Ep. 313: Precession

The Earth is wobbling on its axis like a top. You can’t feel it, but it’s happening. And over long periods of time, these wobbles shift our calendars around, move the stars from where they’re supposed to be, and maybe even mess with our climate. Thank you very much Precession.

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This episode is sponsored by: 8th Light, Swinburne Astronomy Online

Show Notes

  • Precession of the Earth’s Rotation Axis — UTK
  • Gyroscopes — Kenyon College
  • Thuban is a former pole star — EarthSky Blog
  • Earth’s Stabilizing Moon May Be Unique within Universe —
  • Astronomical Theory of Climate Change — NOAA
  • Japan Quake May Have Shortened Earth Days, Moved Axis — Universe Today
  • Transcript

    Transcription services provided by: GMR Transcription

    Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit for more information.

    Fraser Cain: Astronomy Cast episode 313 from Monday, July 1st, 2013: precession. Welcome to Astronomy Cast, our weekly facts based journey through the cosmos where we help you understand not only what we know but how we know what we know. My name is Fraser Cain. I’m the publisher of Universe Today and with me, is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville and the director of CosmoQuest. Hey, Pamela. How you doing?

    Pamela Gay: I’m doing well. How are you doing, Fraser?

    Fraser Cain: Good. And where are you?

    Pamela Gay: I am in Lisbon, Portugal.

    Fraser Cain: And when do you leave there?

    Pamela Gay: I am here until the weekend and then I go to Athens, Greece and then to Volos, Greece and I finally go back to the United States on August 7th. So this is an epic 35 day trip of collaboration, communication and science education.

    Fraser Cain: Go space.

    Pamela Gay: Go space.

    Fraser Cain: But if people wanna hang out with you, where is the next big opportunity? Dragon Con?

    Pamela Gay: Oh, Dragon Con in the United States and if anyone wants to see me in Volos, Greece, we’re going to be doing a bunch of different events. Just drop me a note over on Google+ and we can set up a time. We’re doing a giant science café that one I understand all of the logistics, I will post about and you’re all welcome at the science café. That’s gonna be in the village of Milies, Greece at a big cathedral and yeah. It’s all good.

    Fraser Cain: Cool. And I wanna remind everybody that the Perseid meteor shower is coming up August 11th, 2013 and there’s gonna be no moon so it’s gonna be a really great version of the meteor shower. So if you haven’t already, set some time aside, gather some friends, get organized –

    Pamela Gay: Get bug spray.

    Fraser Cain: Get bug spray and plan to go somewhere nice and dark and far from the city lights and really enjoy this show because it’s just terrific.

    Pamela Gay: I think you and I call that the back yard.

    Fraser Cain: The backyard. Well, for me, it is. Yeah. I live in – there’s the dark sky maps. Have you ever seen those?

    Pamela Gay: Yeah.

    Fraser Cain: You can find out sort of what area you live in, and so if you live in LA, you live in the red area and then orang and yellow and green. So I’m right on the edge of yellow and green, just my backyard, and then I can drive ten minutes and be in the black, no light pollution which is great. So but you’ll want to go and find one of those dark sky finders. Now, I’ve put an event on Google+. So if you just go on and just click yes, then you’ll get a reminder to do it. So just look for events. Search for perseids and you should find that event and then just click yes and then you get a reminder in your calendar and start organizing your friends to go on a trip. Just one night. It’s not gonna kill you.

    Pamela Gay: There will be astrophotography involved on my end.

    Fraser Cain: Yeah. Yeah, me too. Absolutely. Actually, even video. We’re gonna try and actually record us enjoying it.

    Female Speaker: This episode of Astronomy Cast is brought to you by 8th Light Inc. 8th Light is an agile software development company. They craft beautiful applications that are durable and reliable. 8th Light provides disciplined software leadership on demand and shares its expertise to make your project better. For more information, visit them online at Just remember, that’s www., the digit 8, T H L I G H Drop them a note. 8th Light. Software is their craft.

    Fraser Cain: So the Earth is wobbling on its axis like a top. You can’t feel it but it’s happening and over long periods of time, these wobbles shift our calendars around, move the stars from where they’re supposed to be, and maybe even mess with our climate. Thank you very much precession. Okay. So let’s talk about the physics of this and you always sort of relate it to a wobbling top, a wobbling gyroscope. So if I want to imagine precession in my brain, what am I looking at?

    Pamela Gay: What you are looking at is the uneven torque on the Earth due to gravity. So with a top, you have this situation where you have an object that’s not perfectly round that is spinning and if it gets even the slightest bit not completely perpendicular to the force of gravity, it’s gonna end up with more gravity pulling on one side, the side that’s closer to the Earth, the side that’s tilted over than is pulling down on the side that’s off axis. So it’s going to get this uneven tilt that for the most part, is parallel to its rotational axis.

    Well, with the earth, it’s slightly different. You have that same slight tilt. We call it the tilt of the planet, the axial tilt. But in the case of the planet Earth, the gravity from the sun, which is almost perpendicular to the direction of the tilt, is on the side that’s closer to the Earth, pulling a little bit harder on the side that’s tilted away from the Earth, pulling a little bit less, but it’s nonetheless pulling on this not round planet where what’s called an oblate sphere, a flattened sphere. If we were perfect circles, this wouldn’t be an issue. It’s because we’re flattened about our axis of rotation and because we’re rotating at a tilt, that gravity gets the better of us and just ever so slowly rotates the tilt of the planet.

    Fraser Cain: Okay. So I’m gonna imagine I’m gonna take my gyroscope and they’re awesome. If you don’t own a gyroscope and if you have kids, get a gyroscope because they’re awesome. And you pull out string and the gyroscope goes, “Vroom,” and it’s whizzing around and then it’s perfect. For a little while there, it’s just perfectly up, straight up and down, nothing’s –

    Pamela Gay: As far as your eyes can see and your eyes are not an accurate scientific device.

    Fraser Cain: Right. Right. Okay. But from what I can see – yeah, from what you can see, it’s perfect and then as the gyroscope starts to slow down a little, you start to get this wobble, the top axis is starting to form a little circle in the air and then as the gyroscope slows down, that wobble gets bigger and it’s almost like the wobble is starting to effect the gyroscope or maybe it’s just because it’s slowing down and then it just goes completely out of control and then flops over on the side.
    And so in the case of the gyroscope, it’s the gravity pulling it down and you’re saying in the case of the solar system, it’s the sun pulling at it from the side.

    Pamela Gay: The side.

    Fraser Cain: And you’re doing this –

    Pamela Gay: Yeah. In either case, it’s tilted relative to the force. So you end up with an unequal force and it’s that unequal force on the two sides of this tilted world where the tilted top or the tilted gyroscope that causes the precession of the rotational axis.

    Fraser Cain: Does the moon have any impact on the precession?

    Pamela Gay: It’s one of the things that it has a force that’s almost perpendicular to rotation. The moon’s orbit through the sky is very similar to the sun’s. It’s just rotated five degrees off of the sun’s. So yeah. Those two forces are both out there pulling away on the planet Earth.

    Fraser Cain: So let’s imagine that we’re looking down at the Earth from above and obviously we’re gonna see the Earth spinning around and around and around in a, what, counter clockwise direction, but what about that precession? If we could sort of speed up the process, what would we see the Earth doing as we were looking down at it?

    Pamela Gay: So you’re asking me to reverse the way I always think of it.

    Fraser Cain: Oh, okay.

    Pamela Gay: So here we are on the Earth and the way to think of it is as we look at the stars, they appear to move in a retrograde motion, slowly moving from West to East relative to where we’d expect them to be over the course of time. And they’re moving about 50 arc seconds a year.

    Fraser Cain: How much is 50 arc seconds a year? That’s not much.

    Pamela Gay: Well, it adds up to one full degree of motion every 72 years so it adds up fairly quickly. That means that in one normal human lifetime, all of the stars move twice the diameter of the moon across the sky.

    Fraser Cain: Like permanently? Like not permanently but I mean –
    Pamela Gay: For 25,772 years.

    Fraser Cain: Yeah, exactly. So but it’s funny. When I was in high school, and we’ll get to this in a second. Right? Because this has a real impact on the seasons. But when I was in high school and sort of we got to the point of precession and I sort of understood the effect of precession, I was doing the math and I was like, “In my lifetime, the seasons will have shifted.”

    Pamela Gay: Not a lot.

    Fraser Cain: Not a lot but a little bit. And for me, that was kinda mind blowing and I thought even thinking about my grandparents’ generation and the future generations, after a while, the longest day of the year, these will all move. So let’s talk about that then. So you talked about the stars and the stars are moving a couple of degrees over a human lifetime.

    Pamela Gay: Well, a degree-ish over the course.

    Fraser Cain: A degree. Yeah.

    Pamela Gay: Yeah.

    Fraser Cain: What about the seasons? What about the days of the year?

    Pamela Gay: Well, so along with this, if you think about it, what’s happening is we count our normal year based on how long it takes to get the start back in the right place. So that’s something that’s very easy, very accurate to measure but the other thing that we have to look at is how long does it take for the sun to go from straight over the equator to straight over the equator again to back straight over the equator, to go one full cycle all the way North, all the way South, and then back.

    So when you look at that full one year cycle, that’s not quite the same as to line up with the stars and this is where we see relative to the sun’s position, the stars are moving those 50 arc seconds per year. And this causes with our calendar, which is lined up with the starts, this causes the sun to have its solstices, to have its equinoxes gradually on a different date and if you look back through the calendars of the date that’s marked as having the equinox, you can watch over the course of a lifetime. You figure on degree every 72 years. There’s 365 days in a year so there’s basically one day in a lifetime that it all moves.
    Fraser Cain: And that was the math that I had come to was that the first day of summer would have shifted by one day in my lifetime.

    Pamela Gay: And that’s kind of awesome to think about.

    Fraser Cain: Yeah. Yeah, like I said, it blew my mind.

    Pamela Gay: And the thing that gets me about this is the discovery of the precession and the equinox is something that’s been known since ancient times. Exactly how long it’s been known, it’s one of those things that historian love to argue over. There’s those who think that it was probably Hipparchus working probably 100 years before Christ, 200 years before Christ. Figuring out when he was working on things, it was before Christ. It was a long time ago. But he may have been one of the firsts to notice it. It may have even been noticed earlier than that –

    Fraser Cain: Uh oh.

    Pamela Gay: By Aristeleus or Timocrates, one of these individuals that was out there and measuring very carefully what star was the sun lined up with on these different special days of the year. And to think that, well, back when we were basically still using sticks and geometry to measure everything before there was any sort of a clock more fancy than a water clock, they were measuring these one degree per hundred year type of changes in the sky.

    Fraser Cain: How on Earth did they do that?

    Pamela Gay: Well, the neat thing is you just measure what is the star the sun is closest to at sunrise and sunset. It’s fancier than that. You clearly have to figure out, okay, “Sun is setting. What are the stars that you see as they light up?” and there’s math involved. It’s a complex set of measurements but by looking at sunset time and figuring out where things are relative to both sunset and sunrise, you can start to see how things move relative to those stars.

    Fraser Cain: That’s amazing. There’s this idea that – I’m going to go down a rabbit hole for a second here. But there’s this idea that people always though that the world was flat but they really totally knew that it was a sphere for a long time and then just –

    Pamela Gay: The Greeks did.

    Fraser Cain: Yeah. And then just forgot.
    Pamela Gay: Yeah. The dark ages happened.

    Fraser Cain: Yeah. But they had that figured out. They had calculated not necessarily what distance is to the moon but at least they knew sort of percentages of comparison between the distance to the moon and the distance to the sun and they had this all figured out. So yeah.

    Pamela Gay: And there’s also some argued over archaeological indications that maybe the ancient Egyptians were aware of this as well and would tear down buildings when the alignments changed to fix the alignments. And the idea that you would tear a building down because the Earth had rotated such that it was no longer correctly aligned with the stars, that’s a truly Egyptian, historical thing to think about.

    Fraser Cain: Yeah. Now, okay. So the other consequence here is the North Star. Right? So right now, the North pole of the Earth is pointed essentially directly at the North start but as part of this process, that’s gonna shift away.

    Pamela Gay: Well, and also of use, the Southern hemisphere is going to start to get its own pole stars. So yeah. The really cool thing is Vega is actually gonna be about five degrees off of being a pole star in the year 14,080. So we have a while to wait. I don’t think you and I are probably gonna be recording when that happens.

    Fraser Cain: I’ll be on to my third robot body then. No problem.

    Pamela Gay: But we are looking at a future where there is potentially going to be a much brighter pole star. It turns out that about 3,000 BC, Thuban in the constellation Draco was a pole star but it was a much fainter pole star than even the North star is so it would have been quite hard to see but – yeah. All around the circle of Northern precession, we have every few thousands years, a half way reasonable star to look to. Down in the southern hemisphere, choose your expletive, they’re stuck. They don’t really have a nice, happy, bright pole star to look to.

    Currently, there’s really nothing and I mean nothing. Around zero ADBC, pick your way of naming it, there is a star a little bit fainter than the North Star. Things are slowly migrating. Around 4000 AD, there’s gonna be another fainter star but they just don’t ever get any luck.

    Fraser Cain: Now, there are other sort of movements I know that are kind of involved a bit with the Earth’s orbit. So you have this situation where the top of the Earth is rolling around like a top but you also get sort of a bit of a change in variation in its axial tilt as well. Right?

    Pamela Gay: So you have the axial tilt is slowly rotating around a mostly fixed point. So if you thin of the way the top precesses, it’s precessing around a point. In this case, you have that same, it’s precessing about a point. So yeah. You can think of it as the axial tilt changing but it’s all a part of embedded cycles. So this entire thing that it’s rotating around – yeah.

    Fraser Cain: Whoa. Cycles within cycles.

    Pamela Gay: Right.

    Fraser Cain: Right. I know it’s not related exactly to the precession but you get the situation where the wobble is happening and sort of the tilt is changing a bit. And in fact, with the tilt, that’s one of the theories on Mars that it’s sort of quite significant. There’s very strange oceans on Mars, ancient oceans and maybe it could have been a situation where the tilt completely changed over time.

    Pamela Gay: And with Mars, it’s actually much more complicated situation. Here on Earth, we do have our moon to help stabilize us. Mars has two little captured asteroids that do a very sad job at stabilizing its tilt and so when it gets torqued by Jupiter when it undergoes influence from the sun, there’s no extra body there to help stabilize those torques. So over time, its gravitational interactions with the planet can have a much greater effect than the gravitational influence of other planets have here on Earth. The moon is dominant force for us so Jupiter doesn’t manage to get its hands on us quite so badly. Mars doesn’t have the giant moon and it’s also closer to Jupiter so it experiences more torque.

    Fraser Cain: I was sort of gonna move into that. So what about the other planets? So Mars, as we said, experiences quite a lot of torque. What about Jupiter and the other planets? Are they – and poor Uranus.

    Pamela Gay: So when you start looking at those planets is are much more flattened spheres and they do have, again, more precession but it’s the type of thing where it’s a much larger body and they’re further from the sun. So you have less of a pull from the sun on them. They’re harder to pull because they’re much bigger. So the overall effect, it’s not as dramatic as Mars. Mars definitely has the most dramatic effect of the planets that we worry about weather on.

    Fraser Cain: Now, let’s talk a bit about climate because I think this is one of the things that the global warming climate change deniers will sort of jump on and go, “Well, we’ve got this precession.” And I guess there’s a certain part of it that makes a little bit of sense in that we talk about how the Earth gets closer and further away from the sun at various points in its orbit, whether it’s at its aphelion or its perihelion point, and right now, the Southern hemisphere is the one that is sort of pointed, tilted towards the sun when the Earth is at its closest point to the sun. And so the Southern hemisphere gets more heat and you can –

    Pamela Gay: Summer.

    Fraser Cain: Yeah, you can imagine a situation where 26,000 years from now, or whatever, or 13,000 years from now, the Northern hemisphere will be the one that’s getting that. That will be the case. Right?

    Pamela Gay: Right. So 13,000 years from now, right now, it’s the first week of January is when we’re closest to the sun. That’s near winter solstice for the northern hemisphere, so we get the most sun during our winter which is during the Southern hemisphere’s summer which causes the extremes between the seasons to be much greater for the Southern hemisphere. Their winter is worse. Their summers, they get more energy.

    Now, that is going to change over time so the severity of the seasons will change over time. Slight variations in alignments are also going to cause changes in glaciation patterns but right now, I think the dominant concern that we have has nothing to do with precession but rather it’s the carbon in the atmosphere. The last time there was this much carbon in the atmosphere, it led to massive sea rises and so there’s sever concern that Florida is about to get a lot smaller as an Antarctica melts.

    Fraser Cain: But you would expect a situation where you would see the Northern hemisphere warming up and the Southern hemisphere would probably not get those variations. I guess the Northern hemisphere would get those bigger variations. We’d get hotter summers, cooler winters, while the Southern hemisphere would get milder winters and milder summers in relation to what it has now. But sort of overall, across the entire planet, it would just all balance out again.

    Pamela Gay: It balances out. And these are such gradual processes. It takes basically 26,000 years for this cycle to complete, 25,722, for this cycle to complete for the sun to return to lining up with the exact same set of stars on the sky. And over that roughly 26,000 years, our atmosphere can rebound from the slight changes. The glaciers are capable of picking up in one place and reforming through re-deposition of the water in new places. This isn’t normally an issue. It’s when we change the atmosphere of our planet fundamentally that weather changes fundamentally.

    Fraser Cain: So is this precession changing over time? Billions of years ago, was it different than what it is now?

    Pamela Gay: Well, there’s all sort of different effects at play. The fact that our continents are moving, the fact that, well, 4 billion years ago, we weren’t exactly a solid object, all of these things do have an effect. So when we talk about 25, 722 years, that’s ignoring the torques of the other planets. That’s ignoring any effects due to the center of mass of the planet changing ever so slightly as its continents drift.

    The planet’s moment of inertia, the physics that controls how it responds to rotation, that changed when Japan had the severe earthquake a couple years ago. It only changed a small amount but it changed building of the giant dam in China, it changed the moment of inertia of the Earth. All these different things change our rotation and change how we respond to different forms of torque. And it’s hard to make calculations that span millions and billions of years.

    Fraser Cain: I wonder if we’d wanna do some geo engineering for a pointless reason. Could we slowly, carefully, by building gigantic structures and moving Earth around, sort of provide an opposite torque to sort of get our precession in order?

    Pamela Gay: I don’t understand what you’re trying to do to our poor planet.

    Fraser Cain: Just I wanna remove the precession. The precession just bothers me aesthetically and I need it gone. Could I move mass?

    Pamela Gay: You’d need to add 43 kilometers to our North South axis and then change rotation, stop it, restart it and that would kinda kill everything.

    Fraser Cain: But couldn’t I just kinda tweak it a little every year just a little more, move a little more mass so it sort of just balances out nicely?

    Pamela Gay: Well, so the problem is as long as the sucker is rotating, it’s gonna try and be an oblate sphere.

    Fraser Cain: Yeah, and as long as it’s an oblate spheroid, it’s going to get that uneven torque.

    Pamela Gay: Yeah.

    Fraser Cain: I’m gonna have to go after something that’s a little more productive than removing the precession from the planet Earth. Well, that was great, Pamela. Thank you very much.

    Pamela Gay: Thank you, Fraser.

    Male Speaker: Thanks for listening to Astronomy Cast, a non profit resource provided by Astrosphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at You can email us at Tweet us @astronomycast. Like us on Facebook or circle us on Google+. We record our show live on Google+ every Monday at 12:00p.m. Pacific, 3:00p.m. Eastern or 2000 Greenwich Mean Time. If you miss the live event, you can always catch up over at

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    [End of Audio]

    Duration: 27 minutes

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    4 Responses to Ep. 313: Precession

    1. Mark Kelly September 17, 2013 at 1:31 am #

      Hi Nancy.

      Is there any sign of the errant #310 looming on the horizon in the near future?

      • Nancy October 28, 2013 at 1:41 pm #

        That one may have been lost to the ether (or lost in space) — tech difficulties that may be too difficult to overcome. Maybe a redo…

    2. seth September 24, 2013 at 1:48 am #

      diagram seems incorrect. precession direction should be opposite to rotation. i believe it was stated correctly. see nasa video clip enclosed. seth

    3. Genji May 7, 2015 at 1:01 am #

      No. It’s just that there are two ways to ‘see’ the diagram in 3-D. Make the top precession arrow closer than other half. Then precession IS clockwise. A better drawing would be shaded so only one view possible.

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