# How do we know the Sun orbits the Galaxy?

• Dertulm01
In summary, the evidence suggests that the Sun orbits the center of the Milky Way galaxy. Scientists are still trying to figure out how much mass is in the galaxy, and how it is distributed. Measurements of the Sun's orbital velocity using the Galaxy's center as a reference are very robust.f

#### Dertulm01

How do scientists know that the Sun orbits the Milky Way? Is this something more ancient scientists knew or could have known hundreds of years ago?

Nobody really knew what the Milky Way was until the last hundred years, let alone that we orbit its centre. To know this requires precise measurements of the speeds and distances of stars throughout the Galaxy, again, possible only using technology from the last hundred years or so.

Interesting question, and now I wonder just what the experimental evidence is. Not just the fact that it happens (we can appeal to our understanding of gravity for that), but that we actually know the orbital period is 250 million years.

Orbital period is simple if you know radius and velocity. Radius I can understand but velocity... ehhh... My best guess is that they use a rough estimate for the amount of mass in the galaxy, assume it's clumped in the middle, and then figure out the velocity and thus period. Only thing is it ain't all clumped in the middle, so this doesn't work very well!

It doesn't matter whether the mass is clumped in the middle or not, numerical calculations can solved the gravitational force for any known distribution of mass.

The key is that we don't know how much mass there is, or how it is distributed, since much of it is due to dark matter.

I would guess that measuring Doppler shifts of spectral lines in stars is involved with measuring our velocity.

The easiest way to measure the Sun's orbital velocity is to examine the motion of the Galactic centre relative to background stars on the far side of the Milky Way. See for example http://www.chron.com/content/interactive/space/astronomy/news/1999/ds/990602.html [Broken]

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The observational evidence suggesting the sun orbits the center of our galaxy is very robust.

The stars in our galaxy appear to be redshifted on one side of the center of the milky way, and blue shifted on the other. This would be good evidence that stars in our galaxy orbit the center.
The initial unadjusted data for the CMB looks like a ying-yang. It seems like this would be good evidence that we are orbiting our galaxies center.

By far, the coolest books I've ever read on this subject are Galactic Dynamics by Binney and Tremaine, and Galactic Astronomy by Binney and Merrifield.

In the first sections, it is described how the concept of "galaxy" evolved from ancient times to now, including all of the experimental evidence that lead us to the conclusion that the Sun orbits the galactic core at a relatively large distance.

Does the angular velocity of the suns orbit around the center of the galaxy in any way cause an effect for our solar system? Seems like it should (all elliptical orbits immeasurably changed by some small force).
I assume its small.

The stars in our galaxy appear to be redshifted on one side of the center of the milky way, and blue shifted on the other. This would be good evidence that stars in our galaxy orbit the center.
The initial unadjusted data for the CMB looks like a ying-yang. It seems like this would be good evidence that we are orbiting our galaxies center.

How come we can see the doopler effect if we travel at the same angular velocity with the stars of the two sides?

Pixel01,

I had the same question. That was why I only said it was good evidence that those stars were orbiting. By infrence we would be as well. I think I could have said it better by saying it was evidence that either we or they were orbiting the galaxy. Either way it seems to be a pretty good evidence that we are going around the center of the Milky Way. The CMB ying-yang gives additional support.

How come we can see the doopler effect if we travel at the same angular velocity with the stars of the two sides?

Because the Doppler effect depends on line-of-sight velocity, not angular velocity - think of the velocity vectors of the stars involved and you will see how this works.

Just to help clarify, we can't see stars directly across the center of the galaxy from us. Stars in that swath of sky called the Milky Way are ahead of or behind us in our spiral arm or adjacent ones and nearer or further from the center (and therefore orbiting at different speeds) and they will show motion relative to us.