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Comment by marcosscriven | Hacker News Reader

I really don't understand how the solar system can orbit the galaxy faster than the spiral arms and be in accordance with Kepler's 2nd law?

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It is better to view the spiral arms as 'transient' in the same way you would view a momentary queue in traffic - although the jam itself moves slowly or not at all, the individual cars that make it up move through the jam and eventually exit on the other side.

Interesting: "Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain the stability of the spiral structure."

https://en.wikipedia.org/wiki/Spiral_galaxy#Origin_of_the_sp...

Wasn't the higher than expected orbital velocity among the first evidence of dark matter?

Rubin-Ford effect

https://en.wikipedia.org/wiki/Vera_Rubin#Galaxy_rotation_pro...

Kepler's laws don't apply to galactic orbits, because galaxies aren't point masses, nor sufficiently spherical and uniform to be approximated as such.

Spiral arms aren't static objects, they are a denser region of the galaxy as stars pass through them. There's also a small speed variance between different systems, so on average it all evens out. Then there's dark matter which adds an additional factor to the orbital speed.

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