Figure it out
Michael Kentrianakis, an avid eclipse chaser and a member of the American Astronomical Society's Solar Eclipse Task Force, learned about the confusion over the sun's size from his colleague Luca Quaglia, a physicist and eclipse researcher.
"The straw that broke the camel's back," Kentrianakis said, came during an expedition to Argentina in February, where he positioned himself outside what should have been the edge of an annular eclipse — where the moon is circled by a bright "ring of fire." A larger sun would make the "ring of fire" effect visible to a wider area.
"Technically, I should have been outside of annularity, [but the unfiltered photographs show] we were still in the path of annularity, and we have this beautiful chromosphere circling around at the edge," Kentrianakis said. That experience fully convinced him the sun was larger than generally thought.
This upcoming eclipse — which will very likely be the most-watched total solar eclipse in history, NASA officials have said — will provide a chance for others inside and outside the path of totality to help verify its size.
While researchers would ordinarily use the radius of the sun to compute exactly when the moon will cover and uncover the sun for a given location, called contact times, the opposite strategy is required here, Quaglia told Space.com. "If we can measure contact times accurately, everything else being the same, the only thing that can change is the solar radius. We can actually compute the solar radius that way," he said.
Kentrianakis, Jubier, Quaglia and others want to pin it down by positioning researchers inside and outside where totality should be, armed with the equipment for what's called a "flash spectrum" photograph. The process uses a textured grating over a camera, which splits incoming light into component wavelengths — making it easy to determine precisely when the entire photosphere has been covered by the moon, revealing a more limited set of wavelengths emitted by the chromosphere. Combined with accurate timestamps, that process would provide strong evidence for the sun's size. (Such a process has been used before, but on a limited scale, Quaglia said.)
Such measurements would also provide another benefit, Jubier said — investigating what some think is a thin layer in between the photosphere and chromosphere called the mesosphere. That thin layer can be visible for a moment after the photosphere is blotted out during an eclipse, which means observers may make measurements that confuse the mesosphere for more of the photosphere. A flash spectrum can help distinguish between the two, although it must be a high enough resolution so the signals from each can be clearly separated.
A group involving Quaglia, Kentrianakis, and Jubier was unable to get funding for as broad a flash-spectrum experiment as they would have liked — something like 30 separate measurement stations arrayed just inside and just outside the predicted eclipse path. But researchers could still use crowdsourced data and measurements during the eclipse to learn more.
"The more observations we have the better even if they are not providing the kind of quality we expected to get from the cinematographic spectroscopy," Jubier said. "Time will tell what we can make of all this."
Jubier said that flash spectrum measurements would be most useful, but so would (safely!) unfiltered views of the eclipse. Most filters cut out details of the images, making it much more difficult to determine precisely when the sun fully covers the moon.
Other groups will also be using the eclipse to try and measure the sun's diameter, Quaglia said, including the International Occulting Timing Association, which will analyze smartphone videos taken at intervals perpendicular to the eclipse path in Nebraska.
"The more people, the more techniques, the more teams involved will get us there as a whole," Quaglia said. "If, then, the International Astronomical Union makes the decision to change the value, they will probably not change the value lightly."
Understanding the visible sun's exact size will be possible only by combining careful solar measurements with the simulations and precise understanding of the moon's and Earth's elevations that exist now, Jubier said. But the pieces are in place to make that determination, if enough people get on board to measure the most common sight in the sky during those uncommon moments of eclipse.
"It's big, and it will take many eclipses — it may take until 2024 — but at least we're starting it now," Kentrianakis said.
Editor's note: Space.com has teamed up with Simulation Curriculum to offer this awesome Eclipse Safari app to help you enjoy your eclipse experience. The free app is available for Apple and Android, and you can view it on the web.
Original article on Space.com.
The Best ISO-Certified Gear to See the 2017 Solar Eclipse
2017 Solar Eclipse Science Will Star Planes, Radio Waves and Citizen Help
NASA Wants YOU to Be a Citizen Scientist for the 2017 Total Solar Eclipse