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352 points instagraham | 2 comments | | HN request time: 2.45s | source
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nottorp ◴[] No.43543263[source]
So do the galaxies rotate clockwise, or did we make clocks that rotate in the direction the galaxies choose? :)
replies(1): >>43544427 #
1. raattgift ◴[] No.43544427[source]
Sundials are really really cool, and the shadow sweeps in the clockwise direction at mid-latitudes in the northern hemisphere, but

https://en.wikipedia.org/wiki/Sundial#In_the_Southern_Hemisp...

So "clockwise" is because the Earth's axis of rotation about itself is not too far from perpendicular to its orbital plane of rotation around the sun, and because the Earth rotates counter-clockwise from the perspective of someone hovering above the planet's north pole. This is the right-hand rule.

Earth's axis of rotation (and orbital plane) is fairly well aligned with most of the round bodies in the solar system, except notably for oddball tilted Uranus and anti-aligned Venus (and the pretty different orbital planes of the minor planets like Pluto and Eris).

The sun's north pole (following the right-hand rule) and south pole do not point to anywhere particular in the galaxy, and its axis of rotation is highly tilted with respect to the axis through the central bulge of the Milky Way. A nice diagram that seems to have originated from the European Southern Observatory: <https://www.physicsforums.com/attachments/motion-of-earth-an...>

Most disc-like structures we've found tend to be randomly oriented compared to each respective host galaxy's polar axis.

For spiral galaxy rotations we have much better data from mostly edge-on views because we can measure the doppler shifts of molecular clouds at their margins; the advancing side will be less red-shifted than the trailing side. Spectral lines also broaden with the magnitude of rotation. AFAIK there is nothing at all unexpected about the distribution of trailing vs leading edges on our sky; the mystery is in the magnitude of the rotation of these outer gas clouds compared to things like their galaxy's apparent optical brightness or other markers of mass.

Also fun is that for elliptical galaxies, these gas clouds don't rotate around the equatorial bulge of those that have them. They instead tend to move mostly radially deeper and shallower within their host galaxy.

So if galaxies don't choose the rotation of their internal components like star systems or radio-loud objects like pulsars and relativistic-jet-equipped black holes, why (accepting for the sake of this argument that this garbage paper is correct about there being a bias in face-on spiral galaxies) would a distant galaxy affect us more than our own?

replies(1): >>43544767 #
2. nottorp ◴[] No.43544767[source]
Heh I was joking but you can trust HN to have someone come up with a comprehensive explanation for anything :)