# How fast does the Milky Way rotate?

**URL:** <https://boards.straightdope.com/t/how-fast-does-the-milky-way-rotate/63707>\
**Category:** Factual Questions\
**Created:** [April 29, 2001, 5:17am UTC](https://boards.straightdope.com/t/how-fast-does-the-milky-way-rotate/63707 "2001-04-29T05:17:45Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![SPOOFE](https://avatars.discourse-cdn.com/v4/letter/s/c5a1d2/32.png) [@SPOOFE](https://boards.straightdope.com/u/SPOOFE)\
**Post date:** [April 29, 2001, 5:17am UTC](https://boards.straightdope.com/t/how-fast-does-the-milky-way-rotate/63707/1 "2001-04-29T05:17:45Z")

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I usually try to add some interesting commentary to my questions whenever I post to GQ… but, nah, this time I just want to know how long (many millions of years, I’d estimate) for the Milky Way to spin 360°?

I know it’d take a light particle over 300,000 years to travel around the whole circumference, but beyond that, I have no idea how to extrapolate from there.

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**Author:** ![Bob\_Scene](https://avatars.discourse-cdn.com/v4/letter/b/e19adc/32.png) [@Bob\_Scene](https://boards.straightdope.com/u/Bob_Scene)\
**Post date:** [April 29, 2001, 5:26am UTC](https://boards.straightdope.com/t/how-fast-does-the-milky-way-rotate/63707/2 "2001-04-29T05:26:42Z")

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About 200 million.

[http://www.uq.edu.au/~phjross/ph227/galaxy/mwgalaxy.htm](http://www.uq.edu.au/~phjross/ph227/galaxy/mwgalaxy.htm)

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**Author:** ![Terminus\_Est](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/terminus_est/32/3087_2.png) [@Terminus\_Est](https://boards.straightdope.com/u/Terminus_Est)\
**Post date:** [April 29, 2001, 4:23pm UTC](https://boards.straightdope.com/t/how-fast-does-the-milky-way-rotate/63707/3 "2001-04-29T16:23:40Z")

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> [@](#):
>
> \*Originally posted by Bob Scene \*  
> \*\*About 200 million.
> 
> [http://www.uq.edu.au/~phjross/ph227/galaxy/mwgalaxy.htm](http://www.uq.edu.au/~phjross/ph227/galaxy/mwgalaxy.htm) \*\*

This is true for the sun, which is roughtly 2/3 of the way out from the center. Farther out, the orbital period is longer; farther in it’s shorter.

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**Author:** ![bryanmcc](https://avatars.discourse-cdn.com/v4/letter/b/eada6e/32.png) [@bryanmcc](https://boards.straightdope.com/u/bryanmcc)\
**Post date:** [April 29, 2001, 8:10pm UTC](https://boards.straightdope.com/t/how-fast-does-the-milky-way-rotate/63707/4 "2001-04-29T20:10:59Z")

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I just happened to have been watching _Cosmos_ last night. According to Carl Sagan (c. 1980) the Milky Way rotates every quarter billion years. Of course, as implied above, that is just some sort of “average”: the individual stars in different locations may orbit at different rates.

-b

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**Author:** ![Terminus\_Est](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/terminus_est/32/3087_2.png) [@Terminus\_Est](https://boards.straightdope.com/u/Terminus_Est)\
**Post date:** [April 29, 2001, 8:37pm UTC](https://boards.straightdope.com/t/how-fast-does-the-milky-way-rotate/63707/5 "2001-04-29T20:37:17Z")

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Rereading this, I notice that the thread title asks a different question than the OP itself. That is, the title is asking about the _orbital velocity_ whereas the OP is asking about _orbital period_. You already have your answer for orbital period, at least for the sun. In the case of orbital velocity, according to [this page](http://www.physics.gmu.edu/classinfo/astr103/CourseNotes/Html/Lec07/Lec07_pt1_galaxiesOur.htm):

> [@](#):
>
> [ul]  
> [li]Within 300 ly of Galactic center, central regions rotate like a solid wheel so that orbital velocities increase from center outward[/li][li]From 4000 ly through position of Sun, 30,000 ly and on to 60,000 ly velocity ranges from 200 to 300 km/s[/li][li]Beyond 60,000 ly velocity is probably nearer 200 km/s[/li][/ul]