7 Times Solar Eclipses Revolutionized Science

  • Humans have been studying total solar eclipses for millennia to better understand the cosmos.
  • Total eclipses helped prove Einstein’s theory of relativity and led to the discovery of helium.
  • Here are seven cases where total solar eclipses have helped advance our understanding of science.

On April 8, millions of people in the United States will do the same thing: look up.

By mid-afternoon, day will turn to night, as a total solar eclipse affects 15 states.

We now know what causes a total solar eclipse. But our ancient ancestors from millennia ago weren’t so sure, and that was bad news for some scientists.

In the 21st century BCE, the Chinese emperor reportedly had two astronomers beheaded for failing to predict a total eclipse.

Over the centuries, total eclipses have become less disastrous and more of an opportunity to test scientific theories and make new discoveries.

Here are seven times a total solar eclipse helped advance human science.

1. Measure the rotation of the Earth

Aerial view of planet Earth covered in clouds.

The Earth’s rotation has slowed over the millennia.

Aleksandar Georgiev/Getty Images

Some of the earliest alleged records of eclipses date back thousands of years.

Some experts have suggested Petroglyphs or rock carvings found on a monument in Ireland refer to an eclipse that took place on November 30, 3,340 BCE. Others expressed skepticism.

Human-made markings on turtle shells from China and on a Babylonian tablet dating back more than 3,000 years may also contain ancient references to eclipses.

Although these records are open to interpretation, researchers have studied historical descriptions of eclipses for centuries. This is how 18th-century astronomer Edmond Halley first realized that the Earth’s rotation had slowed over the millennia.

2. Discover the causes of eclipses

The engraving of Pericles sits next to Anaxagoras, who sits on steps next to columns in ancient Greece

Anaxagoras (seated) studied eclipses to learn about the sun, moon and Earth.

Icas94 Image Library/De Agostini via Getty Images

Two modern scholars credit the Greek philosopher Anaxagoras of Clazomenes with discovering the Moon’s role in eclipses, calling him “perhaps the first empirical astronomer.”

He probably developed his theories after witnessing an annular eclipse, or “ring of fire”, on February 17, 478 BCE. Although he understood some scientific knowledge well, his overall understanding of the solar system was a product of his times.

Anaxagoras, for example, believed that atmospheric pressure kept the Earth flat aloft, at the center of the rotating sun, moon, and stars. Despite this error, he developed the basic mechanics of eclipses.

Anaxagoras correctly believed that the moon reflected the light of the sun. He was also correct in his theories that when the moon moved in front of the sun, it caused a solar eclipse. Likewise, when the Earth was between the sun and the moon, there would be a lunar eclipse.

He also used the shadow of the moon during an eclipse to estimate its size, but his calculations gave him an answer much lower than reality.

3. Estimation of the distance between the Moon and the Earth

A person sits looking through a telescope with astrological instruments nearby in ancient Greece

Hipparchus of Nicaea is shown observing the sky at the Alexandria Observatory.

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On March 14, 189 BCE, a total solar eclipse swept across what is now northern Turkey. The Greek astronomer Hipparchus was only a child at the time, but it is possible that he witnessed the event.

Years later, Hipparchus may have used second-hand accounts of this same eclipse to make one of the most precise mathematical estimates of the time of the distance between the Moon and Earth.

Although Hipparchus’ direct writings are lost, a 4th-century scholar detailed how he used this information.

The astronomer estimated the distance between where the total eclipse was in modern-day Turkey and Alexandria, Egypt, where one-fifth of the sun was visible, to make his calculation.

Based on his calculations, Hipparchus proposed a few ranges, including an average distance approximately 281,387 miles.

He wasn’t very far away. The moon is about 238,855 miles away.

4. Predict the path of an eclipse

A map of England showing the path of an 18th century eclipse

Edmond Halley’s map showing where and when a solar eclipse would pass over England.

Universal History Archives/Universal Images Group via Getty Images

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 will be centuries before humans make a more accurate prediction. In the 18th century, Edmond Halley, best known for discovering a comet bearing his name, made a map predicting the path of the solar eclipse of May 3, 1715 with extreme precision.

Others made maps before him, but Halley based his prediction on Isaac Newton’s theory of universal gravitation, which helped him pinpoint the time of the eclipse to within four minutes.

5. The discovery of helium

A drawing depicting several people sitting and standing around astronomical equipment in late 19th century India

British astronomers, including Norman Lockyer, prepare for the 1871 eclipse in India.

Science and Society Image Library/Getty Images

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 eclipse of August 18, 1868. He used a spectroscope, a prism-like device to separate 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 to observe the sun even without an eclipse. He saw the same line.

Lockyer called the mysterious element helium. It took scientists a few decades to see it on Earth, during experiments on the lava and uranium of Vesuvius.

6. Proof of Einstein’s theory of relativity

A building houses a large piece of astronomical equipment with a long tube and a rectangular box

The instruments used by the British expedition to observe the total solar eclipse in Brazil helped prove Einstein’s theory.

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Astronomer James Craig Watson was certain he had found evidence of a new planet during the eclipse of 1878. Supposedly located between the Sun and Mercury, Vulcan was only visible when the Moon blocked the giant star.

Several other eclipses occurred without anyone finding evidence of Vulcan’s presence. In 1915, Albert Einstein explained Mercury’s unusual orbit with his theory of general relativity. It fits the data better than a mysterious, hard-to-spot additional planet.

Despite this evidence, Einstein’s theory did not receive scientific proof until the eclipse of May 29, 1919. The physicist had stated that the sun’s gravity would deflect light from nearby stars.

In 1919, expeditions visited Principe, an island off the coast of Africa, and Brazil. As the moon blocked the sun, astronomers took photos.

The stars appeared to have changed location compared to the reference photos. The new apparent locations showed that the sun was bending light by the measures predicted by Einstein.

7. Study eclipses from space

A large satellite dish under a solar eclipse

The “eclipse of the century” took place over Wallops Station in Virginia.

Corbis via Getty Images

Gemini 12 astronauts Jim Lovell and Buzz Aldrin were the first humans to observe a total eclipse from space. The November 12, 1966 eclipse moved from Peru to Brazil, and 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 blurry and a little disappointing.

Four years later, television channels broadcast “the eclipse of the century”, rich in images of the event. NASA also launched more than two dozen rockets to study UV radiation and solar X-rays during the phenomenon.

The agency always uses rockets to collect data during eclipses and will launch three on April 8.


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