Why Your Total Solar Eclipse Map Is Suddenly Wrong

There are dozens of fabulous eclipse maps and eclipse map apps available for April’s total solar eclipse—some of them have even become ultra-successful memes.

But they all have something unexpected in common—they’re wrong.

For those heading far into the path of totality—which about 115 miles wide, on average—it has little consequence. But for locations on the edge, where most maps suggest that totality will be brief, there’s now a risk that it won’t happen at all.

The reason is surprising: scientists don’t know the sun’s diameter, which leaves eclipse maps—projections of its shadow—all askew.

Thankfully, a new map is putting things right, just in time.

Precise Geometry

The precise geometry of the path of totality—the moon’s shadow cone as it tracks across a rotating Earth—traditionally depends on calculations made using a set of values called Besselian elements. Of critical importance is the distance of the moon from the Earth and the exact shape of the moon—but also of the sun.

“The solar radius is one of the parameters that enter into eclipse computations,” said Luca Quaglia, an eclipse calculator based in Australia who has been helping determine the solar radius since 2013, in an email. “But the choice of its value is really left to the eclipse computer.”

The standard value is 959.63 arc seconds (an arc second being one 3,600th of a degree) and has been accepted since the late 1800s. It’s used in most eclipse maps and effectively determines the edge of the path of totality.

But it’s wrong. The sun appears to be larger.

An inaccurate solar radius figure is making eclipse maps out by as much as about 2,000ft./600 meters at their edges. “Measurements and observations in the last decade have demonstrated that this value is slightly too small,” said Quaglia. It’s more in the ballpark of 960 arc seconds, he said, which is enough to affect where the edge of the moon’s shadow is during a total solar eclipse.

The ‘New’ Eclipse Map

Quaglia’s collaborator is John Irwin, a master in eclipse computations who published a new map last week that uses a solar radius of 959.95. It has alarming consequences for cities dissected by the edges of the path of totality, such as Montreal in Canada. The city’s Cité Jardin and its park—which contains the Olympic Village and the Olympic Stadium—are within the path of totality in standard eclipse maps. On Irwin’s, Cité Jardin is just outside.

The variations exist because Irwin doesn’t use Besselian elements, instead starting with the exact topography of both the moon and Earth. Together with the “new” radius of the sun, this means slightly different—but more accurate—eclipse maps.

Using The ‘New’ Eclipse Map

The entire northern edge is narrowed, while the southern edge is extended in Texas and narrowed everywhere else. You can see exactly how if affects your town by using KML files, which are used to display geographic data in an Earth browser such as Google Earth (either online or the desktop software):

Moon’s Topography

Why the difference? The bigger the sun’s diameter, the smaller the moon’s shadow and the narrower the path of totality. The topography of the moon’s limb—now known in great detail thanks to NASA’s Lunar Reconnaissance Orbiter—also makes a difference. “Almost all eclipse limits in published maps are the ‘smooth lunar limb’ ones, and they can be in some cases over five kilometers from the ‘true limb’ ones,” said Quaglia.

So, the path of totality is not only narrower than it looks on most maps, but it also has jagged edges.

Grazing Zones And ‘Double Diamond Rings’

You can stand where the shadow of a mountain on the moon will cross over you, allowing you to see the sun’s corona for a second. Or maybe there’s a valley, and you’re denied any totality. This is a grazing zone. “I think of the grazing zone as the strip between the classic line and the ‘limb line’—but I’m the only person who computes that—I haven’t seen it on any other maps,” said solar eclipse cartographer Michael Zeiler at GreatAmericanEclipse.com, in an interview. Zeiler has taken this to another level by developing maps showing exactly where it will be possible to see a “double diamond ring.”

Fuzzy, Jagged Edges

If accurate maps require eclipse computers to move away from the idea of the moon as a perfect sphere, the same goes for the sun. “Eclipse computations are done, for simplicity, assuming that the sun is a perfect sphere, so the resulting eclipse limits are simple lines,” said Quaglia. “But the edge of the photosphere is likely not as sharp, and there is a transition from the photosphere into the atmosphere of the sun.”

It means fuzzy edges to the path of totality where exactly what spectators will experience is unknown.

Glimpsing ‘Coronality’

In practice, observers a few hundred meters on either side of the limit will probably briefly see the solar corona (what Quaglia and his collaborators call “coronality”). What will differ is whether the Baily’s beads disappear—whether the sun’s disk is ever 100% eclipsed.

“How far outside the eclipse limits the solar corona ceases to be visible is anyone’s guess,” said Quaglia. “There is no experimental data to back any distance up.”

Next week, in a repeat of a 2017 experiment in Vale, Oregon, and Australia last year, Quaglia and collaborator Alessandro Pessi will observe the eclipse about half a kilometer inside the northern limit in Texas. “Total solar eclipses offer unique opportunities to perform measurements in the absence of the glare of the photosphere,” he said.

Just before and after totality, they will put a grating on a regular camera and take images that separate the light from the just-eclipsed sun into its spectral components.

Flash Spectrum Explained

During that time—and just before the end of totality—parts of the sun’s atmosphere, normally obscured by the photosphere, its chromosphere and the corona, are visible down to the sun’s surface. The result will be a time-stamped flash spectrum, light curves in which the exact light sources can be seen within a very condensed region of the sun’s atmosphere.

“We want to probe that region to determine when the photosphere is extinguished to be able to estimate the eclipse solar radius,” said Quaglia. Time-stamped flash spectrum data can determine the solar radius during a solar eclipse more accurately than any other method.

The ‘New’ Solar Radius

As it stands, there is no agreed “new” solar radius, but Quaglia hopes that his and others’ work may, given time, be reproduced by others. One day, perhaps, new values for the “eclipse solar radius” “might be widely adopted—and we’ll finally have an accurate figure for the sun’s radius. Unless, of course, it changes. “There is evidence that the solar radius has changed over the centuries,” said Quaglia. “We need a methodology that reliably, repeatedly and accurately allows for the measure of the solar radius during an eclipse.”

For the latest on all aspects of April 8’s total solar eclipse in North America, check my main feed for new articles each day.

Wishing you clear skies and wide eyes.