Skeptophilia (skep-to-fil-i-a) (n.) - the love of logical thought, skepticism, and thinking critically. Being an exploration of the applications of skeptical thinking to the world at large, with periodic excursions into linguistics, music, politics, cryptozoology, and why people keep seeing the face of Jesus on grilled cheese sandwiches.

Wednesday, June 12, 2024

Cloud collision

Contrary to what the medieval church wanted you to believe, the Earth is in constant motion.

They went to enormous lengths to stand by the principle that we're the center of the universe, motionless, while everything revolves around us in perfect circles.  Arrogant attitude, that.  Also wildly wrong.  Not only is the apparent motion of the stars at night caused by our own rotation, the stars aren't in quite the same positions at a given time one night as compared to the next because we're revolving around the Sun.

Then along came Kepler, and showed that even the "perfect circles" part was wrong; the planets and their moons orbit in ellipses, not circles, some of them quite eccentric (the mathematicians' word for the degree to which an ellipse deviates from a circle).

It's even worse than that.  The Earth's axis precesses, wobbling like a spinning top, drawing out a circle in the sky once every twenty-six thousand years.  So Polaris, hasn't always been the pole star, and at some point won't be any longer.  This fact was discovered by the Greek astronomer Hipparchus, but although the aforementioned church fathers loved the Greek philosophers -- they were especially fond of Aristotle -- they were also excellent at ignoring evidence that challenged their own worldviews, so Hipparchus's studies of axial precession were brushed aside.  The thirteenth century Persian polymath Nasir al-Din al-Tusi studied astronomical records and came up with a value very close to our currently accepted precession rate, but the church fathers didn't much listen to the Muslims, either, so it wasn't until eighteenth century French mathematician Jean le Rond d'Alembert said, "No, really, guys, this precession thing is real" that people in the western world started to accept it.

The path of apparent precession of the pole star. The bright star at the bottom is Vega, which was the pole star twelve thousand years ago (and will be again in fourteen thousand years). [Image licensed under the Creative Commons Tau Ľolunga, Precession N, CC BY-SA 2.5]

But even that's not the end of it, because the Sun (and the rest of the Solar System) are in the edge of one of the spiral arms of the Milky Way, and are traveling at about 230 kilometers per second in orbit around the galactic core.  This is a good clip -- it's only a bit under a thousandth of the speed of light -- but even so, the galaxy is so enormous it will take about 225,000,000 years to complete one orbit.  Put another way, the last time the Solar System was in this spot was the early Triassic Period -- right at the beginning of the "Age of Dinosaurs."

It's this last motion that's what brings the topic up today, because a team led by Boston University astronomer Merav Opher has just found that the motion of the Sun and planets around the galactic center swept it through two successive clouds of cold gas and dust, hitting one about seven million years ago and another a little over two million years ago.  The clouds, which from our current perspective are in the constellation of Lynx, provided enough resistance that the heliosphere -- the region of space dominated by the outward pressure of material thrown off by the Sun -- shrank to the point that the planets were exposed to the dust of the interstellar medium.  This caused a spike of supernova-generated isotopes like iron-60 and plutonium-244 in cosmic dust trapped in sediments and ice layers here on Earth.

Opher's team found this cosmic dust in every place of those ages they looked.  It was the fingerprint of a collision -- between the Solar System and a pair of clouds.

It's an open question what effect that had on the Earth.  The collisions happened just as our hominid ancestors were moving their way out of the African savanna, so any additional flux of cosmic rays from being outside the heliopause didn't seem to do us any harm.  But it's a cool reminder that although we feel like the Earth is solid and unmoving beneath our feet, it's actually being spun around the universe like a little kid on the Tilt-o-Whirl.

But finally, there's even another layer on top of all the above, because the Milky Way and the entire Local Group are moving toward something called the "Great Attractor" at six hundred kilometers per second, over twice as fast as the Solar System's orbital velocity around the galactic center.  Presumably this is because of some sort of gravitational effect, but what sucks is that although we know the general direction where the Great Attractor is located, we don't even know what's there because it's directly on the opposite side of the center of our own galaxy.  In other words, we can't see where we're headed because the Milky Way is in the way.  

What it's in the way of remains to be seen.

So yeah.  The medieval church fathers were kind of spectacularly wrong.  The more we've learned, the weirder the universe gets, and the farther from the center of anything we appear to be.  It's better this way, though, because it gives us constant reminders of how grand and magnificent the universe is -- even if the inevitable consequence is a reminder of how tiny we are by comparison.


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