April 12, 2024

Total eclipses are a cosmic accident

Eclipses are not particularly rare in the universe. One of them occurs every time a planet, its orbiting moon, and its sun align. Nearly every planet has a sun, and astronomers have reason to believe that many of them have moons, so shadows are bound to be cast on one world or the other over the years.

But solar eclipses like the one millions of Americans will watch on April 8 — in which a blood-red ring and a glittering corona emerge to surround a blackened sun — are a cosmic fluke. They are an unlikely coincidence of time, space and planetary dynamics, the result of chance events that happened billions of years ago. And, as far as we know, Earth’s magnificent eclipses are unique in their frequency, an extraordinary case of ordinary stellar spectacle. On April 8, anyone who watches in wonder as the moon passes silently over the sun will witness the planetary version of a lightning strike.

As seen from a planet, a solar eclipse can vary in virtually infinite ways. Everything depends on the apparent size of the star and the planet’s body. Some eclipses, known as annular eclipses or transits, look like nothing more than a small black dot crossing the solar disk. They occur when a moon appears much smaller than the sun in the sky, whether because it is particularly small or particularly far away (or because the star is particularly large or close). Mars, for example, has two small, potato-shaped moons, each too small to block the sun.

On the other hand, if a moon appears much larger in the sky than the sun, in a solar eclipse the small solar disk would be completely obliterated by the much larger moon, as is the case with many of the largest moons of Jupiter and Saturn. Such an eclipse would certainly represent a shocking change from light to darkness, but certainly not the celestial drama seen on Earth. The eerily perfect replacement of the solar disk with a black sphere of equal size, followed by the surprising appearance of previously invisible and dramatic areas of illumination around it – that kind of eclipse requires very special circumstances.

Our sun, like all stars, is a giant ball of superheated plasma. Close to the surface, gigantic fiery flares called prominences shoot up; Behind it extends the corona, the sun’s outer atmosphere, which can measure millions of degrees on any temperature scale. Normally we can’t see any of these details because the sun itself is just too bright. But during a total solar eclipse we can do that: The prominences form an irregular ring of deep red, just around the sun, with the corona behind it. That’s because from our vantage point on Earth’s surface, our moon appears to be almost exactly the same size as the sun: big enough to block out most of the light, but not so big that it obscures the sun’s outer layers.

Relative to Earth’s diameter, our moon is unusually large for a satellite, at least in our solar system. If you were an alien astronomer visiting our corner of space, you would probably think that the Earth-Moon system consists of two planets orbiting each other. And yet our moon, round as it is, is still 400 times smaller in diameter than the sun, but it also happens to be about 400 times closer to Earth. And even that coincidence of space and size is really a coincidence of time. Today, the moon orbits about 250,000 miles from Earth. But 4.5 billion years ago, when it was first born from an apocalyptic collision between Earth and a planet the size of Mars, it was only about 14,000 miles away, and would therefore have appeared in the sky about would have looked 17 times bigger than it does now. Since then, the moon has been slowly drifting away from Earth; currently it moves about 1.5 inches per year. As the size of its orbit increased, its apparent size in Earth’s sky decreased. That means the eclipses we see today were probably only possible about 1 billion years ago, and won’t be possible in 1 billion years. Humanity is fortunate to live in the brief cosmic window of stunning eclipses.

Not every solar eclipse visible from Earth offers a perfect view of the prominences and the corona, while also temporarily plunging the world into night. The slightly non-circular shape of the moon’s orbit means that it grows and shrinks in the sky. But near-perfect total solar eclipses account for about 27 percent of all sun-moon overlaps on Earth — often enough to be noticed every generation or so by someone in a given region. In contrast, eclipses on the other planets in our solar system are almost always either too small to cover the sun or so large that the ring of fire and corona are hidden. Perfect total solar eclipses are rare jewels to our neighbors, but common to us.

That special frequency has caused eclipses to leave deep marks in human myths and history. Total solar eclipses on Earth can last from just a few seconds to as long as seven minutes, but for our ancestors, these brief moments were still a descent into fear. “A great fear overtakes them,” is an Aztec description of the public reaction to a solar eclipse. “The women are crying out loud. And the men cry out: Eternal darkness will fall and the demons will descend.” According to one legend, thousands of years ago a Chinese emperor ordered the execution of two court astronomers who failed to predict a solar eclipse.

Eclipses were so dramatic that they prompted our ancestors, such as the inhabitants of Babylon and China in the millennia before the Common Era, to pay close attention to the sky. They pushed kings and emperors to provide the tools priests needed to create and maintain long-term astronomical data. They contributed to the invention of methods to track the movement of celestial bodies over lifetimes, and in this way the clockwork of the heavens was revealed for the first time. In that long process of observation and recording, something else happened: eclipses helped push humans to develop and reveal our deepest capacity for a new and precise kind of reasoning that could be applied to the world.

I believe that the cosmic misfortune of the perfect eclipses on Earth – with their great drama and hidden patterns, the panic they caused in marketplaces, the danger they posed to those in power, the awe they inspired among the early astronomer priests – may have served as a force that drives people towards nothing less than science itself. And by building science, we have gained the ability to reshape the planet and ourselves. All this might never have happened without the moon and sun appearing to be nearly the same size from Earth. The fortunate circumstances of our heaven may well have been the gift that finally allowed us to become intimate.

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