection can overwhelm our defenses, disabling satellites and causing power outages.

Bill Murtagh is among the scientists watching this space weather for the National Oceanic and Atmospheric Administration in Boulder, Colorado. The biggest event they ever saw came in 2012.

BILL MURTAGH, Scientist: And this is what we saw. All of a sudden that flare occurs, the eruption occurs and that blast was tremendous. Very big. Very, very fast.

MILES O'BRIEN: Fortunately, it did not hit earth, as it would have caused widespread power outages.

A total eclipse is one way scientists try to better understand coronal mass ejections.

BILL MURTAGH: We would love to improve our capability to predict. If we can better model what the magnetic field might look like within an eruption, then we would be in a great place.

MILES O'BRIEN: NASA and the European Space Agency have sent several craft to study the sun over the years. The next big mission, the Parker Solar Probe, is slated for launch next August. It will fly through the corona itself gathering data. But no spacecraft can match the teaching opportunity provided by a total solar eclipse, which occurs when the earth, moon and sun are perfectly aligned, so the moon blocks the sun's light.

The moon is 400 times smaller than sun. But also 400 times closer to the earth. So, from our vantage point, they seem to be the same size.

But total solar eclipses happen rarely because the moon's orbit is tilted five degrees. And it is elliptical, so sometimes it is too far away to completely obscure the sun, causing a so called annular eclipse, with its distinctive ring of fire.

The last total solar eclipse visible in the Continental United States happened in the northwestern corner of the country in 1979.

MILES O'BRIEN: Of course, Jimmy Carter was president back then. That eclipse was in the northwestern U.S., ideally suited for Washington state. This time around, Washington, D.C. wasn't a bad place to watch. President Trump did so, briefly forgetting to put on protective glasses before he finally did the right thing — William.

WILLIAM BRANGHAM: So, Miles, you were there in the actual shadow cast by the moon on the face of the United States. Tell us what — for those of us who were here outside of that shadow, what was it like?

MILES O'BRIEN: You know, William, I've never seen the total eclipse of the sun before. This is my first experience with this. And, of course, we've all seen the pictures in the films.

The experience of being in it is surreal. It's the combination of all the senses that are involved, the temperature dropping, the light becoming this ethereal kind of blue and then, suddenly, darkness at noon for a brief period of time. I stopped looking through the welder's glass and looked at the sun or what was the sun, this disk with this amazing aura around it, and I was truly gobsmacked, I was at a loss for words.

It — you know, we think we're all so advanced and evolved, but I think it appears to us in a very fundamental, kind of limbic brain place. It's sort of an instinctual response that you have, it's difficult to put into words, but it was spectacular.

WILLIAM BRANGHAM: And I understand you were with a rather unique brand of scientists and enthusiasts out in Idaho. Would you tell us who you spent the day with?

MILES O'BRIEN: It was like the United Nations of astronomy here. People from five nations here. Many of them operators of planetariums, some of them amateur astronomers, a few professional astrophysicists, some of them doing some actual science here.

What I like about this and what I like about covering science in general is it does afford opportunities like this that really do bring us together. And we live in a time when things that bring us together seem to be in short supply. So, it was really nice to see us in this particular place come together and really in many respects the country kind of savor this moment together.

WILLIAM BRANGHAM: You reported several times about how crucial this day was for science. And I'm just wondering, why is it that we have to wait for an eclipse to do these sorts of measurements? Can't we put a filter on the telescopes or the devices that we used to measure the sun? Why do we have to wait for the moon to actually block it?

MILES O'BRIEN: You know, it's interesting. You can think about it. You can just cover the sun with your thumb, right, and maybe you get the same thing. It doesn't work that way.

It's important to have something in space that does the blocking because the atmosphere gets in the way of the science. If you have something — coincidently, the moon being 400 times smaller than the sun and yet 400 times closer makes it a perfect disk to occult the sun, creating that clean view of the corona which you really can't get unless you're in space.

And so, this is an opportunity for science. There are probes that have gone to the sun and will go to the sun that will get all kinds of other types of science, but this does give scientists a great opportunity to further understand the corona and its behavior.

WILLIAM BRANGHAM: So, the last one of these was in the late 1970s. Today was obviously a historic event for the U.S. When's the next one? When's the next chance we might have to get a gander at something like this?

MILES O'BRIEN: April of 2024, only seven years away. By quirk, this is happening. It's roughly, as we said, about every 18 months, that eclipse happens somewhere. Any given place on the planet, the odds are one in 365 years that you'll see a total eclipse.

Put that all into the Rubik's cube and you get another American eclipse from Texas all the way up into Pennsylvania, New York and into Maine in seven years' time. And, I having done this one and seen it in person, I can tell you, William, if I'm around, I will be there seven years from now to see it in person.

WILLIAM BRANGHAM: All right. Fantastic. We are always grateful for our Miles O'Brien, especially on days like today. Thank you so much.

MILES O'BRIEN: You're welcome, William.

JUDY WOODRUFF: Just a marvelous today.

And remember to stay with PBS tonight for "Nnova's" special, "Eclipse Across America."