How solar eclipses have been revealing cosmic secrets for centuries

A total solar eclipse occurs somewhere on Earth about every 18 months, and that has been the case for all of human history. Naturally, people have been studying these dramatic events for just as long, with the first known written record of an eclipse dating back more than 3000 years. In all that time, we have learned an astonishing amount from total eclipses about the sun, Earth and even the fundamental laws of physics.

For much of history, totality – the period of time in which the moon covers the entire disc of the sun – has been the only time that humans could see the sun’s faint outermost layer. This wispy shroud of plasma, called the corona, has been central to many of the scientific advances that have come from the study of eclipses.

Solar Eclipse 2024

On 8 April a total solar eclipse will pass over Mexico, the US and Canada. Our special series is covering everything you need to know, from how and when to see it to some of the weirdest eclipse experiences in history.

The corona is home to many of the sun’s most fascinating phenomena, including coronal mass ejections (CMEs), which occur when the sun’s churning magnetic field blasts strands and blobs of material out into space. CMEs that hit Earth can damage satellites and the electrical grid, and they can be extraordinarily dangerous to astronauts out in space, beyond the protection of Earth’s atmosphere.

“The magnetic activity of the sun changes over time and changes across the surface of the star,” says Meredith MacGregor at Johns Hopkins University in Maryland. So far, we don’t have a good way to predict this activity. But we might be able to begin doing so by studying the corona.

A total solar eclipse isn’t the only way to look at the sun’s outermost layers – there is also an instrument called a coronagraph, which uses a shade to block out the disc of the sun in a sort of artificial eclipse. These instruments are important not only for studying our own star, but also for studying other, more distant stars and searching for any planets orbiting them that would otherwise be hidden in the glare of starlight. “The inspiration to use coronagraphs to block out the light of other stars so we can look for their exoplanets comes from natural eclipses,” says MacGregor.

The same dimness that makes the corona difficult to observe outside of totality also makes it an excellent target for spectroscopy. Spectroscopy works by breaking down light into its constituent wavelengths. This allows researchers to determine what elements are present in a material by the unique pattern of wavelengths each element emits or absorbs. Helium was discovered using spectroscopy during an eclipse in 1868, which was the first time any element was discovered by studying the skies.

Soon afterwards, astronomers found what appeared to be another new element in the corona, which they termed coronium, but it turned out to simply be iron heated up to extraordinary temperatures of millions of degrees. Even though it wasn’t a new element, this was a baffling find – the surface of the sun is only about 5600°C, so how could the outermost layer be so sweltering?

“Imagine you’re at a campfire, and you start walking away from the campfire. And it should be getting colder, but it gets far hotter,” says Frederic Bertley at the Center of Science and Industry in Ohio. “That’s what’s going on in the corona, and nobody knows why that is.”

Solar eclipses even provided some of the first proof of Albert Einstein’s general theory of relativity, which governs how gravity behaves on large scales. One of the major predictions of general relativity is that massive objects should bend the trajectory of light as it passes by them. Einstein first presented his theory in 1915, and evidence for its veracity came in 1919, when astronomer Arthur Eddington observed starlight bending around the sun during a solar eclipse.

When a total solar eclipse passes over Central and North America this month, astronomers will continue their long tradition of taking advantage of totality to make precise observations of the sun and how it affects the space around it. The sun still has many secrets to unravel, and an eclipse is one of the best times to study them.

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