Solar Eclipses

It is a staggering coincidence—but a coincidence nonetheless—that, as seen from the surface of the earth, the apparent diameter of the sun is more or less the same as that of the moon. It is only because of this fluke of nature that total solar eclipses appear as spectacular as they do.

A total solar eclipse occurs when the moon passes directly in front of the sun, just covering its disc, but rendering visible the ethereal white light of the surrounding solar corona—the sun's outer atmosphere—and reducing the remainder of the sky to a dusk-like half-light. The same fluke of nature also ensures that total solar eclipses are rare, since if the alignment is not exactly right, then the disc of the sun will never be completely covered and the eclipse will only be partial. As long as only a small part of the sun's disc remains uncovered, its light will continue to light up the sky much as normal and the eclipse might easily pass unnoticed. The same is true of an annular eclipse, which occurs when the alignment is exact but the moon is slightly farther away from the earth than it is on average, so that its disc appears slightly smaller than usual; in this case, a thin ring of the sun's disc remains uncovered.

A total solar eclipse can be seen from somewhere on earth about once every year and a half on average, but each one is only visible from within a narrow track, typically about 160 kilometers (100 miles) in width, though stretching for several thousand kilometers across the surface of the planet. From a typical spot on earth, a total solar eclipse will only occur once every four hundred years on average. Fewer than one in four people in the past, lacking the benefit of modern scientific foreknowledge and travel possibilities, would have experienced a total solar eclipse during their lifetime.

Witnessing a total solar eclipse at first hand is an awe-inspiring experience. In a few moments the sun is transformed from something too bright to be directly visible into a black disc surrounded by a wispy white halo, and the remainder of the sky becomes as dark as early dawn or late dusk, with planets and bright stars visible. All around, nature reacts: the atmosphere cools, birds stop singing, and other animals are in confusion. Then, after no more than a few minutes, and equally rapidly, normality is restored. Even in the modern world, knowing what to expect, one is left with a profound sense of the power and immutability of nature. There can be little doubt that the psychological effect upon people and cultures in the past for whom such an event occurred entirely without warning—upsetting the rhythms of nature known and relied upon since time immemorial—was potentially devastating.

Despite this, hardly any eclipse sightings in prehistoric times have left a definite trace in the archaeological record. Most claims of depictions of eclipses in rock art are purely speculative. For example, on one panel in a large petroglyph field at Pu'u Loa on the Big Island of Hawai'i is a design consisting of a full circle with four adjacent crescents, increasing in size away from it. It has been suggested that these represent the stages of a solar eclipse, up to and including totality. However, the crescents vary in size rather than shape, as one would expect if they were literal depictions, and there are no symmetrical crescents on the other side of the circle, which one might expect to represent the phases after totality. Furthermore, the crescent sun cannot be viewed directly. It is more likely that these symbols have another meaning entirely. Similarly, a group of petroglyphs on a rock outcrop at Ekenburg in Sweden have been interpreted as a literal depiction of the sky during a total eclipse in 1596 b.c.e. However, this interpretation depends upon a number of unsupported assumptions. Given the likely impact of such an unexpected event on the people of the time, it seems unthinkable that, in less than three minutes, they could have recorded the appearance of the sky in enough detail to then reproduce it faithfully in stone.

Even in some of the great ancient empires and city-states, where literacy permitted detailed records to be kept of a variety of astronomical (and meteorological) phenomena, total solar eclipses could not be reliably predicted and remained greatly feared. In ancient China, for example, records of total eclipses of the sun go back at least to about 700 b.c.e., but even two complete millennia after this date there was no means to predict them. When they did happen, they were taken as particularly disastrous omens for the state, or for the Emperor himself. Babylonian astronomers were the first to discover that lunar eclipses tend to recur after certain fixed periods of time. This enabled them to predict lunar eclipses but not solar ones, which remained a sign of an impending calamity.

Across the world in pre-Columbian Mesoamerica, the Maya almanac subsequently known as the Dresden Codex contained a tabulated record of eclipses that, once again, could have been used to provide warnings of further lunar eclipses, with a reasonable (although probably not good) success rate—but not solar ones. The Maya city-states had collapsed by the time the Europeans arrived, but the Aztecs continued to record eclipses, even after the conquest. The tremendous fear felt by those natives who witnessed a total solar eclipse in 1531 is recorded in graphic detail by the chronicler Fray Bernardino de Sahagun.

The problem with predicting total solar eclipses is that regular cycles (such as the Saros cycle of eighteen years and eleven days, which the Babylonians identified) are only crude. They take no account of various additional, more complex irregularities in the motions of the earth and moon. These small irregularities are relatively unimportant in predicting eclipses of the moon, which affect half of the earth, but are critical if we need to predict the narrow path of totality of a solar eclipse.

Many people have been keen to demonstrate that prehistoric people could predict eclipses. Famously, the Aubrey Holes, a ring of fifty-six pits surrounding the sarsen circle at Stonehenge in England, was interpreted by astronomers Gerald Hawkins and Fred Hoyle in the 1960s as an eclipse-pre-

Desperation among Peruvian villagers on the occurrence of a solar eclipse. Copper engraving, Bernard Picardi. (Bettmann/Corbis)

dicting device. The two astronomers proposed slightly different methods, but both schemes involved wooden posts being moved around the holes in prescribed ways. Both methods would have successfully identified "eclipse danger periods"—periods when a solar or lunar eclipse could occur—at roughly six-month intervals. Eclipses of the moon could only occur at full moons, and eclipses of the sun at new moons, falling within these danger periods. However, even partial solar eclipses would only occur on average once every two and a half years, and a total solar eclipse would only happen on average once in several generations. Hawkins's scheme only successfully predicted a fraction of eclipses; Hoyle claimed that his own method would have successfully predicted about half. However, further debate about the relative merits of either scheme is unnecessary. The Aubrey Holes are just one of the several Neolithic pit circles known in Britain, of a range of sizes. It is archaeological nonsense to pick this one as a potential eclipse predictor while ignoring the rest.

Few would doubt that total solar eclipses, when they did occur in the past, could have made a huge impression on the people who witnessed them. Where historical or written records exist they give us concrete information about particular eclipse observations and their social impact. But we must beware of letting our eagerness to find notable accomplishments back in prehistory lead us to invent archaeological evidence that is simply not there.

See also:

Astrology; Eclipse Records and the Earth's Rotation; Lunar Eclipses; Power.

Babylonian Astronomy and Astrology; Chinese Astronomy; Dresden Codex; Stonehenge; Swedish Rock Art.

References and further reading

Aveni, Anthony F. Stairways to the Stars: Skywatching in Three Great Ancient Cultures, 33-37. New York: Wiley, 1997.

-. Skywatchers, 26-29, 173-184. Austin: University of Texas Press, 2001.

Espenak, Fred. NASA/Goddard Space Flight Center Solar Eclipse Page. http://sunearth.gsfc.nasa.gov/eclipse/solar.html.

-. Solar Eclipses of Historical Interest.

http://sunearth.gsfc.nasa.gov/eclipse/SEHistory/SEHistory.html.

Hoyle, Fred. From Stonehenge to Modern Cosmology, 19-54. San Francisco: Freeman, 1972.

Lee, Georgia, and Edward Stasack. Spirit of Place: Petroglyphs of Hawai'i, 97. Los Osos, CA: Easter Island Foundation, 1999.

Littmann, Mark, Ken Willcox, and Fred Espenak. Totality—Eclipses of the Sun (2nd ed.), 1-53. New York: Oxford University Press, 1999.

Schaefer, Bradley E. "Solar Eclipses that Changed the World." Sky and Telescope 87 (1994), 36-39.

Selin, Helaine, ed. Astronomy across Cultures, 450, 547. Dordrecht, Neth.: Kluwer, 2000.

Souden, David. Stonehenge: Mysteries of the Stones and Landscape, 125-126. London: Collins and Brown/English Heritage, 1997.

Thurston, Hugh. Early Astronomy. Berlin: Springer-Verlag, 1994.

Xu Zhentao, David Pankenier, and Jiang Yaotiao. East Asian Archaeoastron-omy: Historical Records of Astronomical Observations of China, Japan and Korea, 13-19, 25-60. Amsterdam: Gordon and Breach, 2000.

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