NASA's predictions for the August 2017 solar eclipse show that about half of Kansas City will see a total eclipse and the other half won't (they'll just see a "near" total eclipse, perhaps even Bailey's Beads?).

I thought this would be a great way of testing how accurate NASA's eclipse path prediction is: have people in Kansas City report, with location, whether they saw a total eclipse or not (of course, many people in Kansas City will probably head into the total eclipse zone just to see it, but hopefully few will remain outside it).

It then occurred to me that someone must've already done this for previous eclipses (I know it's done for lunar occultations of other stars).

However, googling only tells me that NASA's 2017 path predictions are more accurate than ever before, but not actually HOW accurate they are.

Where can I find data about accuracy of eclipse predictions?

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    $\begingroup$ I'd expect NASA is pretty darn accurate. They're very good at their job. Besides, how can you be sure people can be accurate enough to verify NASA? I'm sure NASA can specify the border of where a full eclipse occurs to a much much better accuracy than an individual can distinguish between a full eclipse or a 99.99% full eclipse. $\endgroup$ – zephyr Jul 18 '17 at 17:10
  • $\begingroup$ @zephyr I disagree. The Sun is really really bright. There should be a very noticeable difference between 100% eclipse and 99.999999% eclipse. That's why we have effects like Bailey's Beads $\endgroup$ – user21 Jul 18 '17 at 17:14
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    $\begingroup$ Can you be confident the average person would know that difference though? I'm certainly not. $\endgroup$ – zephyr Jul 18 '17 at 17:19
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    $\begingroup$ You're confusing accuracy and precision. You see that the predictions are precise, but you want to know if they're accurate. $\endgroup$ – AtmosphericPrisonEscape Jul 18 '17 at 17:24
  • $\begingroup$ @zephyr Even if only one billionth of the Sun is showing, it's brighter than Sirius. I'm thinking this is do-able... and, more importantly, it's been done. $\endgroup$ – user21 Jul 18 '17 at 17:29

The path of totality, which eclipse maps show to be 70 miles wide, is actually narrower than that, by up to 1 mile, according to some experts.

An article by the Kansas City Star discusses this:

Those maps, provided by NASA and others, show a crisply defined, 70-mile-wide path of totality where the moon will block 100 percent of the sun. But they are not as precise as they appear, at least on their edges.

The southern edge of the path as shown on the maps could be off by as little as the length of a football field or as much as a half-mile, eclipse mapping experts say. Likewise for the northern edge, meaning the path of totality might be just 69 miles wide.

“This is an issue. This is really an issue, but it’s not advertised. … Yeah, all the maps are wrong,” Mike Kentrianakis, who is the solar eclipse project manager for the American Astronomical Society and who routinely consults with NASA, told The Star.

Xavier Jubier, a French engineer whose calculations have been used to create the interactive Google maps of the eclipse, confirmed to The Star by email that the actual path of the totality is slightly narrower than the 70 miles shown on current maps.

Ernest Wright, who created maps and other multimedia presentations on the eclipse for NASA, said he thought the map might be narrower by about 100 meters, slightly longer than a football field. He also said it’s possible that Kentrianakis is correct in his estimation that the path is narrower by a half-mile or more.

The article explains the reason the path of totality is narrower than the eclipse maps indicate is because they use a 41-year old value for the radius of the Sun, which is now known to be too small a value:

Wright explained that eclipse maps are made based on what is known about the relative sizes and positions of the moon and the sun. “We have really good information about the orbit of the moon, the positions of the sun, the positions of the Earth. All of that is really well nailed down,” Wright said. “In order to get more accuracy, we need to take into account the mountains and valleys on the moon, and the elevations on the Earth. And we’re starting to do that, as well.”

The size of the moon, in fact, has been measured to within a meter, and its position in the heavens has been measured to within a centimeter. “But the last sort of uncertainty might surprise you,” Wright said. “It’s the size of the sun.”

Jubier said that the current maps are accurate using the the 696,000-kilometer radius and other standards agreed upon in 1976 at a meeting of the International Astronomical Union. “This is perfectly accurate but we know it does use a solar diameter that is not large enough. Why don’t we change the value(?)” Jubier wrote. “Well simply because the IAU (International Astronomical Union) has not yet approved a new value. This is part of the research we’re doing and for which we’re looking for funding.”

He continued, “So technically speaking if the Sun is larger than the adopted IAU value, and we know it is, the eclipse path is necessarily narrower and our tools can simulate this, yet the standard maps for the public will still retain the currently adopted solar radius until a new value has been accepted."

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  • $\begingroup$ Thanks! svs.gsfc.nasa.gov/4518 notes that the most precise maps they have "are high-resolution shapes with roughly 100-meter precision", which matches nicely with what you found. If you download data from HORIZONS for the Sun, it says both "Radius (photosphere) = 6.963(10^5) km" AND "Solar Radius (IAU) = 6.955(10^5) km", two slightly different values. $\endgroup$ – user21 Aug 14 '17 at 1:33

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