COn the afternoon of May 29, 1919, a roar echoed across the small volcanic island of Principe off the west coast of Africa. Arthur Eddington, director of the Cambridge Observatory in England, was waiting for the sun to come out. Remnants of a morning thunderstorm can ruin everything.
The island was about to witness the rare and overwhelming sight of a total solar eclipse. The six-minute solar eclipse, the longest since 1416, saw the moon completely block the face of the sun, casting a curtain of darkness over a thin strip of Earth. Eddington traveled the path of eclipse, trying to prove Albert Einstein's new theory of general relativity, one of the most important ideas of his time.
Eddington, a physicist, was one of the few people at the time who understood this theory, which Einstein proposed in 1915. But many other scientists were hampered by the strange idea that gravity is not a mutual attraction, but a distortion of space-time. Light itself is also affected by this distortion. Therefore, a solar eclipse will be the best way to prove whether this theory is correct. Because when the sun's light is blocked by the moon, astronomers can see whether the sun's gravity bends the light of distant stars behind it.
In March 1919, two teams of astronomers boarded a ship departing from Liverpool, England, to observe a solar eclipse and take measurements of the stars. Eddington and his team went to Principe, and another team led by Frank Dyson of the Greenwich Observatory went to Sobral, Brazil.
Total eclipse, or complete occlusion of the sun, will occur at 2:13 local time in Principe. The clouds finally started to part just before the moon slipped in front of the sun. For a moment, it was completely clear. Eddington and his group quickly snapped images of a star cluster in the constellation Taurus called the Hyades, which had been discovered near the sun that day. Astronomers were using the best astronomical technology of the time: photographic plates. Photographic plates were photographed with large exposures on glass instead of film. Stars appeared on seven of the plates, and solar “prominences” – filaments of gas flowing from the Sun – appeared on the other plates.
Eddington wanted to stay on Principe to measure the Hyades when there was no solar eclipse, but a shipworkers' strike forced him to leave early. Eddington and Dyson then compared the glass panes taken during the eclipse with other glass panes that had photographed the Hyades star cluster in different parts of the sky when there was no solar eclipse. In images from Eddington and Dyson's expedition, the stars were not aligned. Einstein, 40 years old, was right.
When the scientific paper was published, the New York Times declared, “All the lights in the sky are slanted.” Solar eclipses were key to discoveries, as so many before and since have revealed new discoveries about our universe.
To understand why Eddington and Dyson traveled so far to see the eclipse, we need to talk about gravity.
At least since the time of Isaac Newton, who wrote in 1687, scientists have believed that gravity is 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. In fact, this is mostly true, but it's a little more nuanced than that.
At larger scales, such as black holes or galaxy clusters, Newtonian gravity is insufficient. It also cannot accurately explain the movements of large celestial bodies that are close to each other, such as how Mercury's orbit is affected by its proximity to the Sun.
Albert Einstein's most important advances solved these problems. General relativity holds that gravity is actually a distortion rather than an invisible force that attracts each other. Large objects like the Sun and other stars react relative to each other because the space in which they exist is changing, rather than in some kind of mutual tug-of-war. Their mass is so great that they bend the fabric of space and time around themselves.
Read more: 10 surprising facts about the 2024 solar eclipse
This was a strange concept, and many scientists thought Einstein's ideas and equations were ridiculous. But others thought it was reasonable. Einstein and his colleagues knew that if their theory is correct and the structure of reality is bent around large objects, then light itself must follow that bend. For example, the light from a distant star will appear to curve around a large object in front of it that is closer to us, such as the Sun. But it is usually impossible to study stars behind the Sun to measure this effect. Enter solar eclipse.
Einstein's theory gives an equation for how much the Sun's gravity displaces the image of a background star. Newton's theory predicts only half that amount of displacement.
Eddington and Dyson measured the Hyades cluster because it contains many stars. The more stars you distort, the better the comparison will be.Both teams of scientists encountered strange political and natural obstacles in making this discovery, which are beautifully documented in the book There's no doubt: the 1919 solar eclipse confirmed Einstein's theory of relativity, by physicist Daniel Kenefick. But it was worth confirming Einstein's ideas. Eddington made a similar statement in a letter to his mother. “His one good plate that I measured gave results consistent with Einstein, and I think his second plate gives us a little bit of confirmation.”
Eddington and Dyson's experiment is not the first time scientists have used a solar eclipse to make profound new discoveries. This idea goes back to the beginning of human civilization.
The careful recording of lunar and solar eclipses is one of ancient Babylon's greatest legacies. Astronomers, actually astrologers, had the same purpose. We were able to predict both lunar and solar eclipses with amazing accuracy. They figured out what we now call the Saros cycle, a cycle of 18 years, 11 days, and 8 hours in which solar eclipses appear to repeat. His one cycle of Saros is equivalent to 223 lunar months, which is the time it takes for the moon to return to the same phase as seen from Earth. They also figured out the geometry that allows solar eclipses to occur, although they may not have fully understood them.
The path we follow around the sun is called the ecliptic. Our planet's axis is tilted relative to the ecliptic plane, and this is why we have seasons and why other celestial bodies appear to traverse the same general path in our sky.
As the moon orbits the Earth, it also crosses the ecliptic plane twice a year. The ascending node is where the moon enters the northern ecliptic. The descending node is where the moon enters the southern ecliptic. A total solar eclipse can occur when the moon passes through a node. Ancient astronomers noticed these points in the sky, and at the height of Babylonian civilization they were very good at predicting when solar eclipses would occur.
2,500 years later, in 2016, astronomers used these same ancient records to measure changes in the slowing rate of Earth's rotation over thousands of years, or how much a day lengthens.
By mid 2019th The 20th century saw an onslaught of scientific discoveries, many of which were powered by solar eclipses. In October 1868, two astronomers, Pierre Jules César Jansen and Joseph Norman Lockyer, independently measured the color of sunlight during a total solar eclipse. Each discovered evidence of unknown elements and presented new discoveries. Helium is named after the Greek sun god. In another solar eclipse in 1869, astronomers found convincing evidence of another new element, which they named coronium, but decades later they discovered that it was not a new element but highly ionized iron. , we learned that the sun's atmosphere was showing an unusually high temperature. This oddity led to predictions in the 1950s of a continuous outflow of what we now call the solar wind.
And during the 1878-1908 solar eclipse, astronomers searched in vain for another proposed planet within Mercury's orbit. The planet, tentatively named Vulcan, was thought to exist because Newtonian gravity could not fully explain Mercury's strange orbit. The problem of the innermost planet's path was finally solved in 1915 when Einstein explained it using the general theory of relativity.
Many eclipse expeditions were aimed at learning something new or proving an idea right or wrong. However, many of these discoveries have major practical implications for us. Understanding why the sun and its atmosphere get so hot can help predict solar outbursts that can disrupt power grids and communications satellites. Understanding gravity at all scales allows us to know and navigate the universe.
For example, GPS satellites provide precise measurements to the nearest inch on Earth. The theory of relativity takes into account the effects of Earth's gravity and the distance between the satellite and the receiver on the ground. According to the special theory of relativity, clocks on satellites with weaker gravity will be slower than clocks on Earth, where gravity is stronger. When viewed from a satellite, Earth's clock appears to be running faster. We can accurately triangulate our position on Earth to the inch by using different satellites and different ground stations in different positions. Without these calculations, GPS satellites would be much less accurate.
This year, scientists will continue the legacy of eclipse science in the skies across North America. Scientists from NASA, several universities, and other research institutions will study Earth's atmosphere. atmosphere of the sun. Sun's magnetic field. and an explosion of the Sun's atmosphere called a coronal mass ejection.
When you look up at the sun and moon on a solar eclipse or lunar day, or when you just observe its shadow darkening the ground under a cloud, it seems more likely that it is just behind the sun's shadow. And think about all the discoveries that are still waiting to happen. Moon.