How, exactly, does planet Earth move through the Universe?
The travel of Planet Earth through space is determined not only by our axial rotation or our motion around the Sun, but also by the motion of the Solar System through the galaxy, the Milky Way’s motion through the Local Group, and the Local Group’s motion through intergalactic space. Only by combining everything and comparing it to the Big Bang’s leftover light can we arrive at a meaningful answer. (Credit: Jim slater307/Wikimedia Commons; background: ESO/S. Brunier)
The Earth rotates on its axis, orbits the Sun, and travels through the Milky Way, which is in motion relative to all other galaxies. We can calculate our cumulative cosmic speed by accurately measuring the objects around us and the light left behind from the Big Bang. Still, there is an element of ambiguity that we will never be able to eliminate. This is why.
Planet Earth is not at rest, but is constantly moving through space.
This image of Earth was provided by NASA’s MESSENGER spacecraft, which had to undertake flybys of Earth and Venus in order to lose enough energy to reach its final destination: Mercury. The round, rotating Earth and its properties are indisputable, as its rotation explains why the Earth bulges at the center, is compressed at the poles, and has different equatorial and polar diameters. (Credit: NASA/MESSENGER)
Each day, the Earth spins on its axis, turning a full 360°.
The Coriolis Force’s impact on a pendulum rotating at 45 degrees North latitude. It is worth noting that the pendulum requires two full rotations of Earth to create a single, complete rotation at this latitude; the rotation angle, like the speed at the Earth’s surface, is latitude-dependent. (Credit: Cleon Teunissen / http://cleonis.nl)
This correlates to an equatorial speed of ~1700 km/hr, which decreases as latitude increases.
The Earth appears to move in a closed, unchanging elliptical orbit around the Sun while rotating on its axis. However, if we look closely enough, we can see that our planet is actually spiraling away from the Sun at a rate of around 1.5 cm per year, and that its orbit precesses on periods of tens of thousands of years. (Credit: Larry McNish/RASC Calgary)
Meanwhile, the Earth orbits the Sun at speeds varying from 29.29 km/s to 30.29 km/s.
Perihelion and the winter solstice coincided only 800 years ago. They are gradually migrating apart due to the precession of Earth’s orbit, completing a full cycle every 21,000 years. The Earth drifts away from the Sun somewhat over time, the precession period lengthens, and the eccentricity changes. (Credit: Greg Benson/Wikimedia Commons)
The perihelion in early January produces the quickest motions, while the aphelion in July produces 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, travels at hundreds of kilometers per second around the galactic center. The speed of the Sun and other stars in our neighborhood has an uncertainty of roughly ~10%, or ~20 km/s, which is the highest factor of uncertainty when it comes to estimating our cumulative motion. (Credit: Jon Lomberg and NASA)
Our heliocentric speed ranges from 200 to 220 km/s and is inclined ~60° to the plane of the planets.
Although the Sun circles within the plane of the Milky Way at 25,000-27,000 light years from its center, the orbital directions of our Solar System’s planets do not align with the galaxy at all. As far as we can tell, planet orbital planes occur at random inside a stellar system, often aligned with the rotational plane of the central star but randomly aligned with the plane of the Milky Way. (Credit: Science Minus Details)
Our motion, however, is not vortical, but rather a simple sum of these velocities.
An accurate depiction of how planets orbit the Sun, which then moves across the galaxy in a separate direction. 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 larger scales, the Milky Way and Andromeda galaxy are moving toward one other at 109 km/s.
A series of stills depicting the Milky Way-Andromeda merger and how the sky will change as it happens. When these two galaxies combine, their supermassive black holes will very certainly merge as well. The Milky Way and Andromeda are currently moving towards one other at a relative speed of ~109 km/s. (Credit: NASA; Z. Levay and R. van der Marel, STScI; T. Hallas; A. Mellinger)
Both appealing clumps and repulsive underdense regions pull on our Local Group.
The Virgo supercluster, which comprises our Local Group, spans more than 100 million light-years and contains the Milky Way, Andromeda, Triangulum, and roughly ~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)
We move 627 22 km/s faster than 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 characteristic known as the dipole repeller, shown below, was discovered very recently and may explain our Local Group’s unusual motion relative to other objects in the Universe. (Credit: Y. Hoffman et al., Nature Astronomy, 2017)
However, the remaining photons from the Big Bang provide a cosmically unique rest frame.
Any observer at any time in our cosmic history will face a uniform “bath” of omnidirectional radiation that originated with the Big Bang. Today, it is only 2.725 K above absolute zero, and so is observed as the cosmic microwave background, peaking at microwave frequencies. (Credit: Earth: NASA/BlueEarth; Milky Way: ESO/S. Brunier; CMB: NASA/WMAP)
The Sun moves at a total speed of 368 km/s in relation to the Cosmic Microwave Background (CMB).
Although the cosmic microwave background is almost the same temperature in all directions, there are 1-part-in-800 variances in one direction, which is consistent with our speed through the Universe. This equates to a motion of around 1-part-in-800 the speed of light, or ~368 km/s, at 1-part-in-800 the overall magnitude of the CMB’s amplitude. (Credit: J. Delabrouille et al., A&A, 2013)
The inherent uncertainty of ±2 km/s comes from not knowing the magnitude of the intrinsic CMB dipole.
We cannot disentangle whatever the intrinsic dipole in the cosmic microwave background is because the dipole we detect from our travel across the Universe is more than a factor of ~100 bigger than whatever the primordial value is. With only one place to test the value of this parameter at, we can’t tell which part is due to our motion and which part is inherent; tens of thousands of such measurements would be required to lower the uncertainties here below their current levels. (Credit: NASA/ESA and the COBE, WMAP, and Planck teams; Planck Collaboration, A&A, 2020)
We can only dream of making such measurements because we are constrained to the Milky Way.
The earliest variations imprinted on our observable universe during inflation may only be ~0.003%, yet those minuscule defects contribute to the temperature and density fluctuations that appear in the cosmic microwave background and seed the large-scale structure that exists today. Measuring the CMB at various cosmic locations would be the only realistic way to separate the CMB’s intrinsic dipole from that caused by our speed through the Universe. (Credit: Chris Blake and Sam Moorfield)