Talk:3156: Planetary Rings

Is my draft right? It's hard to understand a comic that hasn't yet got an explanation! RadiantRainwing (talk) 16:21, 17 October 2025 (UTC)

I came here to say this is one of those xkcds that really doesn't need explaining. I do like the bit about "although it's theorized that [Earth] may have had [a natural planetary ring system] in the past," it's additional information I wouldn't think about just reading the panel. 64.201.132.210 17:48, 17 October 2025 (UTC)

I think you did a good job, you gave depth to the terms used, added related facts, and included comparisons netween the listed rings. 64.114.211.102 18:17, 17 October 2025 (UTC)

Can't wait until our ring evolves via kessler syndrome. 64.114.211.102 18:17, 17 October 2025 (UTC)

I can't wait for the stage after that: Carcinization. 149.22.90.216 23:10, 17 October 2025 (UTC)

I added two notes, but I don't have time now to learn the syntax to make proper notes, so they are just in the middle of the text. I will try to do it later, if someone else doesn't do it. Rps (talk) 18:33, 17 October 2025 (UTC)

Earth's historical rings: Rings of Earth 191.101.157.126 19:20, 17 October 2025 (UTC)

Mars also has a ring, although it consists of just 2 very large objects. SDSpivey (talk) 21:43, 17 October 2025 (UTC)

Nope. Mars has TWO rings. Deimos orbits at three times the distance and four times the period of Phobos. Two rings with one large object each. And if you are taking an areocentric perspective, there's a much more massive ring 1.5 AU away. 181.214.218.33 22:46, 17 October 2025 (UTC)

Regarding the title text: One of those bits of information that aids navigation has long been where on a tree the moss grows (should you be in an area with trees, obviously, and otherwise be without a compass or can see the Sun to do the watch-dial trick (with an analogue watch, or a bit of imagination)). Back in the '90s, I noted that I could augment that, in an urban environment, from the rise in houses having satellite dishes (BSB 'squarials' and Sky dishes, originally, here in the UK) all pointing pretty much directly south. (With enough local knowledge, you might even be able to compare them to TV aerials and possibly triangulate to where you were within a larger urban area - assuming you were 'somewhat lost, but not totally lost'.) And, even today, I find them a reassuring extra bit of info when I'm skirting through the suburbs of cities, knowing that I'm not being twisted too far awa from my chosen direction (working with my in-head memory of the map I'd planned with, which can occasionally get nudged off from reality by an inconvenient twist and turn of road).
Though I must say, I've never ever considered latitude-enumeration as an additional factor. Apart from anything else, the design of the dish normally has the 'receiver arm' sitting at an off-axis focal point, so you need to project out at the complimentary 'up-angle' from the exact angle the dish itself points. But, anyway, I'm not sure I ever could have distinguished 55°N from 60°N, by eye, even sighting upon a centre-axis dish's direction. And yet I'd surely already know if I was as far south as Edinburgh or actually somewhere as far north as Lerwick, before checking out the local dishes... ;) 2.98.65.8 21:48, 17 October 2025 (UTC)

I don't understand why it says "If some of them are pointing straight up, you're probably near the Equator". Surely, if you're on Earth, they're pointing straight down. 86.20.197.254 (talk) 18:12, 18 October 2025 (please sign your comments with ~~~~)

Well, the satellite dishes need to point up to get signal from up there in the sky. MinersHavenM43 (talk) 18:17, 18 October 2025 (UTC)

Sounds like 86...254 thinks "satellite dishes" are the transmitting/transceiving dishes upon the satellites, not the receiving/transceiving dishes aimed at them. 2.98.65.8 18:40, 18 October 2025 (UTC)

Sidenote: I visited my local thrift store and found three nearly new copies of "what if?" on the Humor bookshelves. I bought one (Sorry you won't get paid twice.) and moved one to the Reference bookshelves. These Are Not The Comments You Are Looking For (talk) 20:21, 19 October 2025 (UTC)

I accidentally nerdsniped myself by asking "How many geostationary satellites would we need to have a ring visible from the moon?". I don't even know where to begin with it. Google's AI said it's between 80 million and 540 million (based on the brightness of a single satellite and the lowest brightness visible to the naked eye), but I'm hesitant to trust LLM generated math. Someone please help! Stroopwafel Falafel (talk) 18:16, 5 November 2025 (UTC)

A single metre, at the distance from the Moon to GEO seems like it would subtend ~5.9x10⁵ arcminutes of angle(near-side) to ~4.9x10⁵ (far-side), give or take a little adjustment for exact viewpoint.

Visual acuity is variously quoted as between a few arcseconds and about an arcminute, but that's more distinguishing one thing at one point in vision from another nearly at that point in it. Pinpricks of light distinguish themselves out from a background, of course. But it looks like that Venus is (optimally) from roughly 105,000 to 125,000 times as wide, or square those numbers for the ratio of solid angles, so we'd be talking about the level of far fainter stars.

I think that (as specs of light, spread out) you don't necessarily need even a million of them, but it would be a tenuously-visible definable hairlike feature at best if you put m² 'blocks' edge-to-edge in an 'arc structure'. (Also, if you connected them, wouldn't the 'orbital Centre Of Gravity' be somewhere half way along the chord between the two ends, changing the orbital period it wants to drift round at, or does the non-Newtonian rotating frame of reference let it sit on the 'mid-block' of the arc? Before considering gravitational pull between the blocks, of course.)

But it'd be a 'whisp'. Perhaps folding out to tennis-court size (or larger), plus relying on the perfect orientation for visible reflection from their surface would help. You could leave them drifting hundreds of metres apart, or further, each point notable more for being part of a linear 'asterism' of faint points (like a more compact Milky Way effect), getting away with fewer.

So are we talking of enough satellites to be 'large enough and close enough' to make such a whisp (at near/far extremes) and then extrapolating to what this linear density means a fully-filled orbit would need? (Rather than bunches of them, suggesting the whole orbit only by a 'dashed line', each dash made up of dots.)

Looking at it from the other side, the width of GEO, seen from the Moon, would be about 10-degrees of arc (very visible, assuming the points of the arc are). Theoretically 224 million "metre-cubes" elbow-to-elbow would fill it... in a (thinner-than-)hair-thin structure, but those tennis courts (or, lets get ambitious, football-field sized... any kind of football, but perhaps Aussie Rules would be a useful aspiration), all at handy Sun-reflecting angles..? They could be less dense, far from touching. Be creative with geosynchronous-but-not-geostationary orbital constellations, and it'd thicken-up that hairline feature (at the expense of the satellites needed maybe being a simple multiple, to deal with off-equatorial dispersion of 'density').

I'm most worried about the need to individually control each flat reflector to make best visible to a given Moon-observer, though. Feel free to recalculate for (say) silvered spherical 'inflatable' satellites, or something, of some size probably nowhere near that of a sporting arena. But we definitely need to know more about the albedo-/reflecti ity-profile of whatever we try to spatter around the orbit, before we can tie down everything else about the problem. 82.132.246.185 19:49, 5 November 2025 (UTC)