Earth Doesn’t Move the Way You Think

Every year, we celebrate our birthdays, woohoo! Over that time, Earth completes one full orbit around the sun. But relative to the center of our Milky Way galaxy, the entire solar system travels about 7 billion kilometers in a single Earth year. From childhood, we're given a convenient and simple picture. A big yellow sun hanging motionless in the center while the Earth obediently traces an ellipse around it. But if the sun itself is hurtling through the galaxy, what does Earth's path actually look like? That's exactly where you get those viral 3D animations from. You've probably seen some of them. The solar system portrayed as a giant vortex with the sun racing forward and the planets trailing behind in a long spiral. These animations aren't entirely fabricated. They start with a real fact, the sun is moving. But they combine that with the wrong geometry, creating a false impression that our star is physically dragging the planets behind it through some dense medium. To see where these animations get it right and where they get it wrong, we need to look at the actual orientation of the orbits, and most importantly, the reference frame we're viewing them from. In physics, you can't depict motion without first answering one question. Relative to what? Imagine a fly buzzing around a light bulb on the ceiling of a moving train. Inside the train, its path is a closed circle. But from the platform, the train's forward motion stretches that circle into an open wavy path. Both descriptions are correct, they just use different reference frames. The creators of the animations we're talking about here are trying to show us the view from the platform, that is, relative to the galaxy. That part is correct. If we track Earth in a galactic reference frame, its closed orbit becomes an open, three-dimensional curve. Its exact shape, however, depends on the orientation of the orbital plane, and that's where these viral videos start cutting corners. Let's take a closer look at this famous viral spiral. In the animation, the orbital plane is positioned almost perpendicular to the Sun's direction of motion. Because of this, the planets appear to trail directly behind the Sun, like a comet's tail. In reality, our system's orientation is completely different. The ecliptic, that's the plane of Earth's orbit, with most of the other planets orbiting close to that same plane, is tilted by about 60 degrees relative to the plane of the Milky Way's disk. On a human timescale, this orientation remains practically unchanged. So the solar system doesn't move through the galaxy head-on, as the animation suggests, it cuts through at an angle. Because of this tilt, the planets can't remain permanently behind the Sun and form a tail. In a galactic reference frame, Earth's trajectory becomes a looping, three-dimensional path superimposed on the Sun's journey through the galaxy. Over the course of a year, our planet alternates between being slightly ahead of and slightly behind the Sun along the direction of its galactic motion. Many animations feature another major oversimplification. The Sun itself moves along a perfectly smooth, straight path. Every system of bodies has a common center of mass, the barycenter. In a gravitationally bound system, all of its objects orbit around this shared center of mass. And even though the Sun accounts for 99.8% of the solar system's total mass, those remaining fractions of a percent are more than enough to influence the star. Our gas giants play the main role here, primarily Jupiter, because it's really, really big, more than 300 times as massive as the Earth, and Saturn. Their combined mass is enough to constantly shift the solar system's barycenter. Sometimes the barycenter is deep inside the Sun, and at other times it lies beyond the Sun's surface, depending on the alignment of the giant planets. Because of this, as it moves through the galaxy, the Sun undergoes a continuous gravitational wobble around this invisible point. On the scale of its galactic trajectory, this oscillation is incredibly small. By the way, this wobble on other stars is one way astronomers find exoplanets. They measure periodic, tiny shifts in a star's position, or spectral lines. But gravity doesn't just affect the Sun. The shape of Earth's orbit, and the orientation of its rotational axis, also change slowly over time. These changes are known as Milankovitch Cycles. Over cycles of roughly 100,000 years, Earth's orbit becomes slightly more elongated and then slightly more circular again. The tilt of Earth's axis varies over a cycle of about 41,000 years, and every 26,000 years, the axis slowly precesses, tracing out a circle under the gravitational influence of the Sun and Moon. These orbital cycles can't freeze the planet overnight, but they gradually alter the distribution of solar energy across different latitudes and seasons. Combined with Earth's own climate feedback loops, such as changes in the extent of reflective ice cover, or in greenhouse gas concentrations, these cycles set the long natural rhythm for the onset and retreat of ice ages. And that's just what's happening inside the solar system. Zoom out to the Milky Way, and things get even weirder. Our entire galaxy is rotating, and the solar system is hurtling around its center at a speed of roughly 230 kilometers per second. It takes us about 230 million years to complete just one full orbit of the galaxy. As it orbits the galactic center, the Sun doesn't follow a perfectly circular path around the galaxy. It simultaneously bobs up and down relative to the midplane of the stellar disk. The cause of these oscillations is the gravity of the Milky Way itself. The bulk of the stars and gas is concentrated in the disk. When the solar system rises above this plane, the combined gravity of the disk begins to pull it back down again. The Sun gains speed, then passes straight through the midplane, sinks below it, and then gravity begins to pull it back up again. If we mentally unroll the Sun's galactic orbit into a straight line, this trajectory would look like a wave. A full cycle of these vertical oscillations takes about 60 to 70 million years. Some researchers have suggested that passing through the densest parts of the galactic plane increases the gravitational pull on the Oort Cloud, that hypothesized spherical reservoir of icy bodies on the distant fringes of the solar system. The idea is that this could send more comets plunging into the inner solar system. But researchers have never convincingly linked this cycle to mass extinctions on Earth, so for now it remains only a hypothesis. But Earth's orbital motion also gives physicists something more practical, a possible way to search for dark matter. Our Milky Way galaxy is thought to be surrounded by a vast, roughly spherical halo of dark matter. Since the direction of Earth's orbital velocity changes throughout the year, our speed relative to this hypothesized dark matter halo changes as well. Earth moves along its orbit at about 30 kilometers per second. Early every June, Earth's orbital velocity has its largest component pointing in the same direction as the solar system's motion relative to the halo. In early December, the component points in the opposite direction. Imagine you're driving a car in pouring rain. The faster you go, the more raindrops hit your windshield every second. This so-called dark matter wind follows the same basic idea. To block out background cosmic rays, physicists place ultra-sensitive detectors deep underground. In the Dama-Libra, Cosine-100, and Anais-112 experiments, ultra-pure sodium iodide crystals, a kind of crystalline salt, but definitely not the kind you put on your french fries, serve as the sensitive material. The logic is simple. If dark matter consists of hypothetical particles, like the so-called WIMPs or Weakly Interacting Massive Particles, then they should pass through the Earth almost entirely unimpeded. But very rarely. Such a particle might, just maybe, could, should collide with an atomic nucleus inside a salt crystal. The collision would produce a tiny flash of light that the detector then could register. That's the idea anyway. Under the standard HALO model, these detectors should register slightly more collisions in June than in December. The Dama-Libra experiment in Italy has claimed for years to have detected exactly this seasonal modulation. But combined results from Cosine-100 and Anais-112 failed to reproduce this modulation, casting serious doubt on the idea that DAMA is detecting dark matter. What causes the DAMA signal, however, is still unclear. But the Milky Way galaxy itself is moving too. Our galaxy, together with Andromeda and the rest of the local group, is moving through space under the gravitational influence of enormous concentrations of matter far beyond our own galaxy. One of the best known of these regions is something you might have heard of called the Great Attractor, part of a much larger pattern of galaxy clusters and superclusters whose combined gravity influences the motion of galaxies across hundreds of millions of light years. It might seem like all we have to do now is add up all these movements, the Earth's orbit, the Sun's wobble around the barycenter, its vertical motion through the galactic disc, and the drift towards the Great Attractor to get our true speed. But no such single number exists. First you have to ask, again, like I said before, relative to what? Since there's no universal stationary coordinate grid in the universe, any speed must be measured relative to something else. One of the most convenient cosmological reference frames is provided by the Cosmic Microwave Background, or CMB, the microwave echo left over from the Big Bang that almost uniformly fills the observable universe. Due to the Doppler effect, this ancient radiation background appears slightly hotter or blue-shifted when you're looking at it in the direction we're flying, and slightly colder or red-shifted behind us. By measuring this temperature difference, scientists were able to determine our speed and direction relative to the CMB. In this reference frame, the entire solar system is moving at a speed of about 370 km per second. So, if the textbook diagram shows only the simplest reference frame, why is it still used? The textbook model isn't wrong, it's simply local. For any calculation, you choose the reference frame that makes the problem easiest to solve. When calculating a probe's trajectory to Mars, engineers don't need to account for the solar system's overall movement through the galaxy, or its speed relative to the CMB. The spacecraft, the Earth, Mars, all share the same bulk motion through the galaxy, so all that other stuff gets ignored when we calculate their positions relative to one another. So Earth does not have a single, true trajectory. Relative to the Sun, it's a nearly closed ellipse. Relative to the center of the galaxy, it traces a complex three-dimensional path. Relative to the cosmic microwave background, it's yet another, even more complex path. None of these descriptions contradicts the others, each is valid in a different reference frame. If this breakdown helped you see the familiar model of the solar system in a new light, please do give the video a like, and of course, subscribe to the channel. Let me know in the comments which other popular model we should break down next. See you at the next set of coordinates.