There is an old saying that there is “nothing new under the sun,” but that is not true when it comes to the science of solar eclipses.
Not only do astronomers offer a fantastic celestial spectacle, but have often used the brief moments that solar totality offers to explore the sun and its surroundings. To this end, total solar eclipses in the past and still today provided the opportunity to conduct scientific experiments.
For example, the element helium was first identified in the sun’s lightning spectrum in 1868. Sir Arthur Eddington successfully observed that gravity bends light, as seen in the deflection of stars near the edge of the sun during the total solar eclipse of 1919, and successfully tested one of the predictions of Einstein’s general theory of relativity. This attempt was first made in 1914, but stalled with the advent of World War I.
An annotated version of Edington’s famous photo of the 1919 solar eclipse. Photo credits: ESO / Landessternwarte Heidelberg-Königstuhl / FW Dyson / AS Eddington / C. Davidson.
During the total solar eclipse of 1878 over the American West, astronomers also made one final attempt to spy on the interqueckish world volcano. And although the advent of relativistic physics also explained the anomalous precession of Mercury without inducing the existence of the false world of Vulcan, the existence of a hypothetical group of asteroids in close proximity to the Sun called volcanos has not yet been ruled out. NASA actually hired a pair of converted WB-57 bombers to hunt the moon’s shadow during the 2017 solar eclipse with the third goal of searching the near-sun zone for fake volcanos. So far, however, the searches have come out empty-handed.
In addition, we still manage to tease out scientific results from total solar eclipses. A total solar eclipse presents a unique opportunity on several scientific fronts, including: 1). a chance to directly observe the Sun’s illusory and usually invisible outer atmosphere, including the chromosphere and corona; 2). a chance to search the room very close to the sun for previously unknown objects, and 3)). a chance to use the large distorting gravitational well of our host star for experiments.
An elusive phenomenon that occasionally occurs during totality is coronal mass ejections. Until the advent of space-based coronagraphs, starting with Skylab in the early 1970s, total solar eclipses were the only time astronomers could see this poorly understood phenomenon. Even today, solar totality is the only time that we can see the beginnings of a coronal mass ejection near the sun’s edge, as typical coronagraphs on board solar observation missions such as the Solar Heliosphere Observatory (SOHO) tend to cover several solar diameters. Though short, the totality offers a much better adaptation of the sun covering the moon.
One such event was recently observed by astronomer Guillermo Abramson during the total solar eclipse on December 14, 2020. The path of totality for this eclipse included remote areas on the extreme southern tip of South America. Due to the ongoing pandemic and travel restrictions, few eclipse hunters have made their way to stand in the shadow of the moon this time around.
A developing CME, as seen in the lower left loop of the eclipsed sun. Photo credit: G. Abramson.
“Observations of the solar corona during eclipses, while rare, are extremely valuable and complement those made with coronagraphs from the ground and from space,” Abramson told Universe Today. “The main reason is that during solar eclipses the corona can be observed up to the edge of the sun, while the masks of the coronagraphs tend to be larger.”
The same CME in action, captured by SOHO. Recognition; NASA / ESA / SOHO
Abramson and his team not only succeeded in capturing the entire anatomy of a CME during its formation, but comparing it with observations from space-based facilities such as GOES, ACE and SOHO also made it possible to measure speed and velocity over the course of the CME. The team also got an additional bonus that was verified by SOHO: the Kreutz Group Sonnenweide Komet 2020 X3 SOHO). Comets have made themselves felt in solar eclipses before; a famous example was the Great Eclipse Comet of 1948. It is strange that by the early 1990s fewer than half a dozen sun knives were known. Now, thanks in large part to SOHO, we have recorded 4,182 such comets, and the number is growing.
Comet X3 SOHO, seen from the ground (left) and space (right) during the solar eclipse. Photo credit: G. Abramson / SOHO.
Another elusive mystery eclipse hunter working on the solution is what is known as the corona heating problem. What charges the weak solar corona to temperatures millions of degrees hotter than the layers below?
Step inside the Solar Wind Sherpas, a team of astronomers who have tracked eclipses around the world to understand this phenomenon. The Solar Wind Sherpas, based at the Institute of Astronomy in Honolulu, Hawaii, have tracked the corona during eclipses for over a decade or 11 year solar cycle. Their efforts have led to an unexpected result: although sunspot activity fluctuates over the course of the solar cycle, the temperature of the solar corona remains nearly constant, suggesting that anything that heats the corona to tremendous temperatures is independent of the solar cycle.
“The temperature at the sources of the solar wind in the corona is almost constant during the entire solar cycle,” says Shadia Habbal (University of Hawaii) in a recent press release. “This finding is unexpected because coronal structures are driven by changes in the distribution of magnetized plasmas in the corona, which vary so widely during the 11-year solar magnetic cycle.”
Solar prominences (arrows) interacting with the corona as seen during totality. Source: Habbal et al. 2021.
This fascinating find was published in the Astrophysical Journal Letters. One suspect of solar heat is so-called nanoflares, and missions like NASA’s Parker Solar Probe and ESA’s Solar Orbiter are currently working hard on this problem.
On average, we experience one or two total solar eclipses somewhere on earth every year, although this year’s offer is shy as totality only roams the earth once across the Antarctic continent on December 4th, 2021. However, the United States is on April 8th, 2024 the threat of eclipse fever again, with a total solar eclipse crossing Mexico, the United States and (get your Learjet ready) the Canadian ocean-going ships …
Where will you go in 2024? Photo credit: Michael Zeiler, The Great American Eclipse.
And as always, scientists will be on hand to unravel the sun’s secrets.
-Read Dr. Abramson’s report on the hunt for the ‘Great Patagonian Solar Eclipse’ on his blog.
– Mission statement: A breathtaking view of the total solar eclipse in December 2020. G. Abramson.