Cameron Gibelyou, BHP Science Advisor
Michigan, USA

As many of us (re-)discover whenever we assign our students a Little Big History project, nearly anything can get you thinking about the history of everything. Eclipses, an astronomical phenomenon in which the Moon’s shadow falls on the Earth (solar eclipse) or the Earth’s shadow falls on the Moon (lunar eclipse), are no exception. Contemplating the upcoming solar eclipse of August 21, 2017, can get us thinking about a broad span of human history, as well as the history of life, the Earth, and the Solar System.

What Happens in a Solar Eclipse?

During a solar eclipse, the Earth, Moon, and Sun align in such a way that the Moon appears to block out part or all of the Sun, depending on precisely where you are on Earth. If you are in the “path of totality,” where the darkest part of the Moon’s shadow (the umbra) hits, you will—weather permitting—experience a total eclipse: the Moon will appear to move in front of the Sun, the sky will darken, the temperature will drop, birds will stop chirping, and you’ll be able to see the Sun’s corona, a breathtaking sight.

If we want to understand how people have thought about eclipses historically, however, perhaps we need to consider the most important property of a total solar eclipse, which is that it is really freaky (the technical scientific term, haha). If you have experienced one, you can probably empathize with the feeling that something profoundly “unnatural” is going on. A total solar eclipse tends to get people in touch with a primal wonder, a kind of reverential awe. You are literally in the shadow of the Moon!

Eclipses: Portents of Doom?

Many peoples have thought of solar and lunar eclipses as bad omens or as portents of doom. Even cultures that could predict eclipses—ones that knew ahead of time when they might be coming—often thought this way, or worked hard to predict eclipses so as to make sure that they did whatever they thought necessary to guarantee the normal cosmic order would be restored.

For example, the Maya civilization of Mesoamerica generated sophisticated astronomical knowledge, including eclipse predictions of considerable accuracy. Even though they had such knowledge of celestial phenomena, they still seem to have considered eclipses a bad omen: the Dresden Codex, a well-known Maya text, puts hieroglyphs representing misery, malevolence, and death next to a set of images representing an eclipse. The Maya appear to have been interested in predicting eclipses as a form of forewarning more than forecasting.

How Did People Initially Predict Eclipses?

The ability to predict eclipses, with at least some success, may go back several thousand years. Astronomer Gerald Hawkins claimed that Stonehenge was built to track various astronomical alignments, including the time periods when eclipses were likely to occur. Building on his work, astronomer Fred Hoyle (famous for coining the term “Big Bang” and explaining the origin of elements in stars) wrote of another way in which the “Aubrey holes” of Stonehenge could have been used to track eclipses. These suggestions have come under fairly heavy criticism from other scholars; the mechanisms proposed for eclipse prediction are rather complex and unwieldy. But the idea that eclipses could be at least partially anticipated 4,000 to 5,000 years ago cannot be totally ruled out.

Many civilizations kept written records of eclipse observations. Ancient Chinese and Babylonian astronomers kept sufficiently detailed records of eclipses that they eventually identified eclipse cycles. In particular, the Babylonians knew of the saros, an approximately 18-year cycle of repetition in the occurrence of eclipses. Identifying a cycle that repeats every 18 years requires decades of reliable observations! This detailed understanding of cycles, however, did not immediately demystify eclipses for the Babylonians, who had long attributed astrological significance to them. (The Chinese, meanwhile, traditionally spoke of a dragon devouring the Sun, and would make noise to scare it away. The Chinese Imperial Navy even fired their guns during an eclipse in the nineteenth century in accordance with this tradition, though Chinese astronomers had stopped attributing astrological significance to eclipses many centuries before.)*

Meanwhile, there is no existing record of the ancient Egyptians having made an explicit reference to an eclipse, despite the fact that a total solar eclipse would have happened approximately every 75 years somewhere in ancient Egypt. Given their tendency to revere and even worship the Sun, one might wonder whether they did not wish to acknowledge the darkening of it. But on the other hand, their imagery may also hold a symbolic reference to a solar eclipse: the image of the winged Sun in traditional Egyptian imagery is at least a little reminiscent of the Sun’s corona during an eclipse.

By the time of Ptolemy in the 100s CE, astronomers of the Mediterranean world understood fairly well the detailed motions of the Moon and Sun in the sky; they could predict the timing of eclipse seasons, periods when the Moon’s path takes it straight between the Earth and Sun (rather than above or below the direct line). The Antikythera mechanism is an example of a mechanical model of the Solar System that could have been used as an “eclipse calculator,” dating from approximately the second century BCE.

It is unclear just how far back the Greeks’ ability to predict eclipses goes. Herodotus reports that Thales of Miletus predicted the eclipse that (modern astronomers calculate) probably occurred on May 28, 585 BCE. This eclipse is significant primarily because, according to Herodotus, it stopped a battle between the warring Lydians and Medes, who were so disturbed by the sight of the Sun being “devoured” that they stopped fighting and made peace. (There is no indication that they were aware that someone had previously predicted the eclipse.) Later, in the Peloponnesian War, a lunar eclipse frightened Athenian soldiers and sailors, ultimately leading to a delay in their movements that proved fatal.

Even with an understanding of the motions that produce an eclipse, the phenomenon of an eclipse remains stirring and even terrifying, a byword for awesome or awful things happening. Charlemagne’s son, Emperor Louis the Pious, may have died in the aftermath of the terror he felt due to an eclipse on May 5, 840 CE. Fighting for his throne ended with the Treaty of Verdun in 843, which divided the Carolingian empire into three areas corresponding (very roughly) to modern France, Germany, and Italy. So an eclipse might have played some role, however indirect, in bringing about the Europe we know today.

Another story has Christopher Columbus, during his fourth voyage to the Americas in 1503, getting stranded in Jamaica for months, and using his knowledge of an impending lunar eclipse to intimidate the local inhabitants of Jamaica into providing him and his crew with more food, claiming that the Moon would be eclipsed just before it happened. (This story may have actually happened, or it may be fabricated. Obviously fictional versions of the same idea appear in various places, including Mark Twain’s A Connecticut Yankee in King Arthur’s Court.)

There are many literary references to the phenomenon of eclipses, including (in the Western tradition) Dante in the Paradiso and Milton in Paradise Lost. The first Christian sermon (recorded in Acts 2) appears to make reference to solar and lunar eclipses: “the Sun will be turned to darkness and the Moon to blood” – quoting the book of the prophet Joel, who seems to have used these phenomena as a poetic image of the “wonders” and “signs” that would accompany God’s vindication of the faithful. The apocalyptic vision of the book of Revelation (apocalypse in Greek and revelation in Latin both mean “unveiling”) makes use of similar imagery, the darkened Sun of a solar eclipse and the deep reddening of the Moon that occurs during a lunar eclipse (Revelation 6).

Eclipses: A Good Thing?

There are a few cultures in which eclipses are seen as a good thing, particularly among groups like the Tahitians, or like the Warlpiri people of the Australian Aborigines, for whom an eclipse involves an amorous encounter between Sun and Moon. We might add modern scientific cultures to the list of groups that consider eclipses a positive; amateur astronomers chase eclipses all over the world, and professional astronomers have mounted some seriously epic worldwide expeditions to observe eclipses. A wide variety of scientific knowledge has been gleaned from eclipses, which historically have allowed rare opportunities to study the Sun, including the structure of the corona. Samuel Williams went behind enemy lines during the American Revolution for a scientific expedition to view an eclipse in Maine. Helium was discovered in the Sun (helios is Greek for “Sun,” which is where the element got its name) during an eclipse in 1868; it was not discovered on Earth for another 27 years. Einstein’s theory of gravity, general relativity, was experimentally verified during an eclipse in 1919 by observing how the Sun’s gravity bent the path of light from distant stars in accordance with Einstein’s predictions.**

Eclipses can also be used to fix the exact dates of past events. The date of the battle in 585 BCE, mentioned earlier, is one of the first exact dates known; fixing this date is possible because the regularity of eclipses allows us to determine exactly when they happened in the past as well as predict when they will happen in the future.

Eclipses appear in all manner of ways in human history. What about the history of life, Earth, and the Universe?

How Do Eclipses Affect the Living World, and How Were They Different in Earth’s Past?

Eclipses affect the living world in various ways. During an eclipse, many flowers close as if for the night, temperatures drop, midday twilight emerges, stars and planets appear. Many animals adopt nighttime behaviors or become skittish. Birds stop singing. Cicadas go silent. Not everything reacts, however, and the occasional eclipse probably did not have any specific, concrete influence on how the history of life unfolded.

That said, the experience of an eclipse would not have been the same for organisms in the deep past as it is today. If, for example, a Stegosaurus 150 million years ago had checked out an eclipse, the Moon would have appeared bigger, and a total solar eclipse would (on average) have lasted longer.

At present, total eclipses last at most a few minutes, because the Moon is only just barely big enough to completely cover the Sun, and so only blocks out the entire disk of the Sun for a short while. In fact, some solar eclipses are not total eclipses but rather annular eclipses: if the Moon is near the farthest point in its orbit and the Earth is particularly close to the Sun in its orbit, the Sun can actually appear bigger than the Moon, so that the Sun cannot be completely blocked out.

But the Moon moves away from the Earth by almost 4 cm each year, so it used to be much closer to the Earth and appear bigger in the sky. Does this make anyone besides me think of this image?

 

 

Hence eclipses lasted longer, on average, in the past. Go far enough back in time and you would never be able to see the corona. On the other hand, go hundreds of millions of years into the future and total eclipses may no longer be possible at all. (Depending on exactly how long this takes to come about, the Sun may become a red giant and incinerate the Earth before then.)

Why Do Eclipses Happen Only Occasionally?

A question that might take us back even further in time, to the initial formation of the Solar System, is this: Why do eclipses happen only occasionally, rather than every month? It’s because the Moon’s orbit is tilted (by a little over 5 degrees). If the Moon orbited in the same plane that the Earth orbits the Sun, there would be solar and lunar eclipses every month; there would be a lunar eclipse at every full Moon and a solar eclipse at every new Moon. But the Moon’s shadow usually passes above or below the Earth, and the Earth’s shadow usually misses the Moon too. Why the tilt? No one knows for sure, but it may have been caused by the gravitational pull of large objects coming near the early Earth-Moon system, at a time before the Earth and Moon swept up the debris in the neighborhood of their path.

How Can Eclipses Get Us Thinking Even Bigger?

Thinking about eclipses can bring us back to the very first millions of years of our planet’s and Solar System’s existence. Eclipses of a different sort can broaden our vision in space and time even further, and get us thinking beyond our own Solar System. In studying extrasolar planets or exoplanets—planets around other stars—astronomers often seek to observe transits in which a planet passes in front of its host star, as seen from our vantage point. This kind of “partial eclipse” is a major way of discovering the existence and properties of extrasolar planets, a way used especially by the Kepler satellite.

Eclipses, and eclipse-like phenomena, can get us thinking about everything from the history and future of science, to human cultures and civilizations and literature, to the history of the Earth and Solar System. And pondering these connections may give you another window on Big History, as well as a new source of wonder at the eclipse that you might get to experience firsthand on August 21.

What Should I Know About Upcoming Eclipses?

The US will experience two total solar eclipses in the next seven years, one on August 21 of this year and one on April 8, 2024. If the idea of getting yourself into the path of totality appeals to you at all, go for it. Use the various online resources or Bryan Brewer’s excellent book Eclipse: History. Science. Awe. to figure out what to look for, and how to maximize your chances of getting a look at it. Make sure you are near the center of the path of totality, that you have an unobstructed view of the Sun at the appropriate time of day, that you have a good weather forecast, and that you can move around at the last minute if need be—there may be traffic. Keep safety in mind: use certified and approved eclipse glasses—people really do get their retinas fried and their eyesight permanently damaged during eclipses when they do not take the proper safeguards. (See the NASA site on safe eclipse viewing, or consult any of the other viewing/safety information resources listed below.) Get in contact with your local amateur astronomy group for help. If you want to photograph totality, bear in mind that you will need specialized equipment much more sophisticated than a cell-phone camera.

If you’re curious about how different peoples throughout human history have interpreted eclipses and the stories they have told about them, this Lunar and Planetary Institute article and this one from National Geographic are a solid start.

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*Note that when we talk about predicting eclipses in ancient societies, we are usually talking about lunar eclipses. Although solar eclipses are subject to the same cycles as lunar eclipses, including the saros, lunar eclipses are visible from almost anywhere on Earth where the Moon is visible at the time of the eclipse, while a given solar eclipse is visible only from a rather confined geographical area. (This is because the Earth’s shadow on the Moon is so much bigger than the Moon’s shadow on the Earth.) So predicting a solar eclipse is a much chancier business than predicting lunar eclipses, especially if you are confined to a small portion of Earth’s surface, as ancient skywatchers were.

**It’s also interesting to note in this connection that lunar eclipses were used in ancient and medieval times to show that the Earth is round, because the shadow of the Earth on the Moon is round. (Whatever you may have heard to the contrary, people knew a very long time before Columbus that the Earth is a sphere!)

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References

1. Bryan Brewer, 2017. Eclipse: History. Science. Awe, 3rd ed. Seattle: Earth View Inc.
2. Clive Ruggles, 2005. Ancient Astronomy: An Encyclopedia of Cosmologies and Myth. Santa Barbara: ABC-CLIO, Inc.
3. https://eclipse.gsfc.nasa.gov/SEhistory/SEhistory.html (note the disclaimer near the top—this page is a work in progress).
4. https://eclipse2017.nasa.gov/eclipse-history
5. https://science.ksc.nasa.gov/mirrors/gsfc/omni/eclipse99/pages/traditions_Calendars.html – Egypt
6. http://www.nature.com/nature/journal/v338/n6212/abs/338238a0.html
7. http://www.lpi.usra.edu/education/explore/eclipse/
8. http://news.nationalgeographic.com/news/2013/11/131101-solar-eclipse-myth-legend-space-science/journals.sagepub.com/doi/abs/10.1177/002182869802900305
9. https://www.jstor.org/stable/2742481?seq=18#page_scan_tab_contents
10. http://www.cs.virginia.edu/~robins/Decoding_an_Ancient_Computer.pdf
11. http://ircamera.as.arizona.edu/NatSci102/NatSci102/text/extmayaastronomy.htm
12. http://curious.astro.cornell.edu/about-us/37-our-solar-system/the-moon/the-moon-and-the-earth/111-is-the-moon-moving-away-from-the-earth-when-was-this-discovered-intermediate
13. https://www.wired.com/2008/05/may-28-585-bc-predicted-solar-eclipse-stops-battle/
14. https://www.eclipse-chasers.com/article/papers/wildlife01.html

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Viewing/Safety Information

• https://eclipse.gsfc.nasa.gov/SEhelp/safety.html
• https://eclipse2017.nasa.gov/safety
• https://www.space.com/33797-total-solar-eclipse-2017-guide.html
• https://eclipse2017.nasa.gov/nasas-eyes

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About the author: Cameron Gibelyou, PhD, is a teacher and educator based at the University of Michigan. He holds a doctorate in astrophysics and has taught Big History at high school and college levels, as well as college classes in physics, astronomy, psychology, English, and applied liberal arts. Since 2011, he has worked with the Big History Project as a developer of teaching materials, expert reviewer, and science advisor.