Monday, October 22, 2012

Ancient Moon Shadows

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On October 22, 2136 BC, astronomers in China noted what is now the oldest surviving record of a total solar eclipse, which occurs when the moon passes between the Earth and the sun, causing the moon's shadow to fall upon Earth and block the sun from view.

In ancient China, astronomy was a government-mandated pursuit, and state astronomers had quite sophisticated observatories for their time. Good thing too, for observing solar and lunar eclipses, as well as tracking planetary orbits, were divination tools for predicting the fate of the Emperor.

If an Emperor could predict a solar eclipse, such was a good omen for his health; accuracy was helpful in validating that he was the ordained link between heaven and his subjects on Earth, endorsing his divine right to rule. Imprecise predictions could be seen as evil omens, or even result in a new ruler, whereby rivals for power might use the eclipse as a sign that they could overthrow one who had lost the blessing of the gods. Careful records were made of all solar eclipses. (Lunar eclipses were only haphazardly noted, being so common as to merit lesser import.)

Solar Eclipse
As early as 2650 BC, a star-gazer named Li Shu wrote about celestial bodies, in particular noting that the sun, earth, and moon moved in harmonious ways. Technology in ensuing years revolved around trying to forecast when certain events might occur so as to keep their political successions and societies more stable.

The fascinating field of "Archaeoastronomy" shed light on the Oracle Bones of the Shang Dynasty (1600 – 1050 BC), unearthed in Anyang, Henan Province. Hailed as the bones of dragons (though actually turtles or oxen), they represent some of the earliest Chinese writings. One such gem tells us that the failure to correctly predict the timing of a total solar eclipse resulted in beheadings:
"Here lie the bodies of Ho and Hi,
Whose fate, though sad, is risible;
Being slain because they could not spy
Th' eclipse which was invisible."

Surely these weren't the only two state astronomers to lose their heads, given how erratic solar eclipses can be in any specific geographic location. With so much at stake, precision was well sought after. By 720 BC, some Chou Dynasty astronomers recognized eclipses as "naturally" occurring phenomena, and not heavenly commentary on who held any particular throne. Still, diligent record-keeping continued up through the ages.

Oracle Bone
By the turn of the millennium, the Chinese had a firm grasp of what actually caused eclipses, and by 206 AD, they were predicting cycles by analyzing lunar orbits. Their records show that between 600 and 1300 AD, their solar eclipse timing predictions were often accurate to within about 20 minutes!

To see how it's done in the modern day, see the NASA Eclipse Website.

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