Solar eclipse maps show crisp lines for the path of totality, the narrow strip where a total solar eclipse will be visible. But in reality, the edges of the path are fuzzy, jagged and sometimes wrong by hundreds of meters or more. That might not matter if you’re standing on the path’s centerline, where many eclipse chasers head to experience the longest duration of totality. However, for those at the edge of the path, discrepancies could be the difference between seeing the sun’s corona with the naked eye — one of nature’s most incredible sights — and merely witnessing a crescent sun through solar eclipse glasses.
It’s potentially a big issue for cities whose suburbs will be within the northern or southern limit of the path of totality for the Aug. 12, 2026 total solar eclipse, such as Madrid, Bilbao, and countless small towns and villages in Spain and Iceland.
Why the maps are imprecise
The reason those neat lines on solar eclipse maps don’t always match reality is that the sun’s actual size is still a matter of debate. Eclipse paths were long calculated using Besselian elements, a set of parameters that assume spherical bodies and a fixed solar radius. However, these values are outdated.
“The canonical standard solar radius we have been using for over 100 years in eclipse computation is around 696,000 kilometers [432,000 miles] or 959.63 arcseconds,” eclipse computer Luca Quaglia said June 13 at the Solar Eclipse Conference in Leuven, Belgium. That figure was initially published in 1891 by German astronomer Arthur Auwers. “But if you use that standard value, you will be off,” Quaglia said.
Related: Solar eclipse sights might vary on the edge of totality
Quaglia’s work over the past decade during total solar eclipses, as part of the Besselian Elements Team, suggests that the sun’s apparent size is closer to 959.95 arc seconds, plus or minus 0.05. That’s a difference of just 0.3 arc seconds — less than one-thousandth of a degree. That may sound nitpicky. Yet, on the ground, it shifts the edge of the path of totality by up to 2,000 feet (600 meters). It’s a tiny problem with big consequences for communities on the edge of a path of totality.
Going to the edge
To test and retest that new figure for the solar radius, Quaglia and his colleagues don’t bask in maximum totality at the centerline during total solar eclipses — they go to the edge. By timing the second and third contacts — the moments when the photosphere (the visible surface of the sun) disappears and reappears — with GPS-time-stamped flash spectrum data from frames of video, Quaglia and colleagues can directly test which radius value matches reality. Key to that is the appearance of Baily’s beads — the last and first drops of sunlight streaming through valleys on the moon — that signal the beginning and end of totality.
“Flash spectra are absolutely accurate because they can show you exactly the instant when a Baily’s bead appeared or disappeared,” Dan McGlaun, an eclipse cartographer who uses the new solar radius of 959.95 arc seconds in his eclipse maps (as does Michael Zeiler), told Space.com. “You can use that to back-calculate what the Besselian elements would have had to have been at that location for the beads to disappear at that time, including the radius of the sun. It’s trigonometry.”
The “zone of uncertainty”
Quaglia and colleagues collected their data from a point just outside the edge of the path — as predicted using other measurements of the solar radius — in Stephenville, Texas, on April 8, 2024. Even before that eclipse, Quaglia, working with mathematician John Irwin, had already packaged the math to give new, more accurate local circumstances. Most recently, they’ve produced special new eclipse maps for Spain for Aug. 12, 2026, that diverge from other publicly available maps.
These new maps replace the precise-looking line that marks the limits of the path of totality with a “zone of uncertainty” at the edge. That’s partly because, in addition to being uncertain, the sun’s radius is not sharp. In reality, the sun’s photosphere — its bright face — has fuzzy edges that fade into the chromosphere and corona.
“The photosphere is not a solid object,” Quaglia said. That accounts for the plus-or-minus 0.05-arc-second error bar on any calculation of the solar radius.
That means observers a few hundred meters outside the limits of any calculated path of totality may still glimpse the corona — or not. This “coronality” — the chance of glimpsing the solar corona just outside the path of totality — is unpredictable. It’s impossible to determine how far outside the eclipse limits the solar corona will cease to be visible.
The shape of the moon
The moon also introduces uncertainties. During a total solar eclipse, the path of totality is the projected dark shadow (umbra) of the moon. Traditional maps assume a smooth lunar limb, but valleys and mountains let sunlight leak through as beads. Using the true jagged profile, revealed in detail by NASA’s Lunar Reconnaissance Orbiter since 2009, can shift eclipse timing by up to 3 miles (5 km).
“We need to account for the moon’s complex, jagged shape as accurately as possible if we want to get the most accurate internal contact time predictions,” Quaglia said. Calculations that use a smooth lunar limb can result in path limits that are up to 3 miles different from those calculated using the true limb.
Making 3D eclipse maps
Another issue is that eclipse maps are flat, but the real Earth isn’t, with mountains and valleys blocking or extending visibility. That’s why Irwin has pioneered a new 3D computational method that accounts for not only the sun’s radius and the moon’s topography but also — uniquely — Earth’s terrain. It’s all packaged into an algorithm called Improved Quick Prediction, which powers the new maps and an upcoming Eclipse Countdown smartphone app.
The team’s eclipse map for Aug. 12, 2026, is different from others. It features jagged edges and even gaps where the mountainous terrain alters the view. Sections of the limit hug mountain ridges, sometimes even leaping over peaks where the shadow can’t reach. That’s the reality eclipse chasers must face, Quaglia said — no single line on a map can guarantee what you’ll see.
“We believe these are the most accurate contact times you can compute nowadays by including all the vectors that some other predictions don’t include,” he said.
A variable star and Earth’s changing rotation
Precision in astronomy is everything, but when it comes to the sun, it will always be somewhat elusive. The sun is a variable star. Its magnetic activity waxes and wanes across the 11-year solar cycle, with the effect on the solar radius thought to be tens of milliarc seconds.
“It’s why it would be great if this experiment was done at future eclipses, maybe over a whole solar cycle,” Quaglia said. “That would be the only way to see any variability.”
Another issue for eclipse cartographers is delta T. Even if the exact position, width and shape of the moon’s shadow are known, exactly where it strikes Earth depends not only on its terrain but also on how fast it rotates.
“The rotation of the Earth can change even though the shadow is the same,” McGlaun said. “Even a tenth of a second can change where on Earth the shadow hits.” Although it’s known that Earth’s rotational speed does change by milliseconds, we don’t know what that figure is until it’s measured by the International Earth Rotation and Reference Systems group’s atomic clocks in Paris.
The math is complex, uncertain and evolving. For anyone living close to the limits of a path of totality during a total solar eclipse, the message is simple: Head a mile or two in the direction of the centerline, and a glorious moment of totality is guaranteed under clear skies.
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