To understand why Eddington and Dyson traveled such distances to view the solar eclipse, we need to talk about gravity.
Ever since the time of Isaac Newton, writing in 1687, scientists have thought that gravity was simply a 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 distances between them. This is largely true, but it is a bit 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 that are close together, such as how Mercury’s orbit is affected by its proximity to the Sun.
Albert Einstein’s most consequential breakthrough solved these problems. According to general relativity, gravity is not really an invisible force of mutual attraction, but a distortion. Instead of some sort of tug-of-war, large objects like the sun and other stars interact with 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 them.
Read more: 10 surprising facts about the 2024 solar eclipse
This was a strange concept, and many scientists found Einstein’s ideas and equations ridiculous. But others thought it sounded reasonable. Einstein and others knew that if the theory was correct, and the fabric of reality curves around large objects, light itself should follow that curve. For example, light from a far-away star appears to bend around a large object in front of it, closer to us – like our sun. But normally it is impossible to study stars behind the sun to measure this effect. Enter an eclipse.
Einstein’s theory provides an equation for how much the sun’s gravity would displace the images of background stars. 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 to distort, the better the comparison. Both teams of scientists encountered strange political and natural obstacles in making the discovery, which are beautifully described in the book No doubt: the solar eclipse of 1919 confirmed Einstein’s theory of relativity, by physicist Daniel Kennefick. But the confirmation of Einstein’s ideas was worth it. Eddington said this in a letter to his mother: “The one good plate I measured gave a result consistent with Einstein’s,” he wrote, “and I think I have had a little confirmation from a second plate.”
The Eddington-Dyson experiments were hardly the first time scientists used eclipses 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 rather astrologers, but the goal was the same – were able to predict both lunar and solar eclipses with impressive accuracy. They came up with what we now call the Saros cycle, a repeating period of 18 years, 11 days and 8 hours during which eclipses appear to repeat. 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 makes eclipses possible.
The path we follow around the sun is called the ecliptic. Our planet’s axis is tilted relative to the ecliptic plane. That’s why we have seasons and the other celestial bodies appear to cross the same general path in our sky.
As the moon orbits the Earth, it too passes the plane of 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. When the moon crosses a node, a total solar eclipse can occur. Ancient astronomers were aware of these points in the sky, and at the height of Babylonian civilization they were very good at predicting when eclipses would occur.
Two and a half millenniums later, in 2016, astronomers used the same old data to measure the change in the rate at which Earth’s rotation is slowing – that is, the amount by which the days are getting longer, over thousands of years.
Mid 19e century, scientific discoveries were made at a breakneck pace, and many of these discoveries were driven by eclipses. In October 1868, two astronomers, Pierre Jules César Janssen and Joseph Norman Lockyer, separately measured the colors of sunlight during a total solar eclipse. Each found evidence of an unknown element, indicating a new discovery: Helium, named after the Greek god of the sun. During another solar eclipse in 1869, astronomers found convincing evidence of another new element, which they nicknamed coronium. A few decades later they discovered that it was not a new element, but highly ionized iron, indicating that the sun’s atmosphere is exceptionally, bizarrely 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. This planet, tentatively named Vulcan, was believed to exist because Newtonian gravity could not fully describe Mercury’s strange orbit. The question of the inner planet’s path was finally resolved in 1915, when Einstein used equations from general relativity to explain it.
Many eclipse expeditions were intended to learn something new, or to prove an idea right or wrong. But many of these discoveries have major practical consequences for us. Understanding the sun and why its atmosphere gets so hot can help us predict solar eruptions that could disrupt the power grid and communications satellites. Understanding gravity, at all scales, allows us to know and navigate the cosmos.
For example, GPS satellites provide accurate measurements down to a few centimeters on Earth. Relativity equations take into account the effects of Earth’s gravity and the distances between the satellites and their receivers on the ground. The special theory of relativity holds that the clocks on satellites, which experience weaker gravity, appear to run slower than clocks under the stronger gravity on Earth. From the satellite’s perspective, the Earth’s clocks appear to be running faster. We can use different satellites in different positions and different ground stations to triangulate our positions on Earth accurately to within a few centimeters. Without these calculations, GPS satellites would be much less accurate.
This year, scientists spread across North America and in the skies above the legacy of eclipse science will continue. Scientists from NASA and several universities and other research institutions will study Earth’s atmosphere; the atmosphere of the sun; the sun’s magnetic fields; and the Sun’s atmospheric outbursts, called coronal mass ejections.
When you look up at the sun and moon during the solar eclipse, moon day – or simply see how the shadow darkens the ground beneath the clouds, which seems more likely – think of all the discoveries yet to be made, just behind the shadow of the moon. the moon.
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