A long-exposure photo shows the phases of the moon over a NASA building
A total lunar eclipse over NASA's Armstrong Flight Research Center. Photo by Ken Ulbrich, courtesy NASA

On Oct. 14, the sun will briefly go dark, casting the world into an eerie, brief twilight. On April 8, 2024, it’ll happen again. In an unusual bit of luck, the next six months will see Texas blessed by not one, but two eclipses, an astronomical marvel that’s already drawing fascinated attention—and not a few tourists—to the Lone Star State. But with all this heavenly hubbub, a person might be forgiven for wondering—what are eclipses? And how on earth do astronomers know how to predict them?

First, it’s important to remember that objects within the solar system don’t hang suspended in the heavens: They orbit one another in a vast cosmic dance, looping on their separate paths. Sometimes, those paths cross in front of one another, and as one astronomical object passes another, its shadow blocks out the light. That’s an eclipse. A common example of this is a lunar eclipse, which occurs when the earth passes between the moon and the sun, says Adam Kraus, a professor of astronomy at The University of Texas at Austin. But sometimes, the opposite occurs: The moon passes at just the right spot to shroud the sun, blanketing specific parts of the earth in shadow.  

Sometimes the result is a partial eclipse. Sometimes—as is due to occur in October—the conjunction happens when the moon is at its farthest point from Earth, resulting in an annular eclipse that can’t quite cover the sun. Other times, the moon is close enough to block the sun out entirely.

Two Types of Solar Eclipses

A solar eclipse occurs when the moon passes between the sun and the earth. Sometimes, this conjunction happens when the moon is at its furthest point from Earth. This is an annular eclipse, and will be visible over parts of Texas in October.

Other times, the moon is close enough to block out the sun entirely, causing a few minutes of complete darkness over a portion of Earth. This is called a total eclipse. The phenomenon occurred over the United States in 2017 and will happen again over portions of Texas in April 2024.

In general, someplace on the earth sees an eclipse every six months, Kraus says. However, having two solar eclipses back-to-back at the same location is very rare. “A specific location only sees a total or annular eclipse every few hundred years, pretty much at random,” he explains. “So we’re extremely lucky to have two events occurring within six months of each other.”

According to Kraus, these eclipses are easy to predict because the looping paths of orbiting objects are extremely easy to track. Astronomers have precisely measured the moon’s orbit, thanks to thousands of years of records of the moon’s location in the sky, and some extraordinarily accurate measurements in recent decades. Texas astronomers, specifically at the McDonald Observatory, are partially responsible for that, Kraus says—when astronauts from the Apollo mission visited the moon, they left behind reflective mirrors on its surface. That allowed telescopes around the world, including at the McDonald Observatory in West Texas, to bounce lasers off the mirrors and read the returning signal.

“This means we can predict the precise relative positions of the sun, moon, and Earth out to thousands of years in the future,” Kraus says. “In fact, you can actually go out onto the internet and find tables of all these future eclipses. In particular, the relative arrangement can be used to predict where the sun’s shadow will fall.”

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In this case, the moon’s orbital path means that its shadow will be falling on Texas twice in six months. “Broad swaths across Texas will fall into the shadow of at least one of the events, and lucky folks west of San Antonio will actually find themselves right in the middle of both paths,” Kraus says.

While this year’s eclipse will be an annular eclipse, resulting in a fiery ring around the black moon (thus, why this type of eclipse gets called the “ring of fire”), next year’s will be more spectacular. During the total solar eclipse in April, there’ll be a few minutes at the midpoint where the sun will be completely blocked, allowing people to glimpse the sun’s corona: the gossamer, extended blanket of superheated gasses that sunlight usually renders invisible.

Eyeing the Eclipse

An eclipse is a remarkable natural phenomenon, but it’s important to take care when you look at it. The sun is almost a million times brighter than the full moon, says UT Austin astronomy professor Adam Kraus. He warns that even a tiny sliver of its surface still puts out enough light to do serious damage to your eyes. That’ll be particularly true during October’s annular eclipse, during which the outside rim of the sun will still be visible. The only time it’s safe to look at an eclipse directly will be when the sun is completely covered, which’ll only happen for a scant few minutes during April’s total eclipse. Kraus recommends using eclipse-viewing glasses that filter most of the sun’s light. But don’t just grab them off Amazon: Counterfeits are a serious risk to your eyesight. The American Astronomical Society has a list of reputable manufacturers, including  Lunt Solar Systems, American Paper Optics, and Rainbow Symphony.

The June 2024 cover of Texas Highways: Treasures from the Coast

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