so I wrote a long reply in a thread on someone else's post

[arkuat]

And I thought the long reply I made on someone else's post might make a good post. It's a story to explain how I got so good at identifying northern constellations by naked eye, especially in a sky that is a little bit light-polluted, but not too much so (because that's the environment in which I learned the constellations--finding them in a truly dark sky full of unfamiliar stars is actually more difficult for me than finding the bright ones, the only ones that are visible, in a city sky).

So when I was in second or third grade, I seem to have attained an understanding of inertia with regard to angular momentum. I knew the Earth spun (and pretty much everything else spins) because it's spinning in the first place, and very difficult to stop. But I kept pestering my elementary school teacher with questions about what started it spinning in the first place. (Nowadays, I'd ask, where did the conserved angular momentum come from, and I'm pretty sure that's cosmology and maybe even involves quantum spin, but quadong would know better than I about the latter.)

At the time I lived in Elgin, home of the Elgin Observatory built by the old watch factory, which observatory was at the time owned by school district U-46. So my teacher, or someone at school, suggested I address my question in a letter to Charles Tuttle, the then director of the observatory. I did so (my mom typed it up for me). I don't remember his answer, but I was invited to take after-school classes in astronomy there, which I did during third and fourth grade. We ground mirror-blanks for telescopes, and studied constellations in the planetarium. Now, planetarium constellations are not quite as fascinating to the unlearned as real constellations, and when test-time came, I found that I could only identify Orion and (oddly?) Delphinus. I found this bad test performance humiliating, and that's whence my motivation came in the 1990s when I spent a lot of time outdoors at night studying real constellations.

Now I know the northern constellations so well that the first item on my bucket list is a long stargazing trip to the southern hemisphere, so that I can learn the southern sky as well. I'm mostly a naked-eye and binoculars person, the same way jiawen is such a magnificent telescope and astrophotography person.

Comments

[spiderplanet]

You have reminded me of this only very peripherally related video. If you haven't already seen it, you will enjoy it.

https://www.youtube.com/watch?v=1VPfZ_XzisU

[quadong]

Good video.

Cites something citing my hated classical mechanics textbook. I wonder if I would like gory classical mechanics if I tried to learn/teach it now. It seems to be a thing old people like.

[spiderplanet]

I posit that you are not actually currently old, but you may have been born as an old man and are growing in to your preformed disposition. :) I'm glad you are a young old man.

[quadong]

Excellent.

Yes, Delphinus is "oddly" for sure. I couldn't have even *named* that as a constellation.

Where did the conserved angular momentum come from? I have some outstanding questions about angular momentum myself. Primarily (and not relating directly to yours), why is it that classically angular momentum is a derived quantity, really being just a combination of linear momentum and having two things not traveling co-linearly (and the axiom that space is rotationally invariant), but in quantum mechanics it is a thing that particles can have which is fundamental? For a while I thought that the answer was that quantum mechanical "angular momentum" was just an abstraction or analogy, like the strong force's "color", but no, because you can convert quantum angular momentum to macroscopic angular momentum and vice versa. They really are the same quantity, or anyway the sum of them is the thing that is conserved. There's something deep about this that I haven't grasped yet.

But as far as why the planets, solar system and galaxy spin, I think it's a more mundane thing. If you randomly stir water in a bathtub, some spins some way and some spins the other way at random, adding up to zero (if you stirred equally clockwise and counter-clockwise). I think the universe is just a big stirred pot like this, except there's very little friction to stop the spinning after the initial perturbations. And what are the initial perturbations? Whatever was happening when most of the space stopped being filled by a fluid. I *think* no quantum effects were important at that point, but mutter mutter not an expert. Mutter mutter baryon acoustic oscillations grumble.

https://en.wikipedia.org/wiki/Baryon_acoustic_oscillations