Second in a series of articles on the Great American Eclipse of 2017.
On Aug. 21, the United States will have a front row seat to a total solar eclipse. As the moon’s shadow drifts from Oregon to South Carolina in just over 90 minutes, those fortunate enough to be near the center of the shadow will experience the bizarre occurrences related to a solar eclipse. In a just the course of a few minutes the sky darkens, crickets begin chirping, the sun blackens and becomes engulfed in an eerie glow.
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Solar eclipse occur when the moon passes perfectly between the sun and earth. During these unique moments, the moon’s narrow shadow reaches down and covers a small patch on the Earth’s surface. At least that is the modern explanation.
Ancient civilizations did not see it that way.
In China it was once believed that during a solar eclipse, a dragon would devour the sun. To prevent such a catastrophe, imperial astronomers were charged with tracking celestial motions and predicting eclipses. With enough warning people could organize, bang gongs, beat drums, and even arm themselves with bows and arrows to ward off the dragon. According to one legend, the imperial astronomers Hsi and Ho spent too much time drinking, which prevented them from predicting an eclipse in 2137 BCE. Their punishment? Beheading.
According to Inuit folklore, the moon god Anningan and his sister, the sun goddess Malina, once fought as siblings often do. Malina ran off across the sky. Her brother followed in pursuit in an attempt to apologize. Every now and then Anningan catches up with his sister and when he does there is a solar eclipse.
The earliest record of a solar eclipse dates back over 5,000 years. In 1999 archeoastronomer Paul Griffin discovered Neolithic rock carvings in Ireland that correspond to a solar eclipse that occurred on Nov. 30, 3340 BCE. Like the story of Hsi and Ho, this eclipse may have also resulted in a gruesome ending. A nearby basin held the charred remains of nearly 50 individuals, the possible result of a human sacrifice ritual to the sky god.
Not all solar eclipses end with a horrifying story. Eclipses have also shaped history.
According to the ancient Greek historian Herodotus, a solar eclipse in 585 BCE stopped a long standing war between the Medes and the Lydians in what is now Turkey. The warring nations interpreted the solar eclipse as a sign from the gods about their desire to end the conflict. It is said that immediately following the eclipse, soldiers laid down their weapons and agreed to a truce.
In more modern times a solar eclipse played a prominent role in promoting the career and popularity of a fairly obscure physicist by the name of Albert Einstein. In 1915, Einstein published his general theory of relativity. At first, Einstein’s theory was met with skepticism because of the difficult math involved and the use of thought experiments. Four years later, that all changed.
One of the remarkable predictions from relativity is that a massive object will cause light to deflect form its path as it passes near the object. The effect is small but measurable, especially if the object is very massive – say the mass of the sun.
To test Einstein’s prediction the English astronomer Sir Arthur Eddington led an expedition in 1919 to photograph a total solar eclipse visible from the small island of Principe, located off the west coast of Africa. Eddington’s team measured the position of stars near the limb of the sun during the eclipse. Normally the stars would not be visible, but by using the moon to block out the sun’s glare, Eddington was able to photograph the nearby stars and compare their positions to expectations. Sure enough, the stars’ positions were slightly shifted just as Einstein had predicted. From that moment on, Einstein became a household name.
Eddington’s photographs were not the first time an eclipse was caught on film. That distinction goes to Johann Julius Friedrich Berkowski’s photos of the July 28, 1851 solar eclipse visible in Prussia. It also marks the first accurate recording of the sun’s outer atmosphere known as the solar corona.
Normally the corona is not visible because the sun’s central disk outshines the fainter corona. Only during a total solar eclipse, like the upcoming eclipse on Aug. 21, does the solar corona become visible. Prior to Berkowski’s photograph of the 1851 eclipse, records of the solar corona were made through rapidly drawn sketches.
Astronomers create artificial eclipses all the time by including an instrument called a coronagraph. A coronagraph is a disk placed on a telescope to block out the overly bright sun. However, because the coronagraph is close to the observer, it causes distortions around the edge of the disk, making precise measurements near the sun’s surface difficult. Luckily for astronomers, by a cosmic coincidence the moon and sun are the same size when viewed from earth. During a solar eclipse the moon covers just enough of the sun, allowing astronomers to study the sun’s atmosphere right down to its surface.
For the Aug. 21 eclipse, the National Solar Observatory is working with citizen scientists, high schools, universities, and national laboratories along the path of totality to gather thousands of pictures of the eclipse. By combining the images from the 68 different sites, astronomers plan to produce a 90 minute movie of the inner solar corona, a feat never accomplished before.
What was once considered a bad omen, solar eclipses are now bringing together people all across the country in the name of science.
Dr. Louis J. Rubbo is an associate professor of physics and astronomy at Coastal Carolina University. He has a Ph.D. in physics from Montana State University. During the month leading up to the Aug. 21 eclipse, he will write weekly articles surrounding different aspects of the eclipse. He will also give a public lecture on the science of the eclipse at the Strand Theatre in Georgetown on Aug. 20 at 6 p.m.