Understanding the Science Behind the Total Solar Eclipse

When AISD Trustee Lynn Boswell tells friends why she’s traveled to see multiple total solar eclipses, she explains that she’s going to see “a hole in the sky with fire shooting out of it.”

“It’s magnificent and amazing,” she says. “You just stand there in that darkness awed by it. Everyone is silent, watching together, and the birds and insects change what they’re doing. The temperature drops. The stars come out. If you’re up in a high place looking down, you might see a shadow race across the ground as it’s coming toward you.

“It’s unlike anything else. And then it’s gone.”

A total solar eclipse happens somewhere on Earth about every 18 months. But any given location experiences a total solar eclipse much less frequently – at roughly 375-year intervals, although it’s been more than 600 years since the Austin area saw such an eclipse. This is why Austinites’ opportunity to watch Monday’s eclipse from their own backyard is so rare and valuable.

Total solar eclipses occur when the moon passes between the Earth and the sun, blocking the sun and casting a shadow over a narrow strip of land called the path of totality. The orbits of the moon around the Earth, and the Earth around the sun, are elliptical. But they’re not on the same plane; the moon’s orbit is tilted by five degrees to the Earth’s orbital plane. A solar eclipse only can occur when the moon’s orbit intersects with the Earth’s orbital plane. This is why there’s not a total solar eclipse every month at new moon.

If the sun, moon, and Earth align when the moon is closest to Earth in its orbit (and thus appears largest), those in the moon’s shadow will experience a total solar eclipse. If they align when the moon is farther from Earth, viewers on Earth will see an annular solar eclipse, because the moon does not appear large enough to fully cover the sun. This type of eclipse, in which a ring of bright sun remains visible around the moon, is what Austin experienced last October.

Total solar eclipses are only possible because we live at the right moment in cosmic history. Eclipses happen because the sun is both 400 times larger than the moon and 400 times farther away. This means the two appear to be the same angular size in the sky. When the moon first formed, it was much closer to the Earth and would have appeared larger, had humans been alive to see it. But the moon’s orbit has gradually been enlarging, moving away from the Earth about 1.5 inches each year. Millions of years from now, total solar eclipses will no longer occur.

Multiple ancient cultures left evidence that they were aware of eclipses, although they did not know what they were actually seeing. The Greek philosopher Anaxagoras, born in 500BC, was the first person known to explain eclipses as the result of the moon blocking the sun’s light. Two thousand years before Copernicus propagated the heliocentric model of the universe, Anaxagoras was not aware of the orbital patterns that yielded eclipses. But, as astronomer and artist Tyler Nordgren explains in his book Sun, Moon, Earth, Anaxagoras understood enough to try to calculate the moon’s size based on the shadow it cast during an annular eclipse in 478BC.

Scientists still confront unanswered questions during eclipses. Heliophysicists don’t fully understand the structure of the sun’s corona, or outer atmosphere, or know why the corona is hundreds of times hotter than the sun’s surface. Eclipses present a rare opportunity for scientists to capture images of the entire corona from Earth to try to advance their knowledge.

A total solar eclipse is also the only time the corona is visible to the naked eye. Wait! You can look at the sun with the naked eye during an eclipse? Yes, but only if you are inside the path of totality, and only during the few minutes of totality, when the moon completely blocks out the sun. Every total solar eclipse begins as a partial eclipse, as the moon slides across the face of the sun. In Austin, this will start Monday at 12:17pm. During the entire partial eclipse, it is as dangerous to look at the sun as it is on a normal day. Even a quick glance can cause permanent retinal damage. But once the sun is covered by the moon – in Downtown Austin, this will happen at 1:36pm and 9 seconds – viewers can remove their eclipse glasses. For the minute and 46 seconds of totality (again, in Downtown Austin), everyone can look up at the “hole in the sky with fire shooting out of it” – the moon surrounded by the corona.

If you didn’t get a pair of ISO 12312-2 eclipse glasses, you can still experience the eclipse. You can watch indirectly by creating a pinhole camera, which projects the image of the sun onto a surface. Punch a tiny hole in a piece of paper and – with your back to the sun – hold it over another piece of paper or posterboard. You’ll be able to see the image of the shrinking sun projected onto the second paper. (Anything with a small hole will work, even a colander or slotted spoon.) Set your watch, or listen to people around you who have glasses, to know when totality has begun. Then you can look up and enjoy the darkness until just before totality ends; set a timer to ensure you’re looking away well before any sunlight emerges from behind the moon. If you’re watching from near the edge of the path of totality, be especially careful. Double-check the map (https://eclipse2024.org/eclipse_cities/statemap.html) if you’re south of the river and east of I-35.

Eclipse chasers like Boswell describe the experience of a total solar eclipse as eerie but spectacular. “The two central emotions that everyone describes are awe and primal fear,” writes Kate Russo, an Australian eclipse chaser who helps towns prepare for eclipses. Watching with a group tends to compound the awe, as one scream of delight engenders others: the embedded video on Russo’s site (https://beingintheshadow.com/what-is-it-like), and the NPR video of the 2017 eclipse, “One Nation Under the Sun,” capture the moment.

“It is such a beautiful shared experience,” Boswell says. “The wonder of it connects and unites people. The idea of that coming to us, in our city, and having that amazing experience together is exciting.”

The Austin Public Library is giving away free solar eclipse glasses while supplies last starting on Thursday, April 4. Limit two per person. Once glasses are gone, they will not be replenishing quantities. However, a limited number of glasses will also be handed out at their nine Total Eclipse Watch Parties taking place around Austin on Monday, April 8. Locations and times are as follows:

Central Library: noon-2pm

Cepeda Branch: noon-2pm

Menchaca Branch: 11:30am-2pm

North Village Branch: 11am-2pm

Old Quarry Branch: noon-2pm

Pleasant Hill Branch: 12:30-2:30pm

Twin Oaks Branch: noon-2pm

Willie Mae Kirk Branch: noon-2pm

Windsor Park Branch: noon-2pm