- Humans have observed and theorized about eclipses since antiquity
- Solar eclipses lead to scientific discoveries beyond astronomy
- Einstein’s general theory of relativity confirmed by 1919 solar eclipse
On the day of the new moon, in the month of Hiyar, the Sun was put to shame, and went down in the daytime, with Mars in attendance.
Those words were written on a clay tablet over three thousand years ago by a scribe in ancient Syria to commemorate the solar eclipse of May 3, 1375 BCE. It may be the first eclipse ever recorded.
Humans have been studying the skies for almost as long as we’ve been walking upright. Early civilizations carefully observed the sun and moon to create calendars that could determine optimum planting times and ensure a successful harvest — and the community’s survival.
Which is why an eclipse could inspire what may now seem like outlandish beliefs. When the sky goes dark in the middle of the day, everything early societies depended upon for survival was turned on its head.
“Seeing the usually incredibly bright ball of light we call the sun become a dark black disk surrounded by ethereal light really triggers primordial feelings in people,” says Richard H. Durisen, emeritus professor of astronomy at Indiana University. “It really is a spiritual experience.”
Who was to say that the sun might not stay dark forever and demons descend from the sky to devour men and women? Or that, as the Hindu epic Mahabharata tells, the eclipse was caused by the vengeful demon Rahu swallowing the sun because it had betrayed him to Vishnu.
As time went on, continued observation led to scientific discovery. The ancient Babylonians learned to predict lunar eclipses, but it wasn’t until 1715 that astronomer Edmond Halley (of Halley’s Comet fame) used Newton’s law of universal gravitation to reliably predict where solar eclipses would occur and how long they would last.
Even then, eclipse prediction remained an inexact science. During the American Revolutionary War, Harvard scientist Samuel Williams led a group of eclipse enthusiasts through enemy lines to observe the total solar eclipse of 1780. Unfortunately, he miscalculated and missed the area of totality.
Looking beyond the sky
For most of history, total solar eclipses were the only time astronomers could study the outer layers of the sun.
Most of the light we normally see on earth comes from the sun’s photosphere, which is surprisingly thin (only hundreds of kilometers thick compared to the 2 million km diameter of the sun) but incredibly intense.
When the moon moves in front of the sun, it obscures the photosphere and a soft glowing halo is seen. Initially believed to be the atmosphere of the moon, nineteenth century scientists correctly attributed this glow to the sun itself, now known as the solar corona.
“The coronal light is a beautiful pearly gray and stretches out many millions of miles from the sun,” says Durisen. “The shape of the gas is sculpted by the complex shape of the Sun's magnetic field. It’s one of the most thrilling parts of totality.”
But the sun’s outer layers have more to offer than beauty.
In August 1868, Pierre Janssen, a French astronomer, traveled to India to observe the solar eclipse. While using a spectroscope to study the solar prominences that can be seen with the naked eye during an eclipse, Janssen noticed a yellow line in the chromosphere that didn’t match the wavelength of any known element.
Further analysis led to the discovery of the new element helium, which was independently confirmed two months later by the English astronomer Norman Lockyer.
Perhaps the greatest scientific discovery from an eclipse was confirmation of Einstein’s 1916 general theory of relativity.
Einstein predicted that the gravitational field of large objects causes light to bend by a small amount. Newtonian physics, ascendant at the time, also predicts that light will curve, but only by about half as much. How to measure such a tiny difference?
During the 1919 eclipse, the sun would be passing the Hyades star cluster, giving scientists a perfect opportunity to test Einstein’s theory. Sir Arthur Eddington measured the positions of the Hyades and then several months later traveled to Gabon to measure them again during the eclipse.
His results confirmed Einstein’s predictions, and the bending of light around massive objects is now known as gravitational lensing.
More recently, in June 1973, astronomers viewed a solar eclipse from 55,000 feet, on board a prototype of the Concorde. The supersonic jet raced the lunar shadow across the African continent, allowing five teams of scientists to conduct experiments as they remained in total darkness for 74 minutes.
There’s no limit to what scientists might discover during this eclipse or the next. That’s what science is about: Observing the natural world, testing hypotheses, drawing new conclusions, and creating knowledge.