In August, a total solar eclipse will traverse Ameica for the first time in nearly a century. So many tourists are expected to flood states along the eclipse’s path that authorities are concerned about illegal camping, wildfire risks and even devastating porta-potties shortages. There’s a reason for all this eclipse mania. A total solar eclipse—when the moon passes between the sun and the Earth—is a stunning natural event. For a few breathtaking minutes, day turns to night; the skies darken; the air chills. Stars may even appear.

As awe-inspiring as an eclipse can be, it can also evoke a peculiar fear and unease. It doesn’t seem to matter that science has reassured us that eclipses present no real dangers (aside from looking straight into the sun, of course): When that familiar, fiery orb suddenly winks out, leaving you in an eerie mid-day darkness, apprehension begins to creep in.

So it’s perhaps not surprising that there’s a long history of cultures thinking of eclipses as omens that portend significant, usually bad happenings. The hair-raising sense that something is “off” during these natural events has inspired a wealth of myths and rituals intended to protect people from supposed evils. At the same time, eclipse anxiety has also contributed to a deeper scientific understanding of the intricate workings of the universe—and even laid the foundation for modern astronomy.

The idea of eclipses as omens stems from a belief that the heavens and the Earth are intimately connected. An eclipse falls outside of the daily rhythms of the sky, which has long been seen as a sign that the universe is swinging out of balance. “When anything extraordinary happens in nature ... it stimulates a discussion about instability in the universe,” says astronomer and anthropologist Anthony Aveni, author of In the Shadow of the Moon: The Science, Magic, and Mystery of Solar Eclipses. Even the biblical story of Jesus connects Christ’s birth and death with celestial events: the first by the appearance of a star, the second by a solar eclipse.

Because eclipses were considered by ancient civilizations to be of such grave significance, it was of utmost importance to learn how to predict them accurately. That meant avidly monitoring the movements of the sun, moon and stars, keeping track of unusual celestial events and using them to craft and refine calendars. From these records, many groups—the Babylonians, the Greek, the Chinese, the Maya and others—began to tease out patterns that could be used to foretell when these events occurred.

The Babylonians were among the first to reliably predict when an eclipse would take place. By the eighth century B.C., Babylonian astronomers had a firm grasp of the pattern later dubbed the Saros cycle: a period of 6,585.3 days (18 years, 11 days, 8 hours) in which sets of eclipses repeat. While the cycle applies to both lunar and solar eclipses, notes John Dvorak, author of the book Mask of the Sun: The Science, History and Forgotten Lore of Eclipses, it’s likely they could only reliably predict lunar eclipses, which are visible to half of the planet each time they occur. Solar eclipses, by contrast, cast a narrow shadow, making it much rarer to see the event multiple times at any one place.

Babylonians believed that an eclipse foretold the death of their ruler, leading them to use these predictions to put kingly protections in place. During the period of time that lunar or solar eclipses might strike, the king would be replaced with a substitute. This faux ruler would be dressed and fed like royalty—but only for a brief time. According to ancient Babylonian astronomers’ inscriptions on cuneiform tablets, “the man who was given as the king’s substitute shall die and … the bad omens will not affect that [ki]ng.”

The Babylonian predictions, though accurate, were all based purely on observations, says Dvorak; as far as scholars know, they never understood or sought to understand the mechanism behind planetary motions. “It was all done on the basis of cycles,” he says. It wasn’t until 1687, when Isaac Newton published the theory of universal gravitation—which drew heavily on insights from Greek astronomers—that scientists began to truly grasp the idea of planetary motion.

Surviving records from the ancient Chinese make up the longest continuous account of celestial happenings. Beginning around the 16th century B.C., Chinese star-gazers attempted to read the skies and foretell natural events using oracle bones. Ancient diviners would carve questions on these fragments of tortoise shell or oxen bone, and then heat them till they cracked. Similar to the tradition of reading tea leaves, they would then seek divine answers among the spidery network of fractures.

These methods may not have been scientific, but they did have cultural value. The sun was one of the imperial symbols representing the emperor, so a solar eclipse was seen as warning. When an eclipse was foretold to be approaching, the emperor would prepare himself by eating vegetarian meals and performing sun-rescuing rituals, while the Chinese people would bang pots and drums to scare off the celestial dragon that was said to devour the sun. This long-lived ritual is still part of Chinese lore today.

As far as accurate astronomical prediction, it would be centuries until Chinese predictions improved. By the first century AD they were predicting eclipses with fair accuracy using what is known as the Tritos cycle: a period of eclipse repetition that falls one month short of 11 years. Historians debate how exactly each culture developed its own system of eclipse prediction, says Dvorak, but the similarities in their systems suggest that Babylonian knowledge may have contributed to the development of others. As he writes in Mask of the Sun, “what the Babylonians knew about eclipses was diffused widely. It moved into India and China and then into Japan.”

In ancient India, legend had it that a mythical demon named Swarbhanu once attempted to outsmart the gods, and obtain an elixir to make himself immortal. Everything was going to plan, but after Swarbhanu had already received several drops of the brew, the sun and moon gods recognized the trick and told the supreme god Vishnu, who had taken the form of a beautiful maiden Mohini. Enraged, she beheaded Swarbhanu. But since the beast had already become immortal, its head lived on as Rahu and its torso as Ketu.

Today, according to the legend, Rahu and Ketu continue to chase the Sun and the Moon for revenge and occasionally gulp them down. But because Swarbhanu’s body is no longer whole, the eclipse is only temporary; the moon slides down his throat and resumes its place in the sky.

Eclipses in India were seen as a time when the gods were in trouble, says Dvorak, and to counter these omens land owners donated land to temples and priests. Along with the sun, moon and five brightest planets, they tracked Rahu and Ketu’s movement through the sky. In 499 AD, Indian mathematician and astronomer Aryabhata included these two immortal beings, dubbed “dark planets,” in his accurate description of how eclipses occur. His geometric formulation showed that the beasts actually represent two lunar nodes: positions in the sky in which the paths of sun and moon cross to produce a lunar or solar eclipse.

“They followed the nine wanderers up in the sky, two of them invisible,” says Dvorak. “From that, it was not a big step to predicting lunar eclipses.” By the sixth century A.D.—whether through independent invention, or thanks to help from the Babylonians—the Indians were successfully predicting eclipses.

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Eclipse fears aren't just limited to ancient times. Even in the modern era, those seeking signs of Earthly meaning in the movements of the heavens have managed to find them. Astrologists note that Princess Diana’s fatal car crash occurred in the same year as a solar eclipse. An eclipse darkened England two days before the British King Henry I departed for Normandy; he never graced England’s shores again. In 1918, the last time an eclipse swept from coast-to-coast across the United States, an outbreak of influenza killed up to 50 million people worldwide and proved one of the deadliest pandemics in history.

Of course, there is no scientific evidence that the eclipse had anything to do with the outbreak, nor the other events. Thousands of people are born and die every day—and solar and lunar eclipses are far from rare. In any given year, up to four solar and three lunar eclipses darken the surface of the Earth. Because of this, as Dvorak writes, “it would be surprising if there were no examples of monarchs dying on or close to days of eclipses.”

In their time, ancient Babylonians weren’t trying to create the foundation of modern mathematics. But in order to predict celestial events—and thus, from their perspective, better understand earthly happenings—they developed keen mathematical skills and an extensive set of detailed records of the cosmos. These insights were later adopted and expanded upon by the Greeks, who used them to make a lasting mark on geometry and astronomy as we know it. Today, astronomers still use these extensive databases of ancient eclipses from Babylon, China and India to better understand Earth's movements through the ages.

So if you feel a little uneasy when the sun goes dark on August 21st, you’re not alone. Just remember: It was this same unease that helped create modern astronomy as we know it.