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By Tom Metcalfe

Solar eclipses are among the most dramatic astronomical events around — just ask anyone who’s seen one. For a few minutes, a black disk seems to move across the sun, and daylight slowly gives way to an eerie darkness as the temperature falls.

The awesome spectacle affects wildlife as well as humans, changing the behavior of birds, cows, insects and other animals. But then, as quickly as it came, the darkness lifts and the sun shines again with its usual strength.

Solar eclipses, or eclipses of the sun, are rare events in part because each is visible only from a limited area of Earth’s surface. Parts of the U.S. saw a total solar eclipse on Aug. 21, 2017 — and the next total solar eclipse in the U.S. won’t come until 2024.

What causes a solar eclipse?

Solar eclipses occur when the moon comes between Earth and the sun, blocking sunlight from our host star.

“It's when the moon's orbit brings it directly between us and the sun, so it casts a shadow on us,” says astronomer Caleb Scharf, director of Columbia University's Astrobiology Center in New York City. “Since the moon is much smaller than Earth, the shadow is only about 100 miles across, but it sweeps over the Earth's surface.”

People near the center of the moon’s shadow — what’s called the umbra — see a total solar eclipse, in which almost all of the sun’s light is blocked. People near the edges of the shadow, in a region called the penumbra, see a partial solar eclipse, meaning that the sun is only partly obscured.

During so-called annular eclipses, a ring of sunlight appears around the dark disk of the moon.

A lucky coincidence

Solar eclipses occur only because of a celestial coincidence: though the sun’s diameter is about 400 times that of the moon, the sun also lies about 400 times further away. “The moon appears almost the same size in the sky as the sun, so Earth experiences nearly perfect solar eclipses,” Scharf says.

That wasn’t the case in Earth’s distant past, which stretches back more than 4 billion years. In our planet’s early days, the moon was much closer than it is now. Millions of years from now, as the moon’s orbit gradually widens, our satellite will be farther away. That means eclipses won’t be quite the same.

“It'll look smaller,” Scharf says of the moon, “and so eclipses will never be total again.”

How often do solar eclipses occur?

On average, there are two total solar eclipses somewhere on Earth every three years. But some years there are no total solar eclipses, and the average time between total solar eclipses in one place is more than 300 years. That’s because the moon’s orbit around Earth is slightly tilted compared to Earth’s orbit around the sun. “It's either too high or too low compared to Earth's orbital plane,” Scharf says. “So we have to wait for those times when it does just line up right.”

A related phenomenon occurs when Earth’s shadow falls on the surface of the moon. Because our planet’s shadow is much bigger than the moon’s, lunar eclipses occur more frequently than solar eclipses — usually two times a year.

Solar eclipses in history

The first successful prediction of a solar eclipse is attributed to the Greek philosopher Thales of Miletus in 585 B.C. The eclipse was visible in Asia Minor — roughly the region occupied by Turkey on today’s maps — and was said to occur during a battle between the Medes and Lydians, who were so awed by the spectacle that they agreed to a truce after years of war.

In ancient times, solar eclipses were often interpreted as bad omens for royal dynasties. In Assyria, a substitute king and queen would be installed for the day of an eclipse. The unfortunate pair would be executed after the eclipse.

Solar eclipses have also served to advance scientific research. One of the most notable instances came in 1919, when English astronomer Arthur Eddington used the apparent locations of stars near the eclipsed sun to test Albert Einstein’s theory of general relativity.

“Eddington was just able to measure how the distant stars seemed to shift their positions because of the mass of the sun distorting space,” Scharf says. “It was one of the things that made relativity famous.”