<p>Yes, our (i.e. the Sun's) motion in the "global", or <em>comoving</em>, reference frame can be measured accurately from the dipole of the cosmic microwave background. The latest results from the <a href="https://arxiv.org/abs/1807.06205" rel="noreferrer">Planck Collaboration et al. (2018)</a> yielded a velocity of
<span class="math-container">$$369.82\pm0.11\,\mathrm{km}\,\mathrm{s}^{-1}
$$</span>
in the direction
<span class="math-container">$$
\begin{array}{rcl}
\ell &amp; = &amp; 264.021º\pm0.011º\\
b    &amp; = &amp; 48.253º\pm0.005º
\end{array}
$$</span>
(in <a href="https://en.wikipedia.org/wiki/Galactic_coordinate_system" rel="noreferrer">Galactic coordinates</a>).</p>

<p>Since Earth orbits the Sun with some <span class="math-container">$30\,\mathrm{km}\,\mathrm{s}^{-1}$</span>, there's a small, biannual correction to this result. On much larger timescales (<span class="math-container">$\sim100\,\mathrm{Myr}$</span>) our motion round the Milky Way alters our comoving velocity with the order of <span class="math-container">$\sim100\,\mathrm{km}\,\mathrm{s}^{-1}$</span>.</p>

CC BY-SA 4.0
2019-09-27T22:27:42.593Z
How can the Planck results be precise to 0.11 km/s if there's a 60 km/s correction depending on the time of the year?
Allure
CC BY-SA 4.0
2019-09-27T22:30:38.300Z
@Allure: [The Earth's orbital velocity around the Sun](https://en.wikipedia.org/wiki/Earth%27s_orbit) is quite well known already. And even if it wasn't, they could just average it out over a whole year (or several).
Ilmari Karonen
CC BY-SA 4.0
2019-09-28T03:33:14.837Z
@IlmariKaronen I was thinking more of, if the question is how fast are *we* moving with respect to the CMB, a precision of 0.11 km/s doesn't make sense because of the seasonal variation. Thinking about it, the quoted value of ~370 km/s must be the Sun's velocity with respect to the CMB.
Allure
CC BY-SA 4.0
2019-09-28T06:36:59.513Z
@Allure Yes, exactly; that's why I wrote "_our (i.e. the Sun's)_" :)
pela
CC BY-SA 4.0
2019-09-28T10:00:00.457Z
How can you move relative to the CMB when its expansion is always directly away from you in every direction?
nick012000
CC BY-SA 4.0
2019-09-28T11:46:53.430Z
@nick012000 Because what you've said isn't true. The small departure from this isotropy is what leads to the measurement quoted in the answer.
ProfRob
CC BY-SA 4.0
2019-09-28T14:29:51.343Z
@RobJefferies My understanding is that inflation works like the “ant on an inflating balloon” metaphor where space between points on the balloon continuously increases without there being a “center point” for the expansion, and that motion of the ant relative to other objects is equivalent to the ant remaining stationary while everything else is moving around it.
nick012000
CC BY-SA 4.0
2019-09-28T16:37:51.587Z
@nick012000 Great, you've covered inflation. Now imagine that, in addition to that effect, the ant is simultaneously walking across the surface of the balloon. The sun isn't just plonked in the Universe and doesn't move at all. It's part of a vast network of interacting objects. Expansion of the universe notwithstanding, the Sun is moving in the Universe (e.g. around the Milky Way, and then there's galactic movement to consider, etc)
Lightness Races in Orbit
CC BY-SA 4.0
2019-09-28T16:39:41.083Z
Personally I'm surprised it's so "slow". Nice calculation!
Lightness Races in Orbit
CC BY-SA 4.0
2019-09-28T20:46:47.597Z
@LightnessRacesinOrbit Not just inflation. “that motion of the ant relative to other objects is equivalent to the ant remaining stationary while everything else is moving around it.”
nick012000
CC BY-SA 4.0
2019-09-28T20:51:17.113Z
@nick012000 You're right, this is not just inflation, but the analogy is good enough: Galaxies in an expanding universe (also inflationary expansion, though there were no galaxies around then) are like ants on a balloon surface. But the ants also crawl around slowly, which is exactly this so-called "peculiar" velocity that I quote above.
pela
CC BY-SA 4.0
2019-09-28T23:46:31.097Z
@nick012000 That doesn't mean nothing moves.
Lightness Races in Orbit
CC BY-SA 4.0
2019-09-29T08:50:16.197Z
@LightnessRacesinOrbit That's probably because there are no massively massive superclusters around into which the Home Galaxy can fall.
David Tonhofer
CC BY-SA 4.0
2019-09-29T09:41:19.013Z
@nick I'm sure you've seen CMB images like [this](https://upload.wikimedia.org/wikipedia/commons/thumb/3/3c/Ilc_9yr_moll4096.png/1024px-Ilc_9yr_moll4096.png), which show that the CMB is isotropic to better than one part in 10,000. Such images have been corrected for our peculiar motion; otherwise, the CMB details would be swamped by the Doppler effect of the peculiar motion. A raw map of the CMB (i.e., without that correction), looks like [this](http://astronomy.swin.edu.au/cosmos/C/Cosmic+Microwave+Background+Dipole).
PM 2Ring
CC BY-SA 4.0
2019-09-30T10:33:18.010Z
@DavidTonhofer That's a relief ;)
Lightness Races in Orbit
CC BY-SA 4.0
2019-09-30T15:15:37.937Z
@LightnessRacesinOrbit A bit further there is the [Great Attractor](https://en.wikipedia.org/wiki/Great_Attractor) though, sadly invisible because hidden from view behind the galactic disk; which is also apparently not so great and may be surpassed by other Greats. It's the Nightland out there.
David Tonhofer
CC BY-SA 4.0
2019-09-27T13:41:20.037Z
2019-09-27T13:41:20.037Z
pela
<p>The cosmic microwave background radiation should provide kind of a global reference frame, because you can determine your speed relative to it using the redshift.</p>

<p>Is it known how fast we are moving in relation to the CMB? If you subtract the various orbital motions (Earth around the Sun, Sun around the Galaxy), are we standing still in the expanding universe, or traveling in a certain direction?</p>

CC BY-SA 4.0
2019-09-29T08:50:41.427Z
This is known as galactic peculiar velocity: https://en.wikipedia.org/wiki/Peculiar_velocity
David Tonhofer
<cosmic-microwave-background>
How fast are we moving relative to the CMB?
cuckoo
# How fast are we moving relative to the CMB?

(Asked by: username_0 on 2019-09-27)

The cosmic microwave background radiation should provide kind of a global reference frame, because you can determine your speed relative to it using the redshift.

Is it known how fast we are moving in relation to the CMB? If you subtract the various orbital motions (Earth around the Sun, Sun around the Galaxy), are we standing still in the expanding universe, or traveling in a certain direction?

**Tags:** cosmic-microwave-background
> **Comments:**
> **username_1**: This is known as galactic peculiar velocity: https://en.wikipedia.org/wiki/Peculiar_velocity  

---

## 1 answers

### Answer by username_2 on 2019-09-27. Score: 29 | Accepted answer

Yes, our (i.e. the Sun's) motion in the "global", or *comoving*, reference frame can be measured accurately from the dipole of the cosmic microwave background. The latest results from the [Planck Collaboration et al. (2018)](https://arxiv.org/abs/1807.06205) yielded a velocity of
$$369.82\pm0.11\,\mathrm{km}\,\mathrm{s}^{-1}
$$
in the direction
$$
\begin{array}{rcl}
\ell & = & 264.021º\pm0.011º\\
b & = & 48.253º\pm0.005º
\end{array}
$$
(in [Galactic coordinates](https://en.wikipedia.org/wiki/Galactic_coordinate_system)).

Since Earth orbits the Sun with some $30\,\mathrm{km}\,\mathrm{s}^{-1}$, there's a small, biannual correction to this result. On much larger timescales ($\sim100\,\mathrm{Myr}$) our motion round the Milky Way alters our comoving velocity with the order of $\sim100\,\mathrm{km}\,\mathrm{s}^{-1}$.

> **Comments:**
> **username_3**: How can the Planck results be precise to 0.11 km/s if there's a 60 km/s correction depending on the time of the year?  
> **username_4**: @Allure: [The Earth's orbital velocity around the Sun](https://en.wikipedia.org/wiki/Earth%27s_orbit) is quite well known already. And even if it wasn't, they could just average it out over a whole year (or several).  
> **username_3**: @IlmariKaronen I was thinking more of, if the question is how fast are *we* moving with respect to the CMB, a precision of 0.11 km/s doesn't make sense because of the seasonal variation. Thinking about it, the quoted value of ~370 km/s must be the Sun's velocity with respect to the CMB.  
> **username_2**: @Allure Yes, exactly; that's why I wrote "_our (i.e. the Sun's)_" :)  
> **username_5**: How can you move relative to the CMB when its expansion is always directly away from you in every direction?  
> **username_6**: @nick012000 Because what you've said isn't true. The small departure from this isotropy is what leads to the measurement quoted in the answer.  
> **username_5**: @RobJefferies My understanding is that inflation works like the “ant on an inflating balloon” metaphor where space between points on the balloon continuously increases without there being a “center point” for the expansion, and that motion of the ant relative to other objects is equivalent to the ant remaining stationary while everything else is moving around it.  
> **username_7**: @nick012000 Great, you've covered inflation. Now imagine that, in addition to that effect, the ant is simultaneously walking across the surface of the balloon. The sun isn't just plonked in the Universe and doesn't move at all. It's part of a vast network of interacting objects. Expansion of the universe notwithstanding, the Sun is moving in the Universe (e.g. around the Milky Way, and then there's galactic movement to consider, etc)  
> **username_7**: Personally I'm surprised it's so "slow". Nice calculation!  
> **username_5**: @LightnessRacesinOrbit Not just inflation. “that motion of the ant relative to other objects is equivalent to the ant remaining stationary while everything else is moving around it.”  
> **username_2**: @nick012000 You're right, this is not just inflation, but the analogy is good enough: Galaxies in an expanding universe (also inflationary expansion, though there were no galaxies around then) are like ants on a balloon surface. But the ants also crawl around slowly, which is exactly this so-called "peculiar" velocity that I quote above.  
> **username_7**: @nick012000 That doesn't mean nothing moves.  
> **username_1**: @LightnessRacesinOrbit That's probably because there are no massively massive superclusters around into which the Home Galaxy can fall.  
> **username_8**: @nick I'm sure you've seen CMB images like [this](https://upload.wikimedia.org/wikipedia/commons/thumb/3/3c/Ilc_9yr_moll4096.png/1024px-Ilc_9yr_moll4096.png), which show that the CMB is isotropic to better than one part in 10,000. Such images have been corrected for our peculiar motion; otherwise, the CMB details would be swamped by the Doppler effect of the peculiar motion. A raw map of the CMB (i.e., without that correction), looks like [this](http://astronomy.swin.edu.au/cosmos/C/Cosmic+Microwave+Background+Dipole).  
> **username_7**: @DavidTonhofer That's a relief ;)  
> **username_1**: @LightnessRacesinOrbit A bit further there is the [Great Attractor](https://en.wikipedia.org/wiki/Great_Attractor) though, sadly invisible because hidden from view behind the galactic disk; which is also apparently not so great and may be surpassed by other Greats. It's the Nightland out there.  

---

