Lunar Eclipses

When the moon is full, the side facing the earth is fully illuminated by sunlight, which means that it is also the side of the moon facing the sun. In other words, the earth is between the moon and the sun. More often than not, the earth doesn't pass exactly between them, but when this does hap pen—about once every sixth lunation—the moon passes through the earth's shadow (umbra). At such times an astronaut on the moon would see the earth passing across in front of the sun—a rather different experience from seeing a solar eclipse from the earth, since the earth's disc would appear about four times the size of the sun's.

The situation is actually more complicated than this example suggests, because the earth's shadow has an inner cone (the umbra) and an outer part called the penumbra. If our astronaut on the moon stood within the penumbra, he or she would see the equivalent of a partial solar eclipse. But if he or she stood within the umbra, the earth's disc could entirely cover the sun. Around the disc an eerie glow would be visible—orange, reddish, or brown—due to sunlight reflected in the earth's cloudy, dusty atmosphere.

But how does this appear from the earth? If the moon passes through the penumbra but misses the umbra—a so-called penumbral eclipse—then the moon's disc is slightly dimmed and the eclipse passes unnoticed to the naked eye. Such eclipses have no cultural impact and can be ignored. Umbral eclipses may themselves be partial or total, depending whether the entire moon passes into the umbra.

Lunar umbral eclipses, unlike solar ones, can be seen (weather permitting) from approximately half of the earth: from all places where the sun is below the horizon, it is night, and the moon—opposite to it in the sky—is up and visible. During a partial lunar eclipse—as well as in the stages leading up to a total one—part of the moon is in the earth's shadow and is dark. Sometimes a partially eclipsed moon could be mistaken for a normal crescent moon except that it is in the wrong position in the sky and, to the discerning eye its appearance is odd—the crescent may be at an unusual angle and have an odd shape. A total lunar eclipse, however, can be truly spectacular. As soon as no bright part remains to overpower the rest, the whole lunar disc reappears but now glows dimly with a deep red, brown, or orange color. "Totality" can last as long as one and three-quarter hours.

For modern city dwellers it is easy to miss even a total lunar eclipse, and for this reason we are inclined to think that for ancient peoples the social impact of a total solar eclipse—a truly rare event transforming day into night— would have been far greater. Yet anyone who relies upon the moon to illuminate a dark night or as the basis of their calendar regulating seasonal, subsistence, and ritual activities may be alarmed to watch the full moon being "eaten" away and then totally consumed. And such events are not rare: given clear skies and someone to wake us up when it happens, we would expect to see roughly one total lunar eclipse every three years on average.

To many peoples in the past, then, an eclipse of the moon was just as ominous, potentially frightening, and even calamitous, as a total solar eclipse. The Aztecs, certainly, were every bit as terrified of a lunar eclipse as of a solar one, as we know from the writings of the chronicler Fray Bernardino de Sahagun: when the moon's face darkened, women feared that their unborn children would be born lipless, noseless, or cross-eyed, or turned into mice. Maya, ancient and historical, variously saw the moon as being attacked, bitten, or eaten, and hence sick or dying, just as they saw the sun during a solar eclipse. The impression of the moon literally being eaten is reinforced by its blood-red color at the time of totality. Indeed, the moon being turned into blood was a common metaphor in medieval European chronicles.

The blood-red color was also taken as a portent of death. The AngloSaxon Chronicles record that in c.e. 734 the moon was as though drenched with blood; this lunar eclipse was taken to presage the death of the Venerable Bede in the following year. Similarly, it has been supposed that a prophecy that the moon would "turn to blood" at the crucifixion of Christ might refer to a lunar eclipse (although this is highly questionable). Lunar eclipses were certainly taken very widely as a bad omen. An unexpected lunar eclipse in 413 b.c.e., for example, led the Athenian forces to postpone their departure from Syracuse, which they had held under siege for two years during the Peloponnesian war. This delay resulted in total military defeat. Almost a millennium later, in c.e. 549, two Teutonic armies were preparing to wage battle in the Carpathian basin. It was nighttime. In their two camps, close but out of sight of each other, the chieftains and escorts on each side were likely drunk in their tents. Without warning, the soldiers of both armies—quite independently—simply fled from their camps, leaving their superiors with no choice but to conclude an armistice. The reason, it seems, was a total lunar eclipse.

The Maori had a less calamitous view. According to Maori myth, a woman named Rona was snatched up by the moon after insulting it when it disappeared behind a cloud while she was trying to fetch water at night. Rona can still be seen in the face of the moon. During a lunar eclipse, Rona was attacking the moon and trying to destroy it. The result of the conflict, however, was far from calamitous: the moon ultimately returned reinvigo-rated, young and beautiful.

Once people began to keep systematic records, the possibility emerged of predicting lunar eclipses. Since they can only occur at full moon, the challenge is to predict which full moons are the mostly likely candidates. There are a variety of cycles that, once recognized, provide the means to do this at various levels of reliability. The simplest in the short term involves regular counting in sixes and fives: if a lunar eclipse occurs in month zero, then another is most likely in months six, twelve, eighteen, etc.; might happen in months five, eleven, seventeen, etc.; but cannot happen in any other month. The best-known longer-term cycle is the so-called Saros cycle of eighteen years and eleven days (223 lunations) discovered by the Babylonians, but the Chinese used one of ten years and 334 days (135 lunations), and the Maya recognized one of thirty-two years and 272 days (405 lunations), a time period commensurate with their 260-day sacred calendar round known as the tzolkin (405 lunations = 46 tzolkins to within a fraction of a day). None of these cycles is failsafe, however. Some of the predicted eclipses will only be partial, and some may occur but not be visible because they happen during daylight. Those that occur high in the sky in the middle of the night, and last longest, will be the most conspicuous.

For many ancient peoples, the motivation for systematic astronomical observations—initially, at least—was divinatory. Thus ancient Chinese records of lunar eclipses date back to the Shang dynasty in the second half of the second millennium b.c.e., when they were recorded—together with associated prognostications and verifications—on turtle shells and animal bones that have become known as oracle bones. However, as lunar eclipses became more predictable, around the year 0 and beyond, they began to lose their significance as portents and to fall into the domain of calendrics. They simply became part of the framework of regular and reliable events whose cycles could be calculated using careful record-keeping and mathematics. (Solar eclipses, in contrast, remained unpredictable and so continued to be regarded as bad omens.) The Maya, on the other hand, seem to have been obsessed with establishing exact numerical relationships between natural and entirely manmade cycles, and in particular with the 260-day tzolkin. Each day of this sacred round had particular characteristics that were crucial in fixing rituals and making prognostications. In complete contrast to the ancient Chinese, the desire to incorporate what were effectively astrological considerations into their calendar drove the Maya to ever greater levels of astronomical achievement.

See also:

Astrology; Lunar and Luni-Solar Calendars; Power; Solar Eclipses.

Babylonian Astronomy and Astrology; Chinese Astronomy; Crucifixion of Christ; Dresden Codex; Fiskerton; Mesoamerican Calendar Round.

Lunar Phase Cycle.

References and further reading

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

-. Skywatchers, 28-32, 173-184. Austin: University of Texas Press,

2001.

Aveni, Anthony F., ed. World Archaeoastronomy, 83-91, 389-415. Cambridge: Cambridge University Press, 1989.

Best, Elsdon. The Astronomical Knowledge of the Maori, 19-20. Wellington: Dominion Museum, 1922.

Espenak, Fred. Lunar Eclipses of Historical Interest.

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

-. NASA/Goddard Space Flight Center Lunar Eclipse Page. http://sun-

earth.gsfc.nasa.gov/eclipse/lunar.html.

Ruggles, Clive, ed. Archaeoastronomy in the 1990s, 98-106. Loughborough, UK: Group D Publications, 1993.

Savage, Anne, transl. The Anglo-Saxon Chronicles. London: Pan Macmillan, 1984.

Schaefer, Bradley E. "Lunar Eclipses That Changed the World." Sky and Telescope, 84 (1992), 639-642.

Selin, Helaine, ed. Astronomy across Cultures, 174, 450. Dordrect, Neth.: Kluwer, 2000.

Stephenson, Richard, and David Clark. Applications of Early Astronomical Records, 10, 30-33. Bristol, UK: Adam Hilger, 1978.

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

Xu Zhentao, David Pankenier, and Jiang Yaotiao. East Asian Archaeoastronomy: Historical Records of Astronomical Observations of China, Japan and Korea, 13-19, 61-106. Amsterdam: Gordon and Breach, 2000.

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