On the largest scales, it isn’t just the Earth and the Sun that move, but the entire galaxy and local group, as the invisible forces from gravitation in intergalactic space must all be added up together. Image credit: NASA, ESA; Acknowledgements: Ming Sun (UAH), and Serge Meunier.

How does the Earth move through space?

Now we know, on every scale.

“There is no such thing as perpetual tranquillity of mind while we live here; because life itself is but motion, and can never be without desire, nor without fear, no more than without sense.” -Thomas Hobbes

Ask a scientist for our cosmic address, and you’ll get quite a mouthful. Here we are, on planet Earth, which spins on its axis and revolves around the Sun, which orbits in an ellipse around the center of the Milky Way, which is being pulled towards Andromeda within our local group, which is being pushed around inside our cosmic supercluster, Laniakea, by galactic groups, clusters, and cosmic voids, which itself lies in the KBC void amidst the large-scale structure of the Universe. After decades of research, science has finally put together the complete picture, and can quantify exactly how fast we’re moving through space, on every scale.

Within the Solar System, Earth’s rotation plays an important role in causing the equator to bulge, in creating night-and-day, and in helping power our magnetic field that protects us from cosmic rays and the solar wind. Image credit: Steele Hill / NASA.

Most likely, as you’re reading this right now, you’re sitting down, perceiving yourself as stationary. Yet we know — at a cosmic level — we’re not so stationary after all. For one, the Earth rotates on its axis, hurtling us through space at nearly 1700 km/hr for someone on the equator. That might sound like a big number, but relative to the other contributions to our motion through the Universe, it’s barely a blip on the cosmic radar. That’s not really all that fast, if we switch to thinking about it in terms of kilometers per second instead. The Earth spinning on its axis gives us a speed of just 0.5 km/s, or less than 0.001% the speed of light. But there are other motions that matter more.

The speed at which planets revolve around the Sun far exceeds the rotation speeds of any of them, even for the fastest ones like Jupiter and Saturn. Image credit: NASA / JPL.

Much like all the planets in our Solar System, Earth orbits the Sun at a much speedier clip than its rotational speed. In order to keep us in our stable orbit where we are, we need to move at right around 30 km/s. The inner planets — Mercury and Venus — move faster, while the outer worlds like Mars (and beyond) move slower than this. As the planets orbit in the plane of the solar system, they change their direction-of-motion continuously, with Earth returning to its starting point after 365 days. Well, almost to its same exact starting point.

An accurate model of how the planets orbit the Sun, which then moves through the galaxy in a different direction-of-motion. Image credit: Rhys Taylor of http://www.rhysy.net/, via his blog at http://astrorhysy.blogspot.co.uk/2013/12/and-yet-it-moves-but-not-like-that.html.
An accurate model of how the planets orbit the Sun, which then moves through the galaxy in a different direction-of-motion. Image credit: Rhys Taylor of http://www.rhysy.net/, via his blog at http://astrorhysy.blogspot.co.uk/2013/12/and-yet-it-moves-but-not-like-that.html.

Because even the Sun itself isn’t stationary. Our Milky Way galaxy is huge, massive, and most importantly, is in motion. All the stars, planets, gas clouds, dust grains, black holes, dark matter and more move around inside of it, contributing to and affected by its net gravity. From our vantage point, some 25,000 light years from the galactic center, the Sun speeds around in an ellipse, making a complete revolution once every 220–250 million years or so. It’s estimated that our Sun’s speed is around 200–220 km/s along this journey, which is quite a large number compared both Earth’s rotation speed and its speed-of-revolution around the Sun, which are both inclined at an angle to the Sun’s plane-of-motion around the galaxy.

Although the Sun orbits within the plane of the Milky Way some 25,000–27,000 light years from the center, the orbital directions of the planets in our Solar System do not align with the galaxy at all. Image credit: Science Minus Details / http://www.scienceminusdetails.com/.

But the galaxy itself isn’t stationary, but rather moves due to the gravitational attraction of all the overdense matter clumps and, equally, due to the lack of gravitational attraction from all of the underdense regions. Within our local group, we can measure our speed towards the largest, massive galaxy in our cosmic backyard: Andromeda. It appears to be moving towards our Sun at a speed of 301 km/s, which means — when we factor in the motion of the Sun through the Milky Way — that the local group’s two most massive galaxies, Andromeda and the Milky Way, are headed towards each other at a speed of around 109 km/s.

The largest galaxy in the Local Group, Andromeda, appears small and insignificant next to the Milky Way, but that’s because of its distance: some 2.5 million light years away. It is moving towards our Sun, at the present moment, at around 300 km/s. Image credit: ScienceTV on YouTube / Screenshot.

The Local Group, as massive as it is, isn’t completely isolated. The other galaxies and clusters of galaxies in our vicinity all pull on us, and even the more distant clumps of matter exert a gravitational force. Based on what we can see, measure, and calculate, these structures appear to cause an additional motion of approximately 300 km/s, but in a somewhat different direction than all the other motions, put together. And that explains part, but not all, of the large-scale motion through the Universe. There’s also one more important effect at play, one that was quantified only recently: the gravitational repulsion of cosmic voids.

The various galaxies of the Virgo Supercluster, grouped and clustered together. On the largest scales, the Universe is uniform, but as you look to galaxy or cluster scales, overdense and underdense regions dominate. Image credit: Andrew Z. Colvin, via Wikimedia Commons.

For every atom or particle of matter in the Universe that clusters together in an overdense region, there’s a region of once-average density that’s lost the equivalent amount of mass. Just as a region that’s more dense than average will preferentially attract you, a region that’s less dense than average will attract you with a below-average amount of force. If you get a large region of space with less matter than average in it, that lack-of-attraction effectively behaves as a repellent force, just as extra attraction behaves as an attractive one. In our Universe, opposite to the location of our greatest nearby overdensities, is a great underdense void. Since we’re in between these two regions, the attractive and repulsive forces add up, with each one contributing approximately 300 km/s and the total approaching 600 km/s.

The gravitational attraction (blue) of overdense regions and the relative repulsion (red) of the underdense regions, as they act on the Milky Way. Image credit: Yehuda Hoffman, Daniel Pomarède, R. Brent Tully, and Hélène Courtois, Nature Astronomy 1, 0036 (2017).

When you add all of these motions together: the Earth spinning, the Earth revolving around the Sun, the Sun moving around the galaxy, the Milky Way headed towards Andromeda, and the local group being attracted to the overdense regions and repulsed by the underdense ones, we can get a number for how fast we’re actually moving through the Universe at any given instant. We find that the total motion comes out to 368 km/s in a particular direction, plus or minus about 30 km/s, depending on what time of year it is and which direction the Earth is moving. This is confirmed by measurements of the cosmic microwave background, which appears preferentially hotter in the direction we’re moving, and preferentially colder in the direction opposite to our motion.

The leftover glow from the Big Bang is 3.36 millikelvin hotter in one (the red) direction than average, and 3.36 millikelvin cooler in (the blue) the other than average. This is due to the total motion of everything through space. Image credit: Delabrouille, J. et al.Astron.Astrophys. 553 (2013) A96.

If we ignore the Earth’s motion, we find that the Sun moves relative to the CMB at 368 ± 2 km/s, and that when you throw in the motion of the local group, you get that all of it — the Milky Way, Andromeda, the Triangulum galaxy and all the others — are moving at 627 ± 22 km/s relative to the CMB. That larger uncertainty, by the way, is mostly due to uncertainty in the Sun’s motion around the galactic center, which is the most difficult component to measure.

The relative attractive and repulsive effects of overdense and underdense regions on the Milky Way. The combined effect is known as the Dipole Repeller. Image credit: Yehuda Hoffman, Daniel Pomarède, R. Brent Tully, and Hélène Courtois, Nature Astronomy 1, 0036 (2017).

There might not be a universal frame of reference, but there is a frame of reference that’s useful to measure: the rest frame of the CMB, which also coincides with the rest frame of the Hubble expansion of the Universe. Every galaxy we see has what we call a “peculiar velocity” (or a speed atop the Hubble expansion) of a few hundred to a few thousand km/s, and what we see for ourselves is exactly consistent with that. Our Sun’s peculiar motion of 368 km/s, and our local group’s, of 627 km/s, matches up perfectly with how we understand that all galaxies move through space. Thanks to the effect of the dipole repeller, we now, for the first time, understand how that motion happens for us on every cosmic scale.

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.

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