This article is part special report about a total solar eclipse that will be visible from parts of the United States, Mexico and Canada on April 8, 2024.
Total solar eclipses such as one that will sweep across North America this April.are some of the most sublime and transcendental natural phenomena one can experience. The spectacle of totality – when the Moon completely covers the Sun, casting a dark shadow on the Earth below – is almost surreal, as if the natural rhythm and regular order of the cosmos had been disrupted. It's no wonder that these events have evoked fear, wonder, and reverence throughout history. They also provided astronomers with an excellent opportunity to test cutting-edge physics theories and discover new aspects of our natural world. Here are just three of the many ways a total solar eclipse has changed the way we look at the sky, Earth, and everything in between.
Halley's Eclipse
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If it weren't for Edmond Halley, we might never have had Isaac Newton's revolutionary theory of gravity. In 1684, one of Halley's contemporaries, Robert Hooke, claimed that he could deduce Kepler's laws of planetary motion from simpler principles. However, when his claim was questioned, he was unable to confirm it. Halley also agreed to tackle this problem, but failed himself, so he turned to his old friend Newton. Newton surprised Hooke by declaring that he had already found the solution but had “lost” the notes. To satisfy Halley's insistent support, Newton produced perhaps the greatest work of physical knowledge in history, his Principles of Mathematics.
To say Halley was a big fan of Newton would be an understatement. Halley personally financed the first publication of Newton's work and played a key role in conveying its importance and significance to the public. In doing so, he became the first person known in history to accurately predict the upcoming solar eclipse.
Cultures throughout history have successfully made rough guesses about the timing of eclipses. However, using Newton's newly created laws of gravity, Halley was able to predict with reasonable accuracy the time and trajectory of the total solar eclipse that passed over London on May 3, 1715. The time and route were accurate to within four minutes and 20 miles respectively. (Perhaps Halley was discarded not because of a violation of Newton's laws, but because of inaccuracies in the recordings of the Moon's motion).
Naturally, this event made the news as scientists and laypeople around the world recognized Newton's genius. And the way in which Halley depicted the geographical path of the eclipse (with dark stripes showing its completeness and partiality) was so good that we still use this style.
Janssen Eclipse
By the mid-1800s, chemists, physicists, and astronomers were excited about the new technique of spectroscopy, in which splitting light into a rainbow spectrum of its constituent colors could reveal the elemental composition of the source. (Exactly what these elements were was still a matter of debate, since the existence of atoms had not yet been proven!)
Using spectroscopy, for the first time astronomers were able to look into their telescopes and identify the matter of what they saw as easily as if they could reach out and touch those distant planets and stars. Today, spectroscopy is the basis of modern astronomy. For every eye-catching astronomical image of a celestial object you might encounter, there are probably a dozen published papers about its spectrum.
Since the Sun is the brightest thing in the sky, it has become a natural object for spectroscopy. Using this method, astronomers discovered hydrogen, iron, oxygen, carbon and other substances lurking in the hot atmosphere of the Sun, as well as hints at the presence of one element that defied easy understanding. The earliest observations suggested that it could be some strange kind of iron, but no explanation fully fit the data.
A critical advance occurred on August 18, 1868, when international teams of astronomers observed a total solar eclipse in southern India and Southeast Asia. Among them were Norman Lockyer and Jules Janssen, who together studied the spectra of solar prominences that suddenly appeared around the eclipsing silhouette of the Moon. These spectra allowed them to clear the darkness, clearly revealing the presence in the Sun of a new element previously unknown on Earth.
It would take Earth's chemists decades to isolate the element, which they named helium, after the Greek word Helioswhich means “sun”. Helium was the first and only element to date to be discovered in the heavens before it was found on Earth.
Eddington Eclipse
As beautiful and accurate as Newton's explanation of gravity was, it was incomplete and could not adequately explain some phenomena, such as the precession of Mercury's orbit around the Sun. This incompleteness was a key motivator in Albert Einstein's efforts to create his own new concept of gravity—his general theory of relativity, which views gravity as the curvature of spacetime caused by massive objects. Using general relativity, Einstein was able to explain the mysteries of Mercury's orbit. Technically it was postdiction, however, it is an invention of a theory to explain already known results. He needed forecast– something new to demonstrate how powerful his theory was.
Einstein quickly came up with the idea of ​​using general relativity to predict the extent to which light should be bent by the gravitational field (that is, the curvature of spacetime) around a massive object such as the Sun. The Sun's gravity should bend passing light rays slightly. We usually can't see this effect because it's incredibly small and most light rays from distant stars don't travel close enough to the Sun. But during a total solar eclipse, someone could potentially measure the exact position of the star right at the visible edge of the Sun, and then compare its position at any other time to detect that deviation.
Newton's theory also predicted such a deviation, and Einstein's early studies based on relativity gave identical results. In a 1911 paper, Einstein urged astronomers to search for this effect. Although they attempted several subsequent eclipses, their attempts were thwarted by bad weather.
This turned out well for Einstein: once he fully fleshed out his theory, he realized that his calculations produced a stronger deviation than Newtonian gravity predicted. After Einstein again turned to his fellow astronomers for help, Frank Watson Dyson and Arthur Eddington accepted his challenge. Leading two expeditions—one to the island of Principe and the other to Brazil—these astronomers measured the apparent positions of stars near the Sun during the total solar eclipse of May 29, 1919, and found that they were shifted exactly as Einstein had predicted.
The following year, during lunch at the Royal Astronomical Society, Eddington read the following poem, which he wrote as a parody of Rubaiyat of Omar Khayyam.
Oh, leave to the Wise our measures to compare
At least one thing is certain: light has weight.
One thing is certain, the rest is up for debate.
Rays of light, being close to the Sun, do not travel straight.
Tomorrow's eclipse
Nowadays, astronomers working on Earth usually don't have to wait for the next fateful lunar alignment to study the Sun, because they can make their own “eclipse” on demand using a clever instrument called a coronagraph. Such devices can be as simple as a disk attached to a telescope that precisely blocks the sun. Astronomers often use coronagraphs to study the outer atmosphere of the Sun, where many mysteries remain to be found: no one knows yet exactly why this region is so hot compared to the visible surface of the Sun, why it has such strong and tangled magnetic fields, or why it is capable of launching a never-ending stream of charged particles known as the solar wind.
Naturally occurring eclipses of the past have helped us revolutionize our view of the universe, and today's man-made eclipses are sure to propel us into the future of astronomy. Who knows what new secrets the sun will reveal to us next time?
