(Dis)Orientation: 5. Drift
In which we discuss the precession of the earth, & the impacts of slow drifting change. This is the 5th section in a chapter on how I get my bearings in the night sky, and in life.
Author’s note: this is the fifth section in a six part series, previously titled “Lost in Space.” You can find the first section here, the second section here, the third section here, and the fourth section here.
There are a myriad of reasons we might end up lost, often accumulating like snow on unplowed roads. But over long journeys there’s one other process that, though slow and nearly invisible, threatens to undergird everything we’ve established so far.
It goes by some fancy physics names: it was historically called precession of the equinoxes, which sounds like some great parade of fictional creatures, or gyroscopic motion, which sounds like some sophisticated tool that belongs in a doctor’s office or nuclear hazards lab. For such fancy names, the effect is incredibly subtle. Casual observations over the course of many months or even multiple years would be hard pressed to notice it. Yet the consequences will be, if not dire, at least unsettling: our fixed reference coordinate system, our rigid invisible grid of Right Ascension and Declination, isn’t actually fixed. It is slowly - but perceptibly if you look closely enough - shifting. Said again, our “fixed” star addresses actually drift with time.
How much drift are we talking? Well, the coordinates drift about the thickness of a postcard held at arms length each year (a little less than 1 minute of arc per year, in astronomy jargon). Pretty small. Then again, a star smack dab in the center of an image taken by the Hubble Space Telescope pointed at RA 20.000 in 2022 would be completely off the edge of Hubble’s view if pointed back at RA 20.000 a mere 5 years later. Kinda a big deal.
I mean, perhaps I shouldn’t be surprised. The power of slow drift certainly shouldn’t be underestimated. Consider the Great Lakes surrounding Michigan, or the Finger Lakes in upstate New York - these lakes that defined the geography of so much of my life were all carved over thousands of years by very slowly drifting glaciers. And any sailor knows that currents matter. As the American Practical Navigator notes, “The movement of ocean water is one of the … principle sources of discrepancy between dead reckoning and the actual positions of vessels…. Current models are an integral part of ship routing systems.” Ships take different routes going east and going west across the Atlantic Ocean because of the currents. The drift may seem imperceptible, but the collective impact is huge.
There’s plenty I’ve thought was fixed that turned out to have more drift than I wanted. I think about a girl I dated for two and half years in college. She was great - we had lots of things in common: similar views on faith, similar connection to music, similar cultural upbringing, and much more. We talked about everything. Clearly, we were in more or less the same place, at least emotionally, and socially. But… we went to different universities, and had different friends. Those 1 hour 17 minute drives seemed not a big deal, like we were making it work. Maybe we’re stronger because of it, right?! But there was something I didn’t anticipate: though we thought we had the drive to make things work, we underestimated the speed with which the world spins, and the speed with which our perspectives float, fallen leaves on a meandering stream. During the week our experiences were different - we talked with different people, read different books, took different classes. It’s like trees changing color. You don’t even notice the differences cropping up, and then boom. It’s fall. And at some point, it became too much to correct. Our breakup was difficult, even more so because it was so hard to understand.
So what’s going on? The problem is that the RA/ Dec system is fixed to the earth - centered on the point above Earth’s north pole. But Earth, it turns out, is slowly wobbling as it spins, much like the slow wobbles of a spinning top. To use the fancy lingo, earth’s spin axis precesses. So a laser shot out into space from Earth’s north pole slowly changes which direction it’s pointing. This means that the north star, Polaris, where our imagined laser is currently pointing, eventually won’t be the north star any more. Give it 14,000 years, and a star in the summer triangle, Vega, some 52 degrees from the current north pole, will be our new north star.
So if you want to tell someone where to look, giving coordinates isn’t enough. You have to also give a time. If you look in older astronomy papers all the coordinates have a preface something like B1950. That means they’re in an RA-Dec system fixed to what it was on January 1 in 1950. Of course, as you get into the late 80s early 90s people started switching to “J2000” coordinates - defined based on January of the year 2000. At the time of writing J2000 is still the standard. But time stops for no one. The earth continues to wobble, and soon some 2050 coordinate will be the new fashionable way of telling where to look.
Now I want to emphasize that this drifting is not the daily shifting of perspective from earth’s spin or our moving about. Here the reference system is moving, not you. This is drift of our “ground truth.” It’s a steady march, that happens whether or not you’ve managed to keep up, no matter how well you’ve corrected for our motions. Better analogies might point out the ways our fashions shift, or languages drift, or continents rift. A dollar in 1950 might buy a shirt, in 2000 it’s a candy bar. In 1950, men wore suits and ties to hockey games. In 2000 men and women wore pretty much whatever to hockey games - chicken heads, T-shirts, deep V-cuts, and jerseys.
Come to think of it, what reference do we have that doesn’t shift with time? “On Christ the solid rock I stand” - but even rocks move. Which makes me wonder, though I cast this slow change as a problem, is it a problem if in reality, it is just the way the universe works?
Well, maybe not a problem, but certainly jarring, the way these deviations sneak up on us. I was talking to my friend Sarah about slow drift, and she told me a story about seeing people you haven’t seen in a long time. She told me about her step mom, let’s call her Cindy, who alway used to wear heels - they were a staple part of her outfit, you might even say her identity. But time drifts, and she slowly moved on to other hobbies, other footwear. Yet, Cindy can’t get over recently visiting some old friends, and having them ask, as they stood around in the entryway, off to recreate some mutually remembered adventure, “who’s hiking shoes are these?” “Mine!” said Cindy. Here friends didn’t believe it. “No really, who’s are they?”
It’s not just Cindy and her friends. I recently found an old high school yearbook at home, and realized I could still remember the names of the people pictured, even if I see their posts on instagram now and wonder, who’s that? Even my parents, I still picture, in my minds eye, as if they haven’t aged - perpetually 35, maybe 40. It can’t be their hair is graying now. Fact is: it’s hard to adapt when the waters around us change more slowly than we can see: maybe it’s really like the talking heads on the news say, we’re just frogs in a slowly heating pot.
While jarring, I think that not every connotation of slow drift has to be bad. Slow drifts can be quite useful. Slow drift builds mountains and carves canyons. It melts cultures and evolves new species. Like one of my teachers once said of the western mind: “this fever too will pass.”
More, slow drifts highlight things that we might otherwise never see. If it weren’t for the shifting of our coordinates, we might never know about the earth’s precession. Through inflation economists measure broader forces in the economy, and through changes in vocabulary historians mark deeper cultural currents running through generations. For astronomers, slow drifts in space have turned out to be not just fact, but a boon. For example, Mercury’s orbit, already a slow drift of sorts, also shifts with time. More oval than most orbits, the elongated direction of its orbit (think top to bottom on an egg) points in a slightly different direction each time it goes around. It’s only the tiniest difference - you wouldn’t catch it in just one look. But over years and years and years, it’s all of a sudden different. Or, perhaps a little more esoteric, there’s something called the Hulse-Taylor pulsar. A pulsar is a rapidly spinning dead star that emits regular rapid radio pulses toward the earth with each rotation. These pulsars are like clocks - they spin at amazingly steady and predictable rates. Except the Hulse-Taylor pulsar. This one is special because it has a very close companion: as they dance in orbit, it’s spin slowly slows. Maybe 40 seconds in 30 years. Even our best watches do worse. But a slow drift none the less.
The beautiful thing of both these examples is that the slow drift is actually a clue, telling us something else is at work. Both these drifts are predicted by Einstein’s theory of General Relativity. Einstein’s math said not only that they would occur, but exactly how big and fast the shifts should be. And the match was amazing. Einstein 1, The rest of us 0.
One other small point, but perhaps significant when we wade into it: what is the difference between shifts and drifts? Both represent change, but consider the contrast: shifts give a sense of speed, while drifts are comparatively slow. Shifting, you might feel like you have an element of control. You decide when to bump it into 6th gear, and it’s on you if you stall out. Drifting, on the other hand, is the giving up of control: you must lose traction, allowing the forces of physics and nature to move you as they see fit. Also: shifting is usually linear: up and down, side to side. Drifting cares not for hard edges and straight lines, it prefers bends, and leaves us floating around in swirling eddies and bobbing waves.
Section 4.1 Driving Drift
Now we come to what MIT physics legend Walter Lewin calls “the most non-intuitive… arguably the most difficult part of physics” – that dangerous little question, “but why?” Why does the earth wobble as it walks? What knocks it around, what disorientation has it imbibed?
To get there consider a spinning top, a pretty direct, but more digestible analogy for our lamely pirouetting planet. When you spin a top it seems to twirl flawlessly at first, but soon you notice its spin axis, now just slightly tilted, seems to itself be slowly circling, like the finger of a musician or dancer signaling for a little increase in speed. But here’s the weird thing, unlike a finger, there’s nothing pushing on the top, no visible forces to move it around and around. Have you ever considered how weird that is? It’s not a magnet. And there’s no one blowing on it. It just seems to do it of its own accord.
We know this can’t be, that change requires some sort of push or pull. And indeed, it turns out there is an invisible force: gravity. Gravity, that thing that always pulls down, toward the table; gravity, that in this case, is making the top cycle sideways, though we know it doesn’t pull that way. This, I think, is even more unsettling. I feel like it’s a joke somehow: “gravity” is always the right answer when you’re confused in physics, in the same way “Jesus” is always the right answer in Sunday school. Gravity, pulling down on the tilted top of the top should make it topple. Yet it doesn’t. It just drifts sideways, a little slow circling of the tip of its axis to compliment its rapid spin. Why?
I’m not sure I can give you a completely satisfying answer, but I’ll try. The key is first understanding the most fundamental rule in all of physics. It crops up by several names, most famously Newton’s 1st law, but the concept is this: objects in the universe want to keep doing what they are doing. Everything resists change. We have some intuition of this. Try getting a heavy fridge moving or stoping a moving train. Try getting your dad to stop saying sexist things, or your aunt to support your favorite politician. This rule – resistance to change – is also true of spinning things: a top, a bike wheel, the earth. But with spinning things, our intuition breaks down. Because spinning things not only want to keep spinning, they want to keep spinning in exactly the same direction – exactly the same way. So if the axle I’m turning around is horizontal, it wants to stay horizontal - convenient if you’re riding a bike: a rolling, spinning bike wheel doesn’t tip, a stationary one does. Spin provides a stabilizing force.
Where the problem comes is when we try to manipulate or turn a spinning object. It doesn’t behave how we expect. Have you ever played with those fidget spinner toys? When you try to move them around, they fight you in unexpected ways, yes? So here’s the conflict: our top wants to keep spinning its happy slightly tilted way. But gravity wants to pull down on that top – make it fall. So we have gravity pulling down, and the top wanting to keep spinning with its axis almost straight up. What gives?
Nature’s compromise is wild. The top stays at the same angle to the ground, making its spin happy, but it also start to rotate around, to move in a direction perpendicular to both its spin and the gravitational tug. We have fancy physics words to describe this: the gravity force creates a torque which causes a change in the angular momentum vector, and then, as Professor Lewin would say, the angular momentum chases the torque. But I think that’s mostly me just showing off and using big words to hid my ignorance, because it’s just a mind twisting compromise between the rotation gravity wants (down to the table), and the spinning the top wants to keep doing.
But enough of tops, what of earth? Tops aren’t spheres, it makes sense they might tip. But earth? Well, it turns out that earth’s own spinning is the seed of its discontent. Because earth is spinning, it isn’t actually a perfect sphere, it actually bulges a little bit at the center - the diameter at the equator is some 43 miles wider than the diameter from the north to south poles. This slight asymmetry acts like a lever for the sun, moon, and planets to pull on, trying to tip it so that it’s upright in its orbit rather than slightly tilted. And these forces, just like gravity trying to pull the top down, work to make it precess just like the top. How I think about it: the sun, moon, and other planets are trying to make it get in line, pull that bulge down, to line up with the orbital plane. But our planet knows itself. It won’t be changed that easily. So it responds, like the top, in a non-intuitive way.
Yet respond it does. I think it’s interesting that even our planet is asked to conform, asked to change. And these tiny forces, almost unnoticeable to it, certainly small compared to its overall orbit or tides or anything, cause it to drift, slightly off kilter, orienting itself to an ever different north star. I’ve been there. Trying to be my own person, but also to respond to my parents and my friends and what everyone else thinks. Trying to make my non-conformities get in line while also spinning my own way. No wonder I ended up at a dead end feeling lost. I’d tried so hard to make it all work. I’d been in the “perfect” job. But all this effort responding and responding, and I forgot the strength of gravity; it was only when I finally looked up that I realized: I didn’t know where I was pointed anymore.