Thursday, December 20, 2018

Eclipses?

Obvious though it seems, how and why solar eclipses became important to our civilization is not at all clear. The path of totality is narrow and fleeting, gone in about six or seven minutes. I never experienced a total eclipse, but I have seen three partials: September 20, 1960, May 10, 1994, and August 21, 2017. In every case, had it not been announced long in advance, I would not have known from common experience just what—if anything—had happened.

It is often repeated, citing Herodotus, that Thales of Miletus predicted the solar eclipse of May 28, 585 BCE. How he did that is not recorded. We take for granted our Arabic numerals and positional notation. Absent them, calculation is even more painfully laborious than most of us experience it to be. And the Greeks used geometry, not arithmetic. 
 
Just one construction from
"On the Sizes and Distances of the Sun and Moon" by
Aristarchus of Samos from Aristarchus of Samos:
The Ancient Copernicus
by Sir Thomas Heath
(Oxford University Press; 1977, 1913).
Having long kept records, the Babylonians were aware of the 18-year cycle of eclipses. (18 years 11 days plus a third of a day. See the explanation of the Saros Cycle at Wikipedia.  )

The Chinese also kept records of eclipses, at least back as far as 1302 BCE, the first perhaps in 2159 BCE. (“The eclipse in China”, F. Crawford Brown, Popular Astronomy, Vol. 39, p.567 at The Digital Library for Physics and Astronomy at Harvard  and “Examination of early Chinese records of solar eclipses,” Liu, C., Liu, X., & Ma, L., Journal of Astronomical History and Heritage (ISSN 1440-2807), Vol. 6, No. 1, p. 53 - 63 (2003) at the same archive. )

That being as it may, explanations for the physical events—that the Moon and Earth cast shadows on each other—were lacking until the Greeks, again, beginning perhaps as early as 600 BCE, but certainly known to Aristotle c. 300 BCE. Nonetheless, that knowledge was not widespread.
“The majority of people didn’t really understand what eclipses or shooting stars were until at least the 17th Century,” says Edwin Krupp, director of the Griffith Observatory in California. The ancient Greeks weren’t alone, either.
While a handful of astronomical scholars, from the 8th Century BC onwards, successfully understood the celestial mechanisms behind an eclipse, for another 2,000 years most of the world’s population clung steadfastly to the ancient belief that astronomical events, and particularly solar and lunar eclipses, were the work of the gods. (BBC here.)

Primitives and Moderns both react to solar eclipses.
(Left: "People once feared solar eclipses" 

Right: "Modern astronomers observe eclipses carefully")
The Golden Book of Astronomy: A Child's Introduction to the Wonders of Space
(Simon & Schuster, 1958, 1955).
My best eclipse was May 10, 1994. The sky did grow visibly darker. The air turned cool. The birds were silenced. But it was impossible to look directly at the sun. For that, I built a viewing box to project the image on a sheet of paper. The first partial eclipse I witnessed on September 20, 1960, was an annular, a ring, because the Moon was too close to the Earth to completely block the Sun. What I saw was a partial annular, a geometric “lune” the shape of part of one circle over another. Although I had exposed film to view it through, the cloud cover was just right to view the sun directly. The third time, although some clouds passed by they were not dense enough to allow direct observation of the Sun. If I had not had exposed film to view through, I would not have known that the eclipse occurred at all: no other environmental changes were manifested.

Lunar eclipses are impossible to ignore. The Earth's shadow is large. The Moon often rises red, blue light being absorbed by our atmosphere. The Moon darkens almost to full black. The entire event takes hours.  

Every Lunar eclipse is followed by a Solar eclipse. However, as noted above, the path of totality is narrow and the duration of darkness is just minutes. You would have to be a pretty good sprinter to cover the mile from your field to your home to the village to the church in time to pray before it would all be over. And people 200 miles away might not be aware that anything happened at all. 

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