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FACT: This is the most perfectly timed period in history
At least, when it comes to observing cosmic events. We are about to witness a complete solar eclipse across North America, which is quite a rare occurrence. But if we lived at a different point in cosmic history, it would be more than just rare—it would be impossible.
You see, the fact that the moon is the exact size to block the sun's face in the sky is entirely coincidental. It didn't have to be like that, and in fact, it wasn't always like that.
The the moon initially orbited closer to Earth than it does now, so it would have appeared larger in the sky. It would have been so large that it wouldn't just obstruct the sun, it would also have hidden the solar corona—the glowing atmosphere surrounding the sun that transforms a complete solar eclipse into a magnificent spectacle.
And the moon is continually moving farther away—by about 1.5 inches each year. This movement is a result of the moon's gravitational pull on Earth, which in turn slightly slows down Earth's rotation. To maintain angular momentum, the moon accelerates a tiny amount, thus moving away from us. In another 620 million years, the moon will be far enough away that its face will seem too small to completely obscure the sun as it does now.
FACT: Eclipses have been unsettling humans for essentially forever
Our oldest visual depiction of a solar eclipse might be a relatively nondescript mound of stone in County Meath, Ireland known as the Loughcrew. This grassy hill dates back to around 3,300 BC, making it roughly 1,000 years older than Stonehenge. It contains several large stones with intricate carvings of abstract shapes such as spirals and diamonds. Most importantly for our purposes, one of the cairns displays a large carving of overlapping concentric circles—a common visual representation of the sun being eclipsed and then revealed by the moon.
In 2002, archaeoastronomer Paul Griffin compared the age of the site to calculations of when solar eclipses would have been visible in the area and found a good match for November 30, 3340 BC, just around sunset. He proposed that the other symbols on the cairns might represent stars that became visible due to the darkness of the partial eclipse.
Archaeologists had previously noted the presence of charred human remains from around 50 individuals placed in a chamber just in front of the carving, which of course suggests some form of ceremonial sacrifice.
Some scientists strongly disagree with this explanation of the Loughcrew cairns, because there’s no written record to disprove it. However, the challenge of looking 5,000 years into the past is that we can confidently discuss what was happening in the sky, but we have to make many assumptions to put together what people were doing on the ground.
That said, we can be quite certain that our ancestors had some strong responses to—and descriptions of—total solar eclipses. You can learn about more of them in this week’s episode.
FACT: A total solar eclipse is a great opportunity for examining the sun’s deeply unusual corona
By Lee Billings
One of the most remarkable aspects of a full solar eclipse is the totality, the period during which the moon hangs over the sun to nearly completely block out its light. You might expect the sky to simply be dark around our briefly shaded star, but you’d be mistaken. Instead, the dark sun is surrounded by what seems like a shimmering silvery crown—which is why it's called “corona,” Latin for “wreath” or “crown.” This is a intricate, changing region of hot, rarefied plasma—ionized gas—swirling and billowing in magnetic fields that come from deeper within, and being in the moon’s light-blocking shadow is the best time to see it. The corona wraps around our star like a torn, translucent and constantly renewing cover, constantly shedding pieces at its edges that flow out along magnetic field lines to form the solar wind, which itself creates a larger bubble around our entire solar system that acts as a semi-transparent barrier against the active background of cosmic radiation. At times, the corona releases larger clumps of material in what are called coronal mass ejections, which can hit orbiting planets and cause intense solar storms.
And, for reasons not fully understood, the corona is very hot—a few million degrees. This temperature difference is the so-called “coronal heating problem,” and one reason it's so unusual is because it requires non-thermal energy transfer. Neither simple radiant heat—infrared light—nor convective heat, like the bubbling churn of hot fluids, can provide enough energy to the corona to explain its high temperature. The situation is somewhat like holding a hot incandescent light bulb or a mug of boiling tea and, instead of getting burned, your hand instantly vaporizes into a rapidly expanding cloud of plasma; in other words, the available thermal energy is insufficient. So heliophysicists understand that more unusual processes must be at work, such as heating from a combination of turbulence and the crashing reconnection of immense, twisting loops of the sun’s strong magnetic field. Strange processes that, in turn, must somehow contribute to larger corona-related phenomena such as the solar wind and the huge mass ejections that extend to shape the entire solar system and beyond. Scientists will be thousands of degrees.
attempting to unlock some of these solar mysteries during the upcoming eclipse And other strange things we discovered this week..
