Podcaster: Jay Pasachoff
Title: The July 2, 2019, Total Solar Eclipse
Organization: Williams College and Caltech
Description: On July 2, the path of totality of a total solar eclipse will cross the Pacific Ocean, including Oeno Island, and then cross Chile and Argentina. Scientists and tourists will observe the path of totality, especially from the region of Chile that includes La Serena and the Elqui Valley, with the Cerro Tololo Inter-American Observatory among astronomical facilities in totality. The observations at this minimum phase of the sunspot cycle will provide important information about the solar corona, and will fill in the ordinary gaps between on-disk extreme-ultraviolet imaging and satellite imaging of the outer corona.
Bio: Jay Pasachoff is Field Memorial Professor of Astronomy at Williams College and Chair of the International Astronomical Union’s Working Group on Eclipses. He is on sabbatical at Carnegie Observatories.
The fifth edition of his textbook The Cosmos: Astronomy in the New Millennium (Pasachoff and Alex Filippenko, and a new book on the intersection of art and astronomy, Cosmos: The Art and Science of the Universe (Roberta J. M. Olson and Pasachoff), have just been published.
Bio: Jay Pasachoff is Field Memorial Professor of Astronomy at Williams College and Chair of the International Astronomical Union’s Working Group on Eclipses. He is on sabbatical at Carnegie Observatories. The fifth edition of his textbook The Cosmos: Astronomy in the New Millennium (Pasachoff and Alex Filippenko, and a new book on the intersection of art and astronomy, Cosmos: The Art and Science of the Universe (Roberta J. M. Olson and Pasachoff), have just been published. See http://solarcorona.com
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We are looking forward to the next total solar eclipse, July 2nd, 2019. The path of the eclipse will cross South America—Chile and Argentina—after going across the Pacific and we hope to see what we saw at the ’17 total solar eclipse, the most recent total solar eclipse in which the path of totality crossed the United States.
What happens during an eclipse is that the moon goes in front of the sun and a total eclipse is a little closer than average in its orbit around the earth and entirely covers the everyday sun so that we see the outer parts of the sun, the solar corona. But sometimes the moon is a little smaller than average and a ring of everyday sunlight, an annulus of sunlight, remains visible and that makes the sky too bright. The blue sky remains and we can’t see the corona so we scientists are especially interested in the total solar eclipses when the sky is dark and we can’t see the corona, the outer atmosphere of the sun. It is about a million time darker than a normal day so even if one percent of the everyday sun remains visible that’s still ten-thousand times brighter than it is during a total eclipse so you really have to get into the path of the total eclipse.
The path on July 2nd, 2019, crosses the Pacific. It passes only one island but is mainly over ocean, and just hits South America, where it is low in the sky. It is only about thirteen degrees above the horizon when it reaches the coast of Chile and then at sunset on the horizon just inside the Atlantic coast of Argentina.
18 months later, in December 2020, which is then summertime in South America, there will be another total solar eclipse a little further south, also crossing Chile and Argentina. At that time, the peak will be over Argentina and the sun will be over 70 degrees high in the sky in Argentina. And my group will be located on the Atlantic coast of Argentina.
In any case, we only occasionally get these total solar eclipses—every 18 months or so on the average—and the sun is different every time. Here, for example, is a series of three different total eclipses that were painted almost 100 years ago by Howard Russell Butler in oils. At that time, the cameras couldn’t show with the photographic plates or film the detail of the corona the way they can today, so Butler took notes during the couple of minutes of totality and then used his skills as a portrait painter to translate it into oil paintings. These paintings in particular hung for decades over the entrance of the Hayden Planetarium in New York, part of the American Museum of Natural History. Butler had first been invited by the US Naval Observatory on the 1918 expedition to Oregon, and then he got interested as one might and painted also 1923, 1925 that you just saw, and also in 1932.
In any case, we have many scientific observations that we can make only at total solar eclipses. The corona changes with the sunspot cycle, the roughly 11-year sunspot cycle. We are now at the minimum of the sunspot cycle; most days have no sunspots at all on the surface of the sun and that means there are only a few coronal streamers relatively close to the solar equator and the poles remain visible where there are plumes that come out of the poles looking much like the filings that come out of a bar magnet. All this illustrates how important the magnetic field is for the sun.
The magnetic field in sunspots was discovered by George Ellery Hale at the Mount Wilson Observatory in 1908 and the sunspot regions are regions of especially strong magnetic field. But the coronal magnetic fields, which are weaker, are still strong enough to govern the very hot gas around the sun.
It was discovered at an eclipse in 1869 that there were some special spectral emissions from the corona, in particular a strange one in the green, “the coronal green line.” In 1868, they found a yellow line which was soon called helium because it was only from the sun. The next year, in 1869, the coronal green light was seen and they thought it was from “coronium,” a new element as helium turned out to be. As you know, helium did turn out to be a fundamental element but they didn’t discover that until 1895, when they isolated it on Earth. And coronium took until 1940 to explain, and then it was finally realized that it wasn’t a new element but it was really only a form of iron that only existed when it was so hot—over a million degrees—that half of iron’s normal 26 electrons had been driven off so this is iron that’s 13 times ionized (since the neutral state is Roman numeral I), which is called iron 14. And we now have several spectral lines of different ionization stages and elements.
We will be observing those from our sites in Chile. I will be at a mine at a mountainside at an altitude of about 2200 feet on the centerline as it passes Chile. And unusually, there is actually a couple of major astronomical observatories in the zone of totality. I have permission to have 4 of my colleagues at the Cerro Tololo Inter-American Observatory for the eclipse, and they will have a variety of cameras and telescopes to make images of the sun both in ordinary light—what we call white light, all the sunlight together— and also through some special filters and with some spectrographs. So when we only get a couple of minutes every year and a half or so to see what the corona is like in detail, we want to take advantage of that. And then we can put together these observations at eclipses from what we can get from NASA and European Space Agency spacecraft, which can’t observe the region of the sun that we see best at total solar eclipses. So only on the days of total solar eclipses do we get a complete view of the sun including all of its layers.
And we should also remember that the sun is a typical star and that there are billions and trillions of stars like the sun, so when we are observing the details of the solar corona at an eclipse, we’re learning what the details are of the coronas of all these distant stars that are all fuzzed out without our ability to see detail.
A total solar eclipse remains a scientifically valuable occasion and also a fantastic occasion for individuals to see.
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365 Days of Astronomy
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