by To understand why Eddington and Dyson traveled such distances to watch the eclipse, we need to talk about gravity.
Since at least the days of Isaac Newton, who wrote in 1687, scientists thought that gravity was a simple force of mutual attraction. Newton proposed that every object in the universe attracts every other object in the universe, and that the strength of this attraction is related to the size of the objects and the distance between them. This is mostly true, actually, but it’s a little more nuanced than that.
On much larger scales, such as among black holes or galaxy clusters, Newtonian gravity falls short. Nor can it accurately account for the movement of large objects that are close together, for example the effect of its proximity to the Sun on Mercury’s orbit.
Albert Einstein’s most consequential breakthrough solved these problems. General relativity makes it clear that gravity is not really an invisible force of mutual attraction, but a distortion. Rather than some sort of tug-of-war, large objects such as the Sun and other stars respond relative to each other because the space they are in has changed. Their mass is so great that they bend the fabric of space and time around themselves.
Read More: 10 Surprising Facts About Solar Eclipse 2024
This was a strange concept, and many scientists thought that Einstein’s ideas and equations were ridiculous. But others thought it was reasonable. Einstein and others knew that if the theory was correct, and the fabric of reality bends around large objects, light itself must follow that bend. It would appear that the light of a distant star, for example, would surround a large object in front of it, closer to us—like our Sun. But usually, it is not possible to study the stars behind the Sun to measure this effect. Enter eclipse.
Einstein’s theory gives an equation for how much the Sun’s gravity would displace the images of stars in the background. Newton’s theory predicts only half that amount of displacement.
Eddington and Dyson measured the Hyades cluster because it contains many stars; the more distorted stars, the better the comparison. The two teams of scientists encountered strange political and natural obstacles in making the discovery, which is beautifully chronicled in the book Beyond a Shadow of a Doubt: The 1919 Eclipse Confirmed Einstein’s Theory of Relativity, by physicist Daniel Kennefick. But Einstein’s ideas were worth confirming. Eddington said this in a letter to his mother: “The one good plate I measured gave a result in agreement with Einstein,” he wrote, “and I think I have received a little confirmation from the second plate.”
The Eddington-Dyson experiments were hardly the first time scientists used an eclipse to make profound new discoveries. The idea dates back to the beginning of human civilization.
Careful records of lunar and solar eclipses are one of the greatest legacies of ancient Babylon. Astronomers – or astronomers, really, but the goal was the same – were able to predict both lunar and solar eclipses with remarkable accuracy. They worked out what we now call the Saros Cycle, a repeating period of 18 years, 11 days, and 8 hours when an eclipse appears to repeat itself. One Saros cycle is equal to 223 synodic months, which is the time it takes for the Moon to return to the same phase as seen from Earth. They also discovered, although they may not have fully understood it, the geometry that enables eclipses to occur.
The path we follow around the Sun is called the ecliptic. Our planet’s axis is inclined to the ecliptic plane, which is why we have seasons, and why the other celestial bodies seem to cross the same general path in our sky.
As the Moon orbits the Earth, it, too, crosses the plane of the ecliptic twice a year. The ascending node is where the Moon moves into the northern ecliptic. The descending node is where the Moon enters the southern ecliptic. When the Moon crosses a node, a total solar eclipse can occur. The ancient astronomers were aware of these points in the sky, and by the time of the Babylonian civilization, they were very good at predicting when an eclipse would occur.
Two and a half millennia later, in 2016, astronomers used these same ancient records to measure the change in the rate at which the Earth’s rotation is slowing down—that is, how much days stretching, over thousands of years.
By the middle of the 19thth century, scientific discoveries came at a frenetic pace, and eclipses empowered many of them. In October 1868, two astronomers, Pierre Jules César Janssen and Joseph Norman Lockyer, measured the colors of sunlight separately during a total eclipse. Each of them found evidence of an unknown element, which revealed a new discovery: Helium, named for the Greek god of the Sun. In another eclipse in 1869, astronomers found convincing evidence of another new element, which they nicknamed the corona—before learning a few years later that it was not a new element, but highly ionized iron, which which indicates that the Sun’s atmosphere is exceptionally, extremely hot. This oddity led to the prediction, in the 1950s, of a continuous outflow that we now call the solar wind.
And during solar eclipses between 1878 and 1908, astronomers searched in vain for a proposed additional planet within Mercury’s orbit. Tentatively named Vulcan, this planet was thought to exist because Newtonian gravity could not fully describe Mercury’s peculiar orbit. The matter of the path of the innermost planet was finally settled in 1915, when Einstein used the equations of general relativity to explain it.
The purpose of many eclipse trips was to learn something new, or to prove an idea right or wrong. But many of these discoveries have great practical effects on us. Understanding the Sun, and why its atmosphere gets so hot, can help us predict solar flares that could disrupt the power grid and communications satellites. By understanding gravity, at all scales, we can learn about and navigate the cosmos.
GPS satellites, for example, provide precise measurements down to an inch on Earth. The equations of relativity and the effects of Earth’s gravity represent the distances between the satellites and their receivers on the ground. Due to special relativity, clocks on satellites, which have a weaker center of gravity, appear to run slower than clocks under the stronger force of gravity on Earth. From a satellite perspective, the Earth’s clocks seem to run faster. We can use different satellites in different positions, and different ground stations, to accurately triangulate our positions on Earth down to an inch. Without those calculations, GPS satellites would be less accurate.
This year, scientists across North America and in the skies above will continue the legacy of eclipse science. Scientists from NASA and several other universities and research institutes will study the Earth’s atmosphere; the atmosphere of the Sun; the Sun’s magnetic fields; and outbursts of the Sun’s atmosphere, known as coronal eruptions.
When you look up at the Sun and the Moon on the eclipse, the day of the Moon – or just watching its shadow darken the earth under the clouds, which is more likely than not – think of all the discoveries that remain waiting to happen, just behind the shadow of. the moon.
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