- Humans have been studying total solar eclipses for millennia to better understand the cosmos.
- Total eclipses have helped prove Einstein's theory of relativity and led to the discovery of helium.
On April 8, millions of people across the US will all do the same thing: look up.
In the middle of the afternoon, day will shift to night, as a total solar eclipse touches 15 states.
We know now what causes a total solar eclipse. But our ancient ancestors from millennia ago weren't so certain, and that was bad news for some scientists.
In the 21st century BCE, China's emporer reportedly had two astronomers beheaded for not predicting a total eclipse.
Over the centuries, total eclipses became less dire and more of an opportunity to test scientific theories and forge new discoveries.
Here are seven times a total solar eclipse has helped advance human science.
1. Measuring Earth's rotation
Some of the earliest suspected records of eclipses date back thousands of years.
Some experts have suggested petroglyphs or rock carvings found on a monument in Ireland reference an eclipse that took place on November 30 in the year 3,340 BCE. Others have expressed skepticism.
Human-made markings on tortoise shells from China and a Babylonian tablet from over 3,000 years ago may also contain ancient references to eclipses.
Though these records are open to interpretation, scholars have studied historical descriptions of eclipses for centuries. It's how 18th-century astronomer Edmond Halley first realized the Earth's rotation has slowed over millennia.
2. Discovering what causes eclipses
Two modern scholars give Greek philosopher Anaxagoras of Clazomenae credit for figuring out the moon's role in eclipses, calling him "perhaps the first empirical astronomer."
He likely developed his theories after witnessing an annular, or "ring of fire," eclipse on February 17, 478 BCE. While he got some of the science right, his overall understanding of the solar system was a product of its time.
Anaxagoras, for example, thought that air pressure kept the flat Earth aloft at the center of the rotating sun, moon, and stars. Despite that error, he worked out the basic mechanics behind eclipses.
Anaxagoras correctly believed the moon reflected the sun's light. He was also accurate in his theories that when the moon moved in front of the sun, it caused a solar eclipse. Similarly, when the Earth was in between the sun and moon, there would be a lunar eclipse.
He also used the moon's shadow during an eclipse to estimate its size, but his calculations gave him a much smaller answer than reality.
3. Estimating the moon's distance from Earth
On March 14, 189 BCE, a total solar eclipse swept over what is now northern Turkey. Greek astronomer Hipparchus would have just been a child at the time, but it's possible he saw the event.
Years later, Hipparchus may have used secondhand accounts of that same eclipse to make one of the era's most mathematically accurate estimates of the moon's distance from Earth.
Though Hipparchus' direct writings are lost, a 4th-century scholar detailed how he used the information.
The astronomer estimated the distance between where the total eclipse was in modern day Turkey and Alexandria, Egypt — where a fifth of the sun showed — to make his calculation.
Based on his math, Hipparchus offered a few ranges, including a mean distance of about 281,387 miles.
He wasn't terribly far off. The moon is about 238,855 miles away.
4. Predicting the path of an eclipse
In the 11th or 12th century, Mayan astronomers made a remarkable prediction for their time: They calculated that a total solar eclipse would occur in 1991, and their prediction was accurate to within a day.
It would be centuries until humans made a more accurate prediction. In the 18th century, Edmond Halley, better known for discovering a comet with his name, made a map predicting the path of the May 3, 1715 solar eclipse with extreme accuracy.
Others made maps before him, but Halley based his prediction on Isaac Newton's theory of universal gravitation, which helped him pinpoint the eclipse's timing to within four minutes.
5. The discovery of helium
Helium is very abundant in the universe but rare on Earth. It took an eclipse for an astronomer to discover it.
French astronomer Pierre Jules César Janssen had traveled to Guntur, India just for the August 18, 1868 eclipse. He was using a spectroscope, a prism-like device for separating sunlight into a spectrum.
Janssen saw a yellow line with a wavelength unlike any other element. Around the same time, English astronomer Norman Lockyer developed an instrument for viewing the sun even without an eclipse. He saw the same line.
Lockyer called the mysterious element helium. It took a couple of decades for scientists to see it on Earth, during experiments on Mount Vesuvius lava and uranium.
6. Proof of Einstein's theory of relativity
Astronomer James Craig Watson was certain he'd found proof of a new planet during the 1878 eclipse. Supposedly located between the Sun and Mercury, Vulcan was only visible when the moon blocked the giant star.
Several more eclipses passed without anyone finding evidence of Vulcan. In 1915, Albert Einstein explained Mercury's unusual orbit with his general theory of relativity. It fit the data better than a mysterious, difficult-to-spot extra planet.
Despite this evidence, Einstein's theory didn't receive scientific proof until the May 29, 1919 eclipse. The physicist had said the sun's gravity would bend light from nearby stars.
In 1919, expeditions traveled to Principe, an island off the coast of Africa, and Brazil. While the moon blocked the sun, the astronomers took photographs.
The stars seemed to have shifted locations compared to the reference photos. The apparent new locations showed the sun was bending the light within the measurements that Einstein predicted.
7. Studying eclipses from space
Gemini 12 astronauts Jim Lovell and Buzz Aldrin were the first humans to see a total eclipse from space. The November 12, 1966 eclipse moved from Peru to Brazil, and the astronauts hovered near the path of totality.
It was a coincidence that they were close enough to see it, according to Smithsonian Magazine. Aldrin's photos were unfocused and a bit disappointing.
Four years later, TV networks broadcast the "eclipse of the century," full of images of the event. NASA also launched over two dozen rockets to study UV radiation and solar X-rays during the phenomenon.
The agency still uses rockets to gather data during eclipses and will launch three on April 8.