How solar eclipses have been revealing cosmic mysteries for centuries

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A total solar eclipse is a great opportunity to learn more about the Sun

ESA/Royal Observatory of Belgium

A total solar eclipse occurs somewhere on Earth approximately every 18 months, and has been the case throughout human history. Naturally, people have been studying these dramatic events for a long time, with the first known written records of eclipses dating back more than 3000 years. In all that time, we’ve learned a surprising amount from total eclipses about the Sun, the Earth, and even the fundamental laws of physics.

For most of history, totality – the period of time in which the Moon covers the entire disk of the Sun – has been the only time humans can see the Sun’s outermost layer. This thin envelope of plasma, called the corona, has been central to many scientific advances derived from the study of eclipses.

ER8EXD Solar eclipse.  The Moon is rotating in front of the Sun.  illustration

solar eclipse 2024

On April 8, a total solar eclipse will pass over Mexico, America and Canada. Our special series covers everything you need to know, from how and when to see it to some of the strangest 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 threads and droplets of material into space. CMEs that strike Earth can damage satellites and electrical grids, and they can be exceptionally dangerous to astronauts in space, beyond the protection of Earth’s atmosphere.

“The Sun’s magnetic activity varies over time and across the star’s surface,” says meredith mcgregor At Johns Hopkins University in Maryland. As of yet, we don’t have a good way to predict this activity. But we can start doing this by studying corona.

A total solar eclipse isn’t the only way to see the Sun’s outermost layers — there’s also an instrument called a coronagraph, which uses a shade to block the Sun’s disk in a type of artificial eclipse. These instruments are important not only for studying our own star, but also for studying other, more distant stars and discovering any planets orbiting them that might otherwise be hidden in the brightness of the starlight. Will be. “The inspiration for using coronagraphs to block the light of other stars so we can look for their exoplanets comes from natural eclipses,” says McGregor.

The same blurring that makes the corona difficult to observe outside of totality also makes it an excellent target for spectroscopy. Spectroscopy works by breaking light into its component wavelengths. This allows researchers to determine which elements are present in a material by the unique patterns of wavelengths emitted or absorbed by each element. Helium was discovered using spectroscopy during an eclipse in 1868, marking the first time an element had been discovered by studying the sky.

Soon after, astronomers found another new element in the corona, which they called coronium, but it turned out to be simply iron heated to extraordinary temperatures of millions of degrees. Even though it was not a new element, it was a surprising discovery – the Sun’s surface temperature is only 5600 degrees Celsius, so how could the outermost layer be so hot?

“Imagine you’re at a campfire, and you start walking away from the campfire. And it’s supposed to be cool, but it gets too hot,” says Frederick Bartley At the Center for Science and Industry in Ohio. “This is what is going on with Corona, and no one knows why.”

Solar eclipses also provided the first proof of Albert Einstein’s general theory of relativity, which governs how gravity behaves on large scales. One of the key 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 proof of its validity came in 1919, when astronomer Arthur Eddington observed the bending of starlight 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 precisely observe the Sun and how it affects the space around it. There are still many secrets of the Sun to be revealed and the eclipse is one of the best times to study them.

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