Imagine: you are a man of antiquity, some kind of Neanderthal, and your faithful sun has suddenly and unexpectedly darkened. You are scared. You think, “What if it never comes back? How have we angered God … ancestors? Oh, here it came back. Carried away. But then, years later, it repeats itself. You lose faith in the consistency of the sun and start writing down when these events occur. Centuries pass, and finally a picture emerges through which early civilizations could predict when these strange events would occur.
“The very idea that this is not an accident is incredible,” says Jonathan Seitz, associate professor of history at Drexel. “The Mesopotamians were the first to understand this because they had a habit of writing down everything. They did it because they felt it made sense - that it was not just random natural phenomena."
Thanks to the records that began to be kept back in 700 BC. BC, the Mesopotamians were able to determine the length of the Saros cycle - the interval between when the Moon, Earth and Sun line up for an eclipse. The cycle occurs once every 18 years, 10 days (11 in leap years) and 8 hours, along with it the shadow on the Earth changes. That extra eight hours means that the position of the eclipse changes over time as the Earth rotates.
Although ancient astronomers could not observe all iterations of the Saros cycle (eclipses can occur in the middle of the oceans or uninhabited regions), they were able to quite clearly determine the time intervals when an eclipse could occur. At this point in the story, they just knew when it might happen. Why and how - more on that later.
Greek life
Fast forward to Ancient Greece. For thinkers like Aristotle and others, it was not enough to know that something was happening. It was equally important to know why this is happening. “The Greeks were very interested in causality,” says Seitz. The significance of the eclipse was less important than other factors. "For them, you didn't understand something until you could explain it."
Greek observations helped to figure out how the planets move and that the Earth has a sphere shape. Without telescopes, they still thought of the moon as a luminous celestial body, unlike our solid home, but they already determined its motion relative to the Earth. And although they thought that the Earth was the center of the universe, they realized that an eclipse is the shadow of a new moon thrown by the Sun on the Earth.
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The methods developed by Aristotle and Ptolemy for understanding eclipses were used until Copernicus and Newton took the stage hundreds of years later.
“But that doesn't mean that nothing has happened in the intervening time,” adds Seitz. People accumulated knowledge of ancient cultures, accumulated knowledge and began to improve methods in the Middle Ages. “In the Islamic world, in particular, they paid a lot of attention to astronomy and astrology, developed astrolabes, built angles in the heavens and tried to improve the system,” says Seitz.
Later, thinkers like Tycho Brahe built giant quadrants to make more accurate measurements of the Sun's motion during eclipses, and some used the eclipse measurement methods we still use today. “They used pinhole cameras in the Middle Ages to measure the strength of an eclipse,” says Seitz.
Europe, of course, was not the only place where eclipses were seen. In China, their own predictions of eclipses appeared almost at the same time as those of the people of the Mediterranean, and at the same time, eclipse schemes were discovered, thanks to long annals. There is evidence that the Maya followed eclipses in their own way, but almost all of their records were brutally destroyed by the conquistadors during the European invasion of America.
Despite a good understanding of eclipses, most cultures considered them to be bad omens. Interpretations (slowly) began to change with the advent of telescopes, which showed the topography of the moon and made it possible to predict eclipses more accurately. In fact, in the 1700s, astronomer Edmund Halley made a map of future eclipses and published it in the hopes that the general public would not panic when the Sun disappeared briefly and that observers could gather more data on how long an eclipse would last in different locations. The modern era of eclipse observation has finally begun.
Nowadays
“The method we are using now is based on what humans came up with in the 19th century,” says Ernie Wright, an imaging expert at NASA. The people who started using relatively modern calculation methods to predict eclipses were Friedrich Bessel and William Chauvenet.
"Bessel invented the basic mathematics we use in 1820, and Chauvinet put it in modern form in 1855."
Today we can get even more specific information thanks to our understanding of the shape of the moon. The moon - contrary to all the elementary school drawings you pored over - does not have the shape of a banana or a perfect sphere. Like the Earth, the Moon has mountains and plains, because of which its shape is slightly rough at the edges, which means that the surface itself is laid out unevenly.
“19th century methods suggested that the moon was smooth and that all observers were at sea level,” Wright says. "Such simplifications have to be done if you do the calculations with a pencil on paper."
From the late 1940s to 1963, an astronomer named Charles Burleigh Watts spent countless hours mapping the variations that appeared on the moon's surface and observing the landforms that appeared on the outer edge of the moon as seen from Earth. His detailed maps helped predict eclipses even more accurately. But the shadow of the eclipse was, as it turned out, not an oval, but a polyhedral polygon, in which each angle corresponded to a valley on the body of the Moon.
Then NASA got down to business. The agency's lunar reconnaissance orbiter, based on Watt's work, has detailed the topography of the moon that would have been impossible to compose from images taken on earth.
Wright took this data about the shape of the moon, the topography of the earth, and the positions of the sun, moon, and earth to create an incredibly detailed and accurate picture of where the shadow of an eclipse would fall in the United States.
This eclipse was the most watched total eclipse in history. And after humanity has spent thousands of years observing and recording eclipses, there are still many things that scientists hope to figure out.
“We recently started talking about not knowing the exact size of the sun,” says Wright. “It turned out that eclipses are an extremely sensitive method for measuring the radius of the Sun. The radius of the Sun is about 696,000 kilometers. But if you change it by 125 kilometers, the duration of the total eclipse will also change by a whole second.
Today, when people have the opportunity to accurately observe how the shadow of the eclipse crosses the earth, it is worth thanking all those generations of people who made this possible; from observers who did not know what was happening, who lived for hundreds of years, to people who built modern satellites and made accurate maps of eclipses.
Ilya Khel
