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Comment by rob74 | Hacker News Reader

Cool! One question: what do the planes represent? I thought it was the galactic plane and planes parallel to it, but then I saw that the "band" of the galaxy is (almost?) perpendicular to it, which doesn't fit somehow?

EDIT: TIL that the ecliptic plane of the Solar System is at an 60.2° angle to the galactic plane (https://en.wikipedia.org/wiki/Galactic_plane#/media/File:Mot...) - until now I somehow assumed that they were more or less parallel and never questioned that assumption. So it looks like the "main" plane is the ecliptic plane (which is of course very anthropocentric, after all the ecliptic plane doesn't really matter anymore once you leave the Solar System? But I guess that was they way it was shown in the movie?). Would be interesting to be able to switch to showing the galactic plane instead...

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Thank you for posting this. I wonder if anyone has made a physical galactic orrery. It seems that the concept is used in the video game Warhammer.

Something implicit in the diagrams of the galactic plane but not explicitly stated is that the solar system travels clockwise (retrograde) around the galaxy [0]. I find this unexpected as I thought the same "right hand rules" of planetary motion [1] were somehow connected to those of electromagnetism [2] and would apply upwards in scale.

  The Sun follows the solar circle (eccentricity e < 0.1) at a speed of about 
  255 km/s in a clockwise direction when viewed from the galactic north pole at 
  a radius of ≈ 8.34 kpc about the center of the galaxy near Sgr A*, and has 
  only a slight motion, towards the solar apex, relative to the LSR.

0. https://en.wikipedia.org/wiki/Local_standard_of_rest

1. https://en.wikipedia.org/wiki/Poles_of_astronomical_bodies

2. https://www.arborsci.com/blogs/cool/three-right-hand-rules-o...

I wonder if anyone has made a physical galactic orrery.

Maybe it's harder than it seems. Does a definite galactic plane even exist? The ecliptic is defined by Earth's orbit, not a mean of all the planets. IIRC Sun's rotation plane is not aligned, not should it matter.

If there's a way to measure galactic plane, independently of Sun's orbit around the galaxy center (that also seems difficult to determine) it would involve measuring positions and trajectories of many very distant objects.

Yes, an example is "Local stellar kinematics from Hipparcos data" [0]. Afaict: stars have color (red or blue shift) which represents relative motion; Hipparcos is a large large dataset from an eponymous satellite; fancy math determines relative motion based on position and inferred distance.

0. https://arxiv.org/abs/astro-ph/9710077

I believe such data exists--examine the movement of all galactic objects you can. That will give you a center of mass, the galactic plane is the plane such as to minimize the total distance from objects to the plane.

Go outside on a dark night and look at the Milky Way: you'll see (at the right time of the year) that it's tilted at a large angle to the celestial equator. (There's also the fact that the Earth's axis is tilted relative to the Ecliptic, of course.)

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