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People have been studying total solar eclipses for thousands of years to better understand the cosmos.
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Total eclipses helped prove Einstein’s theory of relativity and led to the discovery of helium.
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Here are seven examples where total solar eclipses have helped advance our knowledge of science.
On April 8, millions of people in the US will all do the same thing: look up.
By mid-afternoon, day shifts to night as a total solar eclipse hits 15 states.
We now know what causes a total solar eclipse. But our ancestors thousands of years ago weren’t so sure, and that was bad news for some scientists.
In the 21st century BC, the Chinese emperor reportedly had two astronomers beheaded for failing to predict a total solar eclipse.
Over the centuries, total solar eclipses became less dire and more of an opportunity to test scientific theories and make new discoveries.
Here are seven times a total solar eclipse advanced human science.
1. Measuring the rotation of the Earth
Some of the earliest suspected records of eclipses date back thousands of years.
Some experts have suggested that petroglyphs or petroglyphs found on a monument in Ireland refer to a solar eclipse that occurred on November 30 in the year 3340 BC. Others have expressed skepticism.
Man-made markings on turtle shells from China and a Babylonian tablet from more than 3,000 years ago may also contain ancient references to eclipses.
While these data are open to interpretation, scholars have studied historical descriptions of eclipses for centuries. For example, 18th-century astronomer Edmond Halley first realized that the Earth’s rotation has slowed over millennia.
2. Discover what causes eclipses
Two modern scholars credit the Greek philosopher Anaxagoras of Clazomenae with discovering the moon’s role in eclipses, calling him “perhaps the first empirical astronomer.”
He probably developed his theories after he died on February 17, 478 B.C. had witnessed an annular or ‘ring of fire’ eclipse. While he got some of the science right, his overall understanding of the solar system was a product of its time.
For example, Anaxagoras thought that air pressure held up the flat Earth at the center of the rotating sun, moon, and stars. Despite that mistake, he worked out the basic mechanisms behind eclipses.
Anaxagoras rightly believed that the moon reflected the light of the sun. He was also accurate in his theories that when the moon moved in front of the sun it caused a solar eclipse. Likewise, if the Earth were between the sun and the moon, a lunar eclipse would occur.
He also used the moon’s shadow during a solar eclipse to estimate its size, but his calculations gave him a much smaller answer than reality.
3. Estimating the distance from the moon to the earth
On March 14, 189 B.C. a total solar eclipse passed over what is now northern Turkey. The Greek astronomer Hipparchus is said to have been just a child at the time, but it is possible that he witnessed the event.
Years later, Hipparchus may have used second-hand accounts of that same solar eclipse to create one of the era’s most mathematically accurate estimates of the Moon’s distance from Earth.
Although Hipparchus’ direct writings have been lost, a 4th-century scholar has detailed how he used the information.
The astronomer estimated the distance between where the total solar eclipse occurred in modern-day Turkey and Alexandria, Egypt – where a fifth of the sun could be seen – to make his calculation.
Based on his calculations, Hipparchus offered a number of distances, including an average distance of about 451,387 miles.
He wasn’t very far away. The moon is about 238,855 miles away.
4. Predicting the path of a solar 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 take centuries before humans made a more accurate prediction. In the 18th century, Edmond Halley, better known for discovering a comet bearing his name, created a map that predicted the path of the May 3, 1715 solar eclipse with extreme accuracy.
Others made maps for him, but Halley based his prediction on Isaac Newton’s theory of universal gravitation, which helped him determine the timing of the solar eclipse within four minutes.
5. The discovery of helium
Helium is very abundant in the universe, but rare on Earth. It took a solar eclipse for an astronomer to discover this.
The French astronomer Pierre Jules César Janssen had traveled to Guntur, India especially for the solar eclipse of August 18, 1868. He used a spectroscope, a prism-like device to divide 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 that allowed him to view the Sun even without an eclipse. He saw the same line.
Lockyer named the mysterious element helium. It took a few decades for scientists to see it on Earth, during experiments with lava and uranium from Vesuvius.
6. Proof of Einstein’s theory of relativity
Astronomer James Craig Watson was confident he had found evidence of a new planet during the 1878 solar eclipse. Vulcan, believed to be located between the sun and Mercury, 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, hard-to-spot extra planet.
Despite this evidence, Einstein’s theory did not gain scientific proof until the solar eclipse of May 29, 1919. The physicist had said that 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. As the moon blocked the sun, the astronomers took pictures.
The stars appeared to have shifted in location compared to the reference photos. The apparent new locations showed that the sun bent light within the measurements Einstein predicted.
7. Studying eclipses from space
Gemini 12 astronauts Jim Lovell and Buzz Aldrin were the first people to see a total solar eclipse from space. The eclipse of November 12, 1966 moved from Peru to Brazil, and the astronauts drifted near the path of totality.
According to Smithsonian Magazine, it was a coincidence that they were close enough to see it. Aldrin’s photos were blurry and a bit disappointing.
Four years later, TV networks broadcast the ‘eclipse of the century’, full of images of the event. NASA also launched more than two dozen rockets to study UV radiation and X-rays from the sun during the phenomenon.
The agency still uses rockets to collect data during eclipses and will launch three on April 8.
Read the original article on Business Insider