A couple weeks ago, I was awake at 4:30 AM with 6-month-old Stanley during the time of the “Blood Moon” eclipse. From the upstairs window, I was able to catch the last bit of the Earth’s shadow passing over the moon. Visually it wasn’t exactly stunning, but I appreciated the reminder that we’re here and we cast a shadow in the universe.
Probably because it was so ridiculously early, I started to wonder what other effects the Earth’s shadow might have on things out there. For instance, if you were some kind of particle floating near Earth, the shadow might be a nice spot to cool down and find shelter from the solar wind.
And maybe long ago, when the solar system was young and less dense, the stray gases and debris that managed to escape the Earth’s atmosphere, gathered together in the shadow to form a nice, comet-like tail. (Seems possible, since apparently the Earth has a vestigial tail even now.)
Maybe so many particles tried to squeeze into this shadow-tail over time, that they melded together and formed enough mass to assert some serious gravity, inviting more and more matter to join the party. Once things really got raging, this heated particle storm could’ve grown to the point that the charged breeze from the sun could no longer confine it to the shadow. Unable to resist the centripetal allure of the sun, but not wild enough to stray too far from mother Earth, it would’ve spun off into it’s own orbit. As this fiery attraction collided with more and more debris, it could’ve boiled over, then congealed and cooled to become the silver rock of inspiration we know and love today.
As I paced back and forth in the hall, trying find the exact patting rhythm that might possibly nudge Stan back to sleep, I tried to figure out if a moon could get ever started just by collecting a small amount of space debris in one place for a few billion years. I can get a pretty big dust-bunny under my dresser in only a week when the ceiling fan is on during the summer. If that dust-bunny managed to increase its diameter by only 1 mm every week, I could have a 1/2 cm dust bunny by the end of the year. But If it started accumulating at the same rate, back when the Earth started forming, some 4 billion years ago, it would now be 200,000 km wide or almost twice as big as Saturn!
But it’s really mass, not size that would have to increase to become a moon. To delve further into that question, I dimmed the brightness on my iPhone and began entering odd sentences into Wolfram/Alpha. “Mass of the Moon divided by Age of the Moon in kilograms” told me you’d need 16.2 trillion kg for every year of development or 514,600 kg/second – which, in our dust-bunny analogy would be like a dust-bunny volcano.
Well… according to pictures like this one, the Earth was crazy with volcanos back in the day. How much stuff could a volcano really produce?
I looked up the first volcano I could think of and discovered that Mt. Vesuvius spewed out 1.3 million kg of rock every second when it buried Pompeii back in 79 AD. That was a huge amount of stuff. Of course not much of that went into space. But then again, when the Earth was still forming, it wasn’t as massive, so it had less gravity so more volcanic debris could’ve escaped into space. Plus there was no atmosphere to hold it in. And besides that, wasn’t there enough free-floating debris and gas to form the solar system? Why wasn’t the moon entitled to some of that? And even if the moon was formed by a giant collision with a Mars-sized object, as the current hypothesis suggests, wouldn’t the Earth’s shadow have been a nice place to get things started?
By the time I had this matter of mass and volcanos worked out to my satisfaction, Stan had drifted back to sleep, and the birds were beginning to chirp. I was now wide awake, stuck with a time-lapse animation running through my head showing the Moon materializing from a comet-like tail in the shadow of the Earth.
I pictured the sun acting as a sort of sand-blaster with the Earth as a stencil protecting the debris in it’s tail from annihilation. Something about this picture got me thinking about another eclipse-related question that has always bothered me: Why are the size of the sun and moon so closely matched when observed from Earth – to the point that they line up almost perfectly during a solar eclipse? It’s always felt like one of those coincidences that’s just too coincidental. With the sun’s rays implicated in the formation of the moon, could this origin story help to explain this peculiarity? Whether or not the trigonometry works out is a question for those who actually know how to do trigonometry.
I found myself facing the day, convinced, despite no serious evidence, that the significance of the Earth’s shadow has been overlooked. I don’t really know the particulars of the particles that were around during the early days of the solar system, or the ones that make up the Moon today (or the Earth, for that matter) and I’m no expert on the solar wind, but there’s a certain elegance to the story that feels compelling.
Having pushed the limits of “soft science” I decided the best course of action would be to create a quick visualization of the concept (see the video at the top of this post) and get it into the minds of people who think about this stuff on a regular basis.
So how ’bout it science? Can we get an accurate particle simulation that takes the Earth’s shadow and the solar wind into account? Can we give the particles some realistic gravitational properties, and plot some orbital trajectories? Can we speed it up over 4.5 billion years and see if we end up with a big rock ball that perfectly blocks out the sun during an eclipse? That would be cool.
If you don’t have the computing power for such a thing, that’s ok too. Maybe just think about it at 4:30 in the morning and post your thoughts or relevant links in the comments.