A total solar eclipse is coming to North America on April 8. The Great North American Eclipse, as it has been dubbed, will be visible across 13 US States, plus parts of Mexico and eastern Canada. But it will not look the same for everybody.
For those living along the path of totality—the projection of the moon’s shadow on the Earth’s surface—the celestial event is bound to delight. For everyone else, experiencing the total eclipse will require extensive planning … which, if you’re organized, should’ve already happened, but let’s face it, it probably has not.
I, for one, was considering Lexington, Kentucky, as my viewing venue. I had read that the eclipse there would be “deep partial,” but having never before witnessed a solar eclipse, I wasn’t sure whether that was good enough. My quest to find out led me to a nifty solar eclipse simulator that helps users visualize what April’s solar eclipse will look like from any city, town, mountain peak, or desolate patch of land in the northern hemisphere. To feed your imagination as you “try out” various locations, the simulator lets you set choose one of more than 50 landscapes to match the vibe you’re looking for—a city skyline, a snowy mountain range, or a placid lakefront. You can drag a slider on a timeline and watch the sun and the moon glide across your screen until they become one as the sky turns a dusky shade of blue.
The tool, which is based on centuries-old astronomical calculations as well as modern data, was built by Dan McGlaun, a retired mathematician from Purdue University and a self-professed geek who has been chasing eclipses since he was 10 years old. To date, McGlaun has witnessed 15 eclipses from incongruous places like airplanes and cruise ships. “I went to Kenya for an 11-second eclipse and it was the best day of my life,” he says.
The solar eclipse simulator is a side feature of McGlaun’s main website, through which he sells eclipse safety glasses. (These protective shades are an absolute must if you’re planning to look up at the sky during the eclipse.) But it only takes a minute on the phone with McGlaun to understand that the simulator is an absolute labor of love and the safety glasses business exists to fund it.
In 2017, McGlaun built a smartphone app that allowed people to choose an eclipse-viewing location on a map of the US to find out whether their chosen spot was in the path of totality for the eclipse during August of that year. Now, he’s furthered his effort to “evangelize eclipses” by building a more complex simulator, this one with an educational bent.
It took less than a minute of tinkering for me to understand that I would not be spending April 8 in Lexington, Kentucky, for the simple reasons that a partial eclipse—however “deep”—was nowhere near as impressive as the total eclipse I saw (on my screen) in places like Dallas, Texas, or Mazatlan, Mexico. Other notable locations in the path of totality are Russellville, Arkansas; Carbondale, Illinois; Indianapolis, Indiana; Cleveland, Ohio; Erie, Pennsylvania; Buffalo, Rochester, and Syracuse in New York; northern Vermont; central Maine; central New Brunswick, Canada; and central Newfoundland, Canada.
As I travel-hopped in the virtual shadow of the moon, I learned that, on top of the sky going dark during a total eclipse, the horizon turns amber. I learned to watch out for Baily’s Beads, the arc of bright spots visible immediately before and after totality that form when the sun’s rays shine between the mountains on the moon’s surface. And I learned that the corona, the sun’s outer atmosphere, is typically obscured by the sun’s glare but becomes visible during an eclipse. The simulator lets you visualize about 20 different patterns that look like wispy stars but are in fact streamers of charged gas. McGlaun says the corona is a “living, breathing atmosphere,” the shape of which is extremely hard to predict until about two weeks before the eclipse.
Cosmic Calculations McGlaun spent two years planning and creating the simulator, plus several months doing the math needed to run it. What might raise an eyebrow is the fact that the basis of those calculations date back centuries.
“We’ve known [there’s going to be an eclipse in 2024] for hundreds of years,” says McGlaun. “The Babylonians could’ve predicted it.” Babylonians were keen astronomers who lived in ancient Mesopotamia and kept meticulous records of the heavens on clay tablets for hundreds of years. Beginning around the 7th or 8th century BCE, they discovered a recurring pattern in solar and lunar eclipses known as the Saros Cycle, which they used to predict future eclipses.
The Babylonians’ understanding laid the foundation for later advances in astronomy. Today’s astronomers use sophisticated mathematical models known as astronomical ephemerides. These help us predict the positions of the sun and the moon—and therefore both lunar and solar eclipses—thousands of years into the future.
To build the solar eclipse simulator, McGlaun pulled from several databases, including astronomical ephemerides, but also NASA’s Lunar Reconnaissance Orbiter, which has been circling and mapping the moon for 15 years and helped him simulate exactly which slice of the moon will be visible at any given time during the eclipse. He also used an elevation map of the Earth, because if you’re viewing the eclipse from Alma, Colorado (elevation 10,522 feet), you will be closer to the moon than if you’re viewing it from Providence, Rhode Island (elevation 10 feet).
The end result is as accurate as scientifically possible, though McGlaun notes there are many variables to contend with, like the fact that the sun pulsates in a way that causes its diameter to change ever so slightly, or that the climate crisis—and the resulting massive melting of glaciers—has slightly shifted the planet’s rotation and influenced the speed at which it spins. These factors could influence the orientation of the Earth relative to the Moon’s shadow and upset McGlaun’s calculations—only by a matter of milliseconds, but that’s a lot for a mathematician.
So, unless you’re a hairsplitting scientist, you’ll find the solar eclipse simulator to be just fine, especially if, like me, you still can’t decide where to go and still haven’t booked your travel. And if you really can’t make up your mind, or if you’re not graced with clear skies, McGlaun invites you to use the simulator as a “livestream” and watch an animation of the eclipse on the website as it’s happening. You won’t need the safety glasses for that.
Update, April 6 at 7:30 am: This article was updated to include more details about the specific locations that can be found in the path of totality.
