NASA
On July 21, there will be a full 90 minutes of totality as the moon's shadow sweeps across the country. The moon moves in front of the sun and briefly snatches it away from us.

SALT LAKE CITY — When millions of Americans see a total solar eclipse Monday, or a partial eclipse in Utah, they'll join the ranks of people throughout history who have been amazed — or terrified — by standing in the shadow of the moon.

It's a phenomenon that continues to thrill people — and engage their scientific curiosity.

"It's just so miraculous, you can't really describe it," said Cory Bauman, a Salt Lake resident who has traveled around the world to see three total solar eclipses.

A simple, stunning coincidence makes it possible: The sun is 400 times larger than the moon, but it's also 400 times farther away. So when an eclipse is viewed from Earth, the moon just about perfectly covers the sun and snatches it away, creating what some eclipse watchers call "a hole in the sky."

The phenomenon tends to give most people a sense of wonder.

"We look to the sky for answers for all of the questions that we have here on Earth," said Samuel Singer, executive director of Wyoming Stargazing. "I think solar eclipses as well as dark skies have captivated people for tens of thousands of years."

The earliest observers evidently feared eclipses — and some cultures still do. But early on, some sky watchers figured out how to predict them with uncanny accuracy, and many ancient people were eager to see one. Socrates warned Greeks more than two millennia ago that they could injure their eyes by staring at the sun during an eclipse.

The science of predicting solar and lunar eclipses advanced so much over the centuries that by the time Christopher Columbus visited the New World he could use his knowledge of eclipses to intimidate indigenous people.

In 1504, Columbus' ships were stranded for repairs on the island of Jamaica. When local natives tired of providing supplies, Columbus told the tribe that God would take away the moon if they didn't provide food. He knew a lunar eclipse was coming because he was carrying documents that had detailed predictions of future eclipses.

When the lunar eclipse took place right on schedule, the natives trembled in fear and agreed to supply food.

Scientists have traveled to eclipses for generations, often to far-flung places around the world under difficult conditions.

Famed inventor Thomas Edison had it relatively easy in 1878 when he traveled on the new transcontinental railroad to visit Wyoming for a total solar eclipse. That same eclipse drew numerous scientists to the summit of Pike's Peak in Colorado so their instruments would have a better view.

To this day, scientific tools are still trained on nearly every solar eclipse.

"There's high-altitude balloons, there's chains of cameras, there's orbiting spacecraft," said Seth Jarvis, director of Clark Planetarium. "There's always something left to learn. So studying the sun's atmosphere, the corona, is really best done when the moon covers up the sun. And then you can see that very fine wispy (solar) atmosphere."

Continuing scientific curiosity about the corona is what has prompted an unusual experiment being conducted during Monday's eclipse.

Julia Kamenetzky, assistant professor of physics at Westminster College, will lead one of 68 teams participating in the Citizen CATE (Continental-America Telescopic Eclipse) experiment. They'll try to film a full 90 minutes of totality as the moon's shadow sweeps across the country.

"The unique part of the eclipse that we're going to be studying in the Citizen CATE experiment is the solar corona," Kamnetezky said. "And it's really only visible during a total eclipse."

Each team will shoot two or three minutes of totality at their individual locations, and it will all be stitched into a single movie so scientists can observe how the corona changes over an extended period.

Although eclipses have always been a subject of scientific curiosity, an eclipse a century ago provided one of the greatest moments in science history.

Einstein stunned the world in the early 1900s with his startling theories of relativity. One of Einstein's bizarre conclusions was that light must bend around the sun because the gravity of large objects in space must warp the fabric of space-time.

Sure enough, scientists during an eclipse in 1919 were able to observe stars normally hidden behind the sun. Einstein was right.

"Starlight from behind the sun comes past the sun," Jarvis said. "But because the sun is bending the fabric of space, the star appears to have shifted in its location."

That observation helped confirm a theory that later led to important breakthroughs in science and technology.

"It was a huge deal," Jarvis said.

Now, thanks to modern mobility, almost anyone has a shot at seeing a total eclipse someday.

"I think the rarity of it is what makes it such an exciting thing," Singer said. "I've never seen a total solar eclipse before. And the chances are this might be the only one I get to see."

From northern Utah, a dedicated eclipse watcher would have to travel to Wyoming or Idaho to enjoy the sight that made the ancients tremble: totality, a hole in the sky where the sun is supposed to be.