To understand why Eddington and Dyson traveled such distance to watch the eclipse, we need to talk about gravity.
At least since the days of Isaac Newton, who wrote in 1687, scientists have thought that gravity is simply a force of mutual attraction. Newton proposed that every object in the universe attracts all objects in the universe, and that this gravitational force is related to the size of the objects and the distances between them. This is actually mostly true, but it’s a little more nuanced than that.
On much larger scales, such as black holes or galaxy clusters, Newtonian gravity falls short. It also cannot accurately explain the motion of large objects close together, such as how Mercury’s orbit is affected by its proximity to the Sun.
Albert Einstein’s most important invention solved these problems. General relativity holds that gravity is actually a distortion, not an invisible force of mutual attraction. Large objects such as the Sun and other stars react relative to each other due to changes in the space in which they exist, rather than 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
It was a strange concept, and many scientists thought Einstein’s ideas and equations were nonsense. But others thought it seemed reasonable. Einstein and others knew that if the theory was correct and the fabric of reality curved around large objects, light itself must follow that bend. For example, the light of a very distant star may appear to bend around a large object in front of it, such as our Sun, that is closer to us. However, it is normally impossible to examine stars behind the Sun to measure this effect. Enter an eclipse.
Einstein’s theory gives an equation for how much the Sun’s gravity can alter the appearance of background stars. Newton’s theory predicts only half this amount of displacement.
Eddington and Dyson measured the Hyades cluster as it contained many stars; The more stars that are distorted, the better the comparison. Both teams of scientists encountered strange political and natural obstacles in making the discovery, and these are beautifully described in the book. No Room for Doubt: The 1919 Eclipse That Confirmed Einstein’s Theory of RelativityBy physicist Daniel Kennefick. But the confirmation of Einstein’s ideas was worth it. Eddington said as much in a letter to his mother: “The only good plate I measured gave a result that agreed with Einstein,” he wrote, “and I think I got a little confirmation from a second plate.”
The Eddington-Dyson experiments were not the first experiments in which scientists used eclipses to make new and profound discoveries. This 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 astrologers, actually, but the goal was the same) were able to predict both lunar and solar eclipses with impressive accuracy. They calculated what we now call the Saros Cycle, a recurring period of 18 years, 11 days and 8 hours in which eclipses appear to recur. One Saros cycle equals 223 synodic months; this 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 makes eclipses happen, although they didn’t fully understand them.
The path we follow around the sun is called the ecliptic. Our planet’s axis is tilted relative to the ecliptic plane, which is why we have seasons, and why other celestial bodies appear to cross the same general path across our sky.
As the Moon orbits the Earth, it crosses the ecliptic twice a year. The ascending node is where the Moon moves towards 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 were aware of these spots in the sky and were very good at predicting when eclipses would occur at the height of Babylonian civilization.
Two and a half millennia later, in 2016, astronomers used the same old records to measure the change in the rate at which Earth’s rotation slows down, the amount by which days lengthen over thousands of years.
mid 19’sThis Scientific discoveries occurred at a frenetic pace in the 19th century, and eclipses powered much of it. In October 1868, two astronomers, Pierre Jules César Janssen and Joseph Norman Lockyer, separately measured the colors of sunlight during a total eclipse. Each found evidence of an unknown element that pointed to a new discovery: Helium, named after the Greek god of the Sun. At another eclipse in 1869, astronomers found convincing evidence of another new element, which they named coronium; A few decades later they learned that it was not a new element but highly ionized iron; This showed that the Sun’s atmosphere was extraordinarily, strangely hot. This peculiarity 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 extra planet orbiting Mercury. This planet, tentatively named Vulcan, was thought to exist because Newtonian gravity could not accurately describe Mercury’s strange orbit. The issue of the path of the innermost planet was finally resolved in 1915, when Einstein used the equations of general relativity to explain it.
Most eclipse expeditions were aimed at learning something new or proving an idea right or wrong. But many of these discoveries have major practical implications for us. Understanding the Sun and why its atmosphere is getting so hot could help us predict solar flares that could disrupt the power grid and communications satellites. Understanding gravity at every scale allows us to know and manipulate the universe.
GPS satellites, for example, provide precise measurements down to inches on Earth. Relativity equations take into account the effects of Earth’s gravity and the distances between satellites and their receivers on the ground. Special relativity suggests that clocks on satellites subjected to weaker gravity run slower than clocks on Earth with stronger gravitational forces. From a satellite perspective, Earth clocks appear to be running faster. We can use different satellites and different ground stations in different locations to triangulate our positions on Earth accurately down to centimeters. Without these calculations, GPS satellites would be much less accurate.
This year, scientists have spread out across North America and will continue the legacy of eclipse science in the skies above. Scientists from NASA, various universities, and other research institutions will study the Earth’s atmosphere; The Sun’s atmosphere; Magnetic fields of the Sun; and the Sun’s atmospheric explosions, called coronal mass ejections.
When you look at the Sun and Moon during the eclipse, the day of the Moon—or simply observe its shadow darkening the ground beneath the clouds, which seems more likely—think of all the discoveries yet to be realized, just behind the Moon’s shadow. moon.
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