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Starts With A Bang

How, exactly, does planet Earth move through the Universe?

The Solar System isn’t a vortex, but rather the sum of all our great cosmic motions. Here’s how we move through space.
Earth move
Planet Earth's motion through space isn't just defined by our axial rotation or our motion around the Sun, but the Solar System's motion through the galaxy, the Milky Way's motion through the Local Group, and the Local Group's motion through intergalactic space. Only with everything combined, and by comparing to the Big Bang's leftover glow, can we arrive at a meaningful answer. (Credit: Jim slater307/Wikimedia Commons; background: ESO/S. Brunier)
Key Takeaways
  • The Earth spins on its axis, orbits the Sun, and travels through the Milky Way, which itself is in motion relative to all the other galaxies around us.
  • By correctly measuring the objects around us and the light left over from the Big Bang, we can determine our cumulative cosmic motion.
  • Still, there remains an uncertainty that we’ll never be able to get rid of. Here’s why.

Planet Earth isn’t at rest, but continuously moves through space.

Earth move
This view of the Earth comes to us courtesy of NASA’s MESSENGER spacecraft, which had to perform flybys of Earth and Venus in order to lose enough energy to reach its ultimate destination: Mercury. The round, rotating Earth and its features are undeniable, as this rotation explains why Earth bulges at the center, is compressed at the poles, and has different equatorial and polar diameters. (Credit: NASA/MESSENGER)

The Earth rotates on its axis, spinning a full 360° with each passing day.

Earth move
The effects of the Coriolis Force on a pendulum rotating at 45 degrees North latitude. Note that the pendulum takes two full rotations of Earth in order to make a single, complete rotation at this particular latitude; the rotation angle, just like the speed at Earth’s surface, is latitude-dependent.. (Credit: Cleon Teunissen /

That translates into an equatorial speed of ~1700 km/hr, dropping lower with increasing latitudes.

The Earth, moving in its orbit around the Sun and spinning on its axis, appears to make a closed, unchanging, elliptical orbit. If we look to a high-enough precision, however, we’ll find that our planet is actually spiraling away from the Sun by about 1.5 cm per year, and precesses in its orbit on timescales of tens of thousands of years. (Credit: Larry McNish/RASC Calgary)

Meanwhile, the Earth revolves around the Sun, at speeds ranging from 29.29 km/s to 30.29 km/s.

Just 800 years ago, perihelion and the winter solstice aligned. Due to the precession of Earth’s orbit, they are slowly drifting apart, completing a full cycle every 21,000 years. Over time, the Earth drifts slightly farther from the Sun, the precession period increases, and the eccentricity varies as well. (Credit: Greg Benson/Wikimedia Commons)

Early January’s perihelion causes the fastest motions, while July’s aphelion yields the slowest.

All of the major planets orbit the Sun in ellipses that are nearly circles, with only a few percent deviation among even the most eccentric planets. The rotational speed of any planet is tiny compared to its orbital speed, but the orbital speeds of the planets are small compared to the Solar System’s motion through the galaxy. This animation shows our future gravitational encounter with asteroid 99942 Apophis, scheduled for 2029. (Credit: ESA/NEO Coordination Centre)

Atop that, the entire Solar System travels around the Milky Way.

The Sun, like all the stars in our galaxy, orbits around the galactic center at speeds of hundreds of km/s. In our neighborhood, the speed of the Sun and the other stars around the galactic center have an uncertainty of around ~10%, or ~20 km/s, which is the largest factor of uncertainty when it comes to calculating our cumulative motion. (Credit: Jon Lomberg and NASA)

Our heliocentric speed of 200 to 220 km/s is inclined ~60° to the plane of the planets.

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. As far as we can tell, the orbital planes of the planets occur randomly within a stellar system, often aligned with the central star’s rotational plane but randomly aligned with the plane of the Milky Way. (Credit: Science Minus Details)

However, our motion isn’t vortical, but a simple sum of these velocities.

An accurate model of how the planets orbit the Sun, which then moves through the galaxy in a different direction-of-motion. The speeds of the planets around the Sun are only a small fraction of the Solar System’s motion through the Milky Way galaxy, with even Mercury’s revolution around the Sun contributing only ~20% of its total motion through our galaxy. (Credit: Rhys Taylor)

On grander scales, the Milky Way and Andromeda travel toward each other at 109 km/s.

A series of stills showing the Milky Way-Andromeda merger, and how the sky will appear different from Earth as it happens. When these two galaxies merge, their supermassive black holes are fully expected to merge together as well. At present, the Milky Way and Andromeda move towards one another at a relative speed of ~109 km/s. (Credit: NASA; Z. Levay and R. van der Marel, STScI; T. Hallas; A. Mellinger)

Attractive clumps and repulsive underdense regions both tug on our Local Group.

This illustrated map of our local supercluster, the Virgo supercluster, spans more than 100 million light-years and contains our Local Group, which has the Milky Way, Andromeda, Triangulum, and about ~60 smaller galaxies. The overdense regions gravitationally attract us, while the regions of below-average density effectively repel us relative to the average cosmic attraction. (Credit: Andrew Z. Colvin/Wikimedia Commons)

Combined, we move 627 ± 22 km/s relative to the cosmic average.

Because matter is distributed roughly uniformly throughout the Universe, it isn’t just the overdense regions that gravitationally influence our motions, but the underdense regions as well. A feature known as the dipole repeller, illustrated here, was discovered only recently, and may explain our Local Group’s peculiar motion relative to the other objects in the Universe. (Credit: Y. Hoffman et al., Nature Astronomy, 2017)

However, the Big Bang’s leftover photons offer a cosmically unique rest frame.

At any epoch in our cosmic history, any observer will experience a uniform “bath” of omnidirectional radiation that originated back at the Big Bang. Today, from our perspective, it’s just 2.725 K above absolute zero, and hence is observed as the cosmic microwave background, peaking in microwave frequencies. (Credit: Earth: NASA/BlueEarth; Milky Way: ESO/S. Brunier; CMB: NASA/WMAP)

The Sun moves at a cumulative 368 km/s relative to the Cosmic Microwave Background (CMB).

Although the cosmic microwave background is the same rough temperature in all directions, there are 1-part-in-800 deviations in one particular direction: consistent with this being our motion through the Universe. At 1-part-in-800 the overall magnitude of the CMB’s amplitude itself, this corresponds to a motion of about 1-part-in-800 the speed of light, or ~368 km/s. (Credit: J. Delabrouille et al., A&A, 2013)

An inherent uncertainty of ± 2 km/s comes from not knowing the intrinsic CMB dipole’s magnitude.

Although we can measure the temperature variations all across the sky, on all angular scales, we cannot disentangle whatever the intrinsic dipole in the cosmic microwave background is, as the dipole we observe, from our motion through the Universe, is more than a factor of ~100 larger than whatever the primordial value is. With only one location to measure the value of this parameter at, we cannot disentangle which part is due to our motion and which part is inherent; it would take tens of thousands of such measurements to reduce the uncertainties here below their current values. (Credit: NASA/ESA and the COBE, WMAP, and Planck teams; Planck Collaboration, A&A, 2020)

Being confined to the Milky Way, we can only dream of making such measurements.

The initial fluctuations that were imprinted on our observable universe during inflation may only come into play at the ~0.003% level, but those tiny imperfections lead to the temperature and density fluctuations that appear in the cosmic microwave background and that seed the large-scale structure that exists today. Measuring the CMB at a variety of cosmic locations would be the only feasible way to disentangle the intrinsic dipole of the CMB from that induced by our motion through the Universe. (Credit: Chris Blake and Sam Moorfield)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.


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