This is the most cosmically perfect time in history

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FACT: This is the most cosmically perfect time in history

By Clara Moskowitz

At least, in terms of observing cosmic phenomena, it is. We’re about to see a total solar eclipse over North America, which is a pretty rare phenomenon. But if we were living at a different point in cosmic history, it would be more than rare—it’d be impossible.

See, the fact that the moon is the perfect size to cover up the face of the sun in the sky is a total coincidence. It didn’t have to be that way, and in fact, it didn’t used to be that way.

The moon started off closer to Earth than it is now, so it would have looked bigger in the sky. It would have been so big that it wouldn’t just block the sun, it would also have covered up the solar corona—the glowing atmosphere around the sun that turns a total solar eclipse into a beautiful spectacle.

And the moon’s getting farther away all the time—by about 1.5 inches each year. This movement is a consequence of how the moon tugs on Earth to create the tides, which in turn drag Earth’s spin down minutely. To conserve angular momentum, the moon speeds up a teensy tiny amount, and thus moves away from us. In another 620 million years, the moon will be far enough away that its face will appear too small to completely block out the sun like it does now.

FACT: Eclipses have been freaking humans out for pretty much forever

By Rachel Feltman

Our oldest visual representation of a solar eclipse could be a fairly innocuous looking mound of stone in County Meath, Ireland called the Loughcrew. This grassy hump dates back to around 3,300 BC, making it a good 1,000 years older than Stonehenge. It features a number of large stones with intricate carvings of abstract shapes like spirals and diamonds. Most importantly for our purposes, one of the cairns shows 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 should have been visible in the area, and found a good match for November 30 3340 BC, just around sunset. He argued 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 basin just in front of the carving, which of course evokes some kind of ceremonial sacrifice.

Now, some scientists vehemently disagree with this interpretation of the Loughcrew cairns, because there’s no written record to disprove it. But that’s kind of the issue with looking 5,000 years into the past: We can say confidently what was going on in the sky, but we have to make a lot of inferences to piece together what people were doing on the ground.

That being said, we can be pretty certain that our ancestors had some wild reactions to—and explanations for—total solar eclipses. You can hear about more of them in this week’s episode.

**FACT: A total solar eclipse is a perfect opportunity for scrutinizing the sun’s deeply weird corona**

By Lee Billings

One of the most striking aspects of a full-blown solar eclipse is the totality, the period in which the moon hangs over the sun to almost perfectly blot out its starlight. You might expect the sky to simply be dark around our briefly shadowed star, but you’d be wrong. Instead the dark sun is wreathed by what looks like a wavering silvery crown—hence the name, “corona,” Latin for “wreath” or “crown.” This is a complex, dynamic region of hot, rarefied plasma—ionized gas—swirling and billowing in magnetic fields that emanate from deeper within, and being in the moon’s star-blocking shadow is by far the best time to see it. The corona envelopes our star like a tattered, diaphanous and ever-regenerating shroud, constantly shedding pieces at its edges which flow out along magnetic field lines to make the solar wind, which itself forms a larger bubble around our entire solar system that serves as a semipermeable barrier against the seething background of cosmic radiation. Sometimes the corona unleashes larger clumps of material in what are known as coronal mass ejections, which can strike orbiting planets to raise potent solar storms.

And, for reasons no one fully understands, the corona is quite hot—a few million degrees. Which may not seem so strange until you realize the sun’s apparent “surface,” which lies just beneath, is only some thousands of degrees.

This temperature difference is the so-called “coronal heating problem,” and one reason it’s so deeply weird 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 pump enough energy into the corona to explain its high temperature. The situation is a bit like holding a hot incandescent light bulb or a mug of boiling tea and, instead of suffering a burn having your hand flash-vaporize to a rapidly expanding cloud of plasma; that is, the available thermal energy is insufficient to do the deed. So heliophysicists know that more bizarre processes must be at play, such as heating from some combination of turbulence and the crashing reconnection of immense, writhing loops of the sun’s powerful magnetic field. Bizarre processes that, in turn, must somehow contribute to larger-scale corona-connected phenomena such as the solar wind and the giant mass ejections that reach out to shape the entire solar system and beyond. Scientists will be trying to unlock some of these solar secrets during the upcoming eclipse.