<p>The rotation of our Milky Way in general is derived from observations of the kinematics of gas and stars throughout our galaxy. See for instance a recent <a href="https://www.sciencedirect.com/science/article/pii/S2352711016300437" rel="nofollow noreferrer">compilation of Milky Way rotation curve data</a> from which this figure is taken
<a href="https://i.sstatic.net/d2hfH.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/d2hfH.jpg" alt="enter image description here" /></a></p>
<p><em>The rotation curve of the Milky Way as derived from gas kinematics (blue), star kinematics (orange) and masers (black).</em></p>

CC BY-SA 4.0
2021-12-04T09:54:25.273Z
What is the difference between the filled and unfilled points? The error bars are huge here, and most of the data points are not even visible by eye. How can one deduce the speed of the Sun from this plot, which is the actual question posed by the OP?
Daddy Kropotkin
CC BY-SA 4.0
2021-12-04T11:34:30.083Z
@DaddyKropotkin Yes, many of the error bars are very big, especially for the outer regions, and generally more so for data relating to stellar kinematics. As for the different symbols, see the explanations in the top image here https://ars.els-cdn.com/content/image/1-s2.0-S2352711016300437-gr2_lrg.jpg
Thomas
CC BY-SA 4.0
2021-12-04T12:34:59.483Z
It would be best if you could explain the figure in your answer. How can one deduce the speed of the Sun from this plot, which is the actual question posed by the OP? As is, your answer does not explain the figure nor does it answer the OP's question.
Daddy Kropotkin
CC BY-SA 4.0
2021-12-04T14:50:04.877Z
@DaddyKropotkin The OP apparently assumed that the speed of the Sun can be calculated from a simple formula similar to Kepler's laws. This is not possible as the mass distribution within the galaxy is poorly known. In fact, if you accept the dark matter hypothesis, it is practically completely unknown. So on the contrary, measurements of the velocities in the galaxy are needed to derive the mass distribution. The figure shown gives results of such measurements. The link provided gives more details how these have been obtained. SE would not be the place to discuss this in detail.
Thomas
CC BY-SA 4.0
2023-02-23T22:26:18.897Z
The original paper listed does not provide evidence to estimate the speed of the Sun, rather it makes a "reasonable choice" of those parameters from the literature and uses those values in their models of galactic rotation (pg 56), though from what I can tell the value they use (230 km/s) isn't actually mentioned in the papers they cite. Those papers give [240](https://dx.doi.org/10.1088/0004-637X/783/2/130), [246](https://dx.doi.org/10.1088/0004-637X/704/2/1704), [200-279](https://doi.org/10.1111/j.1365-2966.2009.15932.x), and [218](https://dx.doi.org/10.1088/0004-637X/759/2/131) km/s
AJ Biffl
CC BY-SA 4.0
2021-11-22T22:19:35.727Z
2021-11-22T22:19:35.727Z
Thomas
<p>It seems the 220~250 km/s value was calculated from &quot;Oort Constants&quot;, originally published in 1927 (IAU 1964: <span class="math-container">$R_0 = 10\ kpc, θ_0=250\ km\ s^{−1}, A = 15\ km\ s^{−1} kpc^{−1}, B = −10\ km\ s^{−1} kpc^{−1}⁠$</span>).</p>

CC BY-SA 4.0
2021-12-07T08:00:24.543Z
And where did the Oort constants come from?!
ProfRob
CC BY-SA 4.0
2021-12-07T16:56:18.150Z
I still don't know how Oort Constants are calculated, but I am learning: https://en.wikipedia.org/wiki/Oort_constants
Julio Otuyama
CC BY-SA 4.0
2021-12-07T17:40:06.267Z
The Oort constants are measured (or derived from measurements) and that offers one route to answering this question.
ProfRob
CC BY-SA 4.0
2021-12-04T01:00:13.837Z
2021-12-04T01:00:13.837Z
Julio Otuyama
<p>The Sun travels around the Milky Way Galaxy with a speed of 220 km/s. The question is: where did this value come from? Is there any article about the calculation?</p>
<p>Reformulated question: What are the different approaches that can be used to estimate the speed of Sun (around the Milky Way Galaxy)?</p>

CC BY-SA 4.0
2021-11-17T03:30:56.947Z
Did you tried Googling? https://en.m.wikipedia.org/wiki/Galactic_year
Nilay Ghosh
CC BY-SA 4.0
2021-11-17T03:38:09.117Z
References (in Wikipedia, and others) does not take to calculations, only the value 220~230 km/s: https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html https://www.scientificamerican.com/article/how-fast-is-the-earth-mov/
Julio Otuyama
CC BY-SA 4.0
2021-11-17T17:22:38.037Z
So, "how is the galactic year calculated?" seems to be the more relevant question.
Daddy Kropotkin
CC BY-SA 4.0
2021-11-17T23:08:59.643Z
The article in Scientific American gives details of how was calculated the speed of Earth (and Solar System: 390 km/s), using Cosmic Microwave Background Radiation (CBR). For instance, I could find some ways to calculate the speed of Earth (rotation, and orbit around the Sun). But the speed of the Solar System (220 km/s) around the Milky Way is somewhat different. Was it calculated a hundred years ago, or is it a computer simulation solution?
Julio Otuyama
CC BY-SA 4.0
2021-11-18T01:05:47.873Z
The most accurate method uses observations to Sgr A*, the (radio source associated with the) supermassive black hole in the Galactic centre. From its proper motion and distance follows the Solar angular speed (which is 250km/s btw). The distance is obtained in various ways, for example by modelling the motion/orbits of the so-called S-stars orbiting Sgr A*.
Walter
CC BY-SA 4.0
2021-11-18T04:22:14.067Z
Thanks for your help.
Julio Otuyama
CC BY-SA 4.0
2021-11-19T19:17:27.573Z
@Walter Do you have a reference?
Daddy Kropotkin
CC BY-SA 4.0
2021-11-20T00:05:50.387Z
@Walter: This would tell us about the movement of Sgr A*, but not that of the Sun. We can’t infer how the Sun moves this way. Also, I’m quite certain the motion of the Sun was known before the discovery of Sgr A*…
Pierre Paquette
CC BY-SA 4.0
2021-12-07T08:07:39.987Z
@PierrePaquette The centre of Galactic motion is Sgr A* and the motion of the Sun w.r.t. Sgr A* *is* what is required here and it is the most accurate method. Of course other methods were used before that.
ProfRob
<orbit><solar-system><milky-way><history>
How was the speed of the Sun (around the Milky Way Galaxy) calculated?
Julio Otuyama
# How was the speed of the Sun (around the Milky Way Galaxy) calculated?

(Asked by: username_0 on 2021-11-17)

The Sun travels around the Milky Way Galaxy with a speed of 220 km/s. The question is: where did this value come from? Is there any article about the calculation?

Reformulated question: What are the different approaches that can be used to estimate the speed of Sun (around the Milky Way Galaxy)?

**Tags:** orbit, solar-system, milky-way, history
> **Comments:**
> **username_1**: Did you tried Googling? https://en.m.wikipedia.org/wiki/Galactic_year  
> **username_0**: References (in Wikipedia, and others) does not take to calculations, only the value 220~230 km/s: https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html https://www.scientificamerican.com/article/how-fast-is-the-earth-mov/  
> **username_2**: So, "how is the galactic year calculated?" seems to be the more relevant question.  
> **username_0**: The article in Scientific American gives details of how was calculated the speed of Earth (and Solar System: 390 km/s), using Cosmic Microwave Background Radiation (CBR). For instance, I could find some ways to calculate the speed of Earth (rotation, and orbit around the Sun). But the speed of the Solar System (220 km/s) around the Milky Way is somewhat different. Was it calculated a hundred years ago, or is it a computer simulation solution?  
> **username_3**: The most accurate method uses observations to Sgr A*, the (radio source associated with the) supermassive black hole in the Galactic centre. From its proper motion and distance follows the Solar angular speed (which is 250km/s btw). The distance is obtained in various ways, for example by modelling the motion/orbits of the so-called S-stars orbiting Sgr A*.  
> **username_0**: Thanks for your help.  
> **username_2**: @Walter Do you have a reference?  
> **username_4**: @Walter: This would tell us about the movement of Sgr A*, but not that of the Sun. We can’t infer how the Sun moves this way. Also, I’m quite certain the motion of the Sun was known before the discovery of Sgr A*…  
> **username_5**: @PierrePaquette The centre of Galactic motion is Sgr A* and the motion of the Sun w.r.t. Sgr A* *is* what is required here and it is the most accurate method. Of course other methods were used before that.  

---

## 2 answers

### Answer by username_6 on 2021-11-22. Score: 5

The rotation of our Milky Way in general is derived from observations of the kinematics of gas and stars throughout our galaxy. See for instance a recent [compilation of Milky Way rotation curve data](https://www.sciencedirect.com/science/article/pii/S2352711016300437) from which this figure is taken
[![enter image description here](https://i.sstatic.net/d2hfH.jpg)](https://i.sstatic.net/d2hfH.jpg)

*The rotation curve of the Milky Way as derived from gas kinematics (blue), star kinematics (orange) and masers (black).*

> **Comments:**
> **username_2**: What is the difference between the filled and unfilled points? The error bars are huge here, and most of the data points are not even visible by eye. How can one deduce the speed of the Sun from this plot, which is the actual question posed by the OP?  
> **username_6**: @DaddyKropotkin Yes, many of the error bars are very big, especially for the outer regions, and generally more so for data relating to stellar kinematics. As for the different symbols, see the explanations in the top image here https://ars.els-cdn.com/content/image/1-s2.0-S2352711016300437-gr2_lrg.jpg  
> **username_2**: It would be best if you could explain the figure in your answer. How can one deduce the speed of the Sun from this plot, which is the actual question posed by the OP? As is, your answer does not explain the figure nor does it answer the OP's question.  
> **username_6**: @DaddyKropotkin The OP apparently assumed that the speed of the Sun can be calculated from a simple formula similar to Kepler's laws. This is not possible as the mass distribution within the galaxy is poorly known. In fact, if you accept the dark matter hypothesis, it is practically completely unknown. So on the contrary, measurements of the velocities in the galaxy are needed to derive the mass distribution. The figure shown gives results of such measurements. The link provided gives more details how these have been obtained. SE would not be the place to discuss this in detail.  
> **username_7**: The original paper listed does not provide evidence to estimate the speed of the Sun, rather it makes a "reasonable choice" of those parameters from the literature and uses those values in their models of galactic rotation (pg 56), though from what I can tell the value they use (230 km/s) isn't actually mentioned in the papers they cite. Those papers give [240](https://dx.doi.org/10.1088/0004-637X/783/2/130), [246](https://dx.doi.org/10.1088/0004-637X/704/2/1704), [200-279](https://doi.org/10.1111/j.1365-2966.2009.15932.x), and [218](https://dx.doi.org/10.1088/0004-637X/759/2/131) km/s  

---

### Answer by username_0 on 2021-12-04. Score: 1

It seems the 220~250 km/s value was calculated from "Oort Constants", originally published in 1927 (IAU 1964: $R\_0 = 10\ kpc, θ\_0=250\ km\ s^{−1}, A = 15\ km\ s^{−1} kpc^{−1}, B = −10\ km\ s^{−1} kpc^{−1}⁠$).

> **Comments:**
> **username_5**: And where did the Oort constants come from?!  
> **username_0**: I still don't know how Oort Constants are calculated, but I am learning: https://en.wikipedia.org/wiki/Oort_constants  
> **username_5**: The Oort constants are measured (or derived from measurements) and that offers one route to answering this question.  

---

