rather, they are obscured in different ways. 

 Astronomically speaking, a solar eclipse 

 can be called an occultatiori: a masking of a 

 celestial body by another that passes in front 

 of it. During a total lunar eclipse, in contrast, 

 the full Moon passes completely into Earth's 

 shadow. From another point of view — that 

 of an observer on the Moon, for instance 

 — a total solar eclipse amounts to a partial 

 (very partial) eclipse of the Earth. 



Contrary to popular belief, total solar 

 eclipses are not particularly rare. Astron- 

 omers predict sixty-eight to take place 

 during the twenty-first century — one about 

 every eighteen months. That's not counting 

 annular or "ring" eclipses (in which the 

 Moon is too far from Earth to completely 

 cover the Sun, and the tip of the umbra 

 doesn't quite reach Earth's surface); hybrid 

 eclipses (which are annular along one part of 

 the path, total along the rest); and a goodly 

 number of partial solar eclipses. 



But if total solar eclipses aren't all that 

 rare, seeing one is — mainly because (even 

 assuming clear weather) you have to be at 

 the right place at just the right time, and the 

 Sun and Moon do not arrange their assignations for human 

 convenience. The track traced by the Moon's umbra on 

 Earth can run for many thousands of miles, but it's also very 

 narrow, at most about 170 miles wide. It has been calculated 

 that on average, a total eclipse of the Sun is visible from the 

 same spot on Earth only once in about every 375 years. If 

 you think back to a time when human societies were small 

 and communications and transportation limited, not only 

 were the chances of seeing a total solar eclipse slight, but 

 the fact that anyone else had ever seen one would not be 

 common knowledge. The phenomenon would have been 

 regarded with awe and, likely, fear — as a sign that the gods 

 were angry, an omen of impending disaster. Many people 

 believed that the Sun disappeared because it was being 

 eaten by a dragon. And, of course, as soon as the priests — 

 the only astronomers (or rather, astrologers) of the earliest 

 civilizations — were able to forecast such happenings, they 

 could use their skills as a means to impress the multitudes. 



Old Chinese bone inscriptions provide one of the 

 earliest records of an eclipse — probably the one that, by 

 our Western calendric reckoning, occurred October 22, 

 21 34 13. c. Hsi and Ho, astronomers to the Emperor Chung 

 K'ang, had failed to predict that eclipse, and as the Sun 

 faded, pandemonium broke loose. The Son of Heaven had 

 his court astronomers decapitated. 



And there is at least one apparent reference to an eclipse 

 in the Bible. In Amos 8:9, we read: "I will cause the Sun to 

 go down at noon, and I will darken the Earth in the clear 



Above: Eclipse of September 10, 1923, as seen near Lompoc, 

 California, shows the diamond ring effect. It was painted by 

 Howard Russell Butler, based on his sketches and notes of the 

 colors. Totality lasted 140 seconds. Opposite page: Composite 

 of five digital camera images of the eclipse of August 1, 2008, 

 processed to bring out detailed structure that cannot be revealed 

 in a single photograph. The shape of the corona resembles that in 

 Butler's painting; both eclipses occurred during quiescent years of 

 the eleven-year sunspot cycle. 



day." Most likely that was the Eclipse of Nineveh, which 

 has been dated June 15, 763 B.C. An Assyrian tablet also 

 attests to the event. 



Modern-day astronomers have learned much by 

 studying solar eclipses, such as determining the precise 

 relative positions of the Sun, Moon, and Earth. Certain 

 kinds of studies — especially measurements of the Sun's 

 searing corona — can best be made during a total solar 

 eclipse. The corona, or crown of the Sun, is a delicate halo 

 of pearly white light that is always present but whose pale 

 glow is normally overpowered by the Sun's brilliance. 

 (Because it is only visible during totality, it wasn't until 

 the eighteenth century that astronomers were certain that 

 the corona surrounded the Sun and not the Moon!) It's 

 composed of highly diffuse, superheated, ionized gases; 

 most scientists believe those gases extend all the way to 

 Earth as the solar wind. Thus, understanding the corona is 

 relevant to understanding our own environment. 



During a total solar eclipse, a few ruby-red spots may 

 seem to hover around the jet-black disk of the Moon. 

 Those are solar prominences, tongues of incandescent 



38 



natural history October 2008 



