"Nothing can be sworn impossible since Zeus made night during mid-day, hiding
the light of the shining Sun."
- Archilochus 648 BC
Throughout history, solar eclipses have inspired awe and fear. Until relatively recent
times, the cause of a solar eclipse was not understood and could not be predicted.
Imagine, if you can, what it must have been like. Day after day, year after year,
you see the Sun rise every morning and set every night. Then one day, the Sun vanishes.
What would you think? What would you feel?
Mark Littmann and Ken Willcox note, in their book Totality, that in ancient
times the solar eclipse was generally explained in one of four ways:
A celestial being, usually a monster, attempts to destroy the Sun
The Sun fights with its lover the Moon
The Sun and the Moon make love and discreetly hide themselves in darkness
The Sun grows angry, sad, sick, or neglectful
Littmann and Willcox write, "Within these myths is a great truth. The harmony and
well-being of the Earth are dependent on the Sun and the Moon. Abstract science cannot
convey this profound realization as powerfully as a myth in which the celestial bodies
come to life."
In Norse mythology, the wolflike giant Sköll follows the Sun waiting for his chance
to devour it. In ancient Egypt, the evil god Set was thought to have leapt into the
eye of the Sun god, Horus. In ancient China it was a heavenly dog that ate the Sun.
In many places around the world, it was thought that screams would help the Sun and
Moon escape the affliction of an eclipse. The Chippewa indians in North America shot
flaming arrows at the Sun hoping to rekindle the flames.
In 430 B.C., the Greek historian Herodotus records that
War broke out between the Lydians and the Medes , and continued for
five years, with various success. In the course of it the Medes gained many victories
over the Lydians, and the Lydians also gained many victories over the Medes....As,
however, the balance had not inclined in favour of either nation, another combat took
place in the sixth year, in the course of which, just as the battle was growing warm,
day was on a sudden changed into night. This event had been foretold by Thales, the
Milesian, who forewarned the Ionians of it, fixing for it the very year in which it
actually took place. The Medes and Lydians, when they observed the change, ceased
fighting, and were alike anxious to have terms of peace agreed upon.
Using modern knowledge of astronomy, the date for this eclipse is thought to be May
28, 565 B.C.
Herodotus also reports that, as Xerxes and his Persian army were marching on Greece,
At the moment of departure, the Sun suddenly quitted his seat in the
heavens, and disappeared, though there were no clouds in sight, but the sky was clear
and serene. Day was thus turned into night; where-upon Xerxes, who saw and remarked
the prodigy, was seized with alarm, and sending at once for the Magians, inquired
of them the meaning of the protent. They replied--"God is foreshadowing to the Greeks
the destruction of their cities; for the Sun foretells for them, and the Moon for
us." So Xerxes, thus instructed, proceeded on his way with great gladness of heart.
As it happened, Xerxes' navy was destroyed by the Greeks and he was forced to withdraw.
Considering the impact that an unexpected solar eclipse had on society, it is no wonder
that ancient peoples struggled to find a way to predict eclipses. There is some evidence
that holes in the ground around Stonehenge gave a limited ablity to forecast eclipses
even 4000 years ago. Mayan astronomers in Mexico also had a method for predicting
eclipses. Using historical records, ancient astronomers searched for patterns in the
occurences of solar eclipses, the most notable being the Saros
cycle of 18 years 11.3 days. Eclipses tend to repeat themselves with this
interval. As you might expect, there are many (in fact, 42) Saros cycles progressing
at the same time and we do not have to wait 18.3 years from one eclipse to the next,
we only have to wait 18.3 years for the next eclipse in each Saros cycle. The Saros
cycle was first recorded by the Chaldeans on clay tablets in cuneiform writing.
The geometry of the Sun, Moon, and Earth very nearly repeats itself every 18 years
and this is the basis for the Saros cycle. Indeed if you take the date of a solar
eclipse and add 6585.32 days to it, you will have a good forecast for the occurance
of another eclipse. Each Saros cycle lasts about 1300 years, so, with this method
of forecast you would only make one error in 1300 years (just after the cycle dies
out). However, since the Saros cycle does not include an integral number of days (18
years 11 and 1/3 days, rather than 18 years 11 days exactly), the next eclipse in
the cycle will occur one third of the way around the world from the previous eclipse
in the cycle. So, the Saros cycle allowed ancient astronomers to predict when a
solar eclipse could be expected, but it did not help them figure out where the
eclipse would occur. Thus, it was very hard to verify the prediction and to instill
confidence in their ability to make predictions.
In more modern times, a very famous solar eclipse was used to verify Einstein's General
Theory of Relativity. In this theory light is affectd by gravity in much the same
way as matter is affected by gravity. Hence, light coming from a distant star should
be bent a little bit by the Sun's gravity if the light passes very close to the Sun
during its journey to the Earth. The only way to verify this was to very accurately
measure the position of stars which appear close to the Sun when seen during a total
solar eclipe. If Einstein's theory is correct, the Sun's gravity should move the apparent
positions of the stars a little bit compared to their locations at other times of
the year when they do not appear close to the Sun. This measurement could only be
made during a total solar eclipse when the relatively faint stars could be seen as
the light from the Sun itself was blocked by the Moon.
The experiment took place in 1919 when eclipse expeditions were sent to two small
islands, Sobral, off the northeast coast of Brazil, and Principe, in the Gulf of Guinea.
About a dozen stars were studied and the results agreed with Einstein's predictions.
It was perhaps the most dramatic scientific result obtained from a solar eclipse.
Oh leave the Wise our measures to collate
One thing at least is certain, LIGHT has WEIGHT
One thing is certain, and the rest debate --
Light-rays, when near the Sun, DO NOT GO STRAIGHT.
- Arthur S. Eddington (1920)
Source: Littmann and Willcox, Totality, University of Hawaii Press, 1991.
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