for people using non-medieval units of measure, √(GM/r) is a fairly precise approximation to circular orbital speed, and units(1) is good at calculating things like this:
You have: (G earthmass / earthradius)**.5
You want:
Definition: 7909.7861 m / s
it was amusing when the sound cut out at 2'59" (sumerian units of measure). for a while i thought the microphone had been destroyed by the sound at that point
the original title said 'at 17000 mph', and as you can see, that's very significantly low, about 4%:
You have: (G earthmass / earthradius) ** .5 / 17000mph - 1
You want: %
* 4.0805157
so if you, like any rational person, want to know how fast the capsule was entering in meters per second, you're better off using units(1) to calculate it from first principles rather than converting it from a 4% wrong quantity in medieval units
i'm pretty much a raw beginner with orbital dynamics but my vague idea was that you'd have to ellipticize the orbit by hundreds of kilometers to change the orbital speed by more than a fraction of a percent, and that would be expensive enough that you'd only do it if you had a really good reason
but the only satellites i've ever troubleshot had only extremely limited maneuverability (though i'm not totally sure how much more than that i can say)
Sound came back later in the video, so I’m pretty sure they just turned the volume way down. The sound was probably very annoying for that period of time, but they actually wanted people to be able to enjoy the beauty of that scene.
I find it interesting people are pretty hardcore that meters is a superior measure based on its lack of human foot reference, but still hold onto Celsius. Basing a unit on the temperatures water phase changes at 1atm is as barbaric and anthropomorphic. That’s why I only measure with a zero based on actually zero. Negative heat?? Wtf.
Your argument doesn't make that much sense to me when Kelvins, while 0=absolute 0, have the same magnitude as a degree Celsius. I.e. they're still based on water phase changes at 1 atm.
The language defining the Celsius unit originally was:
"The arbitrary points which coincide on the two scales are 0° and 100°"
Those distances are arbitrary, and not specifically moored on the anchors of 0 and 100. Kelvin units are moored however on a fundamental anchor of zero heat energy, with arbitrary sized units above. It happens they were later calibrated to the Boltzmann constant, which itself is anchored on the triple point of water. None of this is based on human experience.
it's water because the humans are mostly made of water. rumor has it that on 𐍨𐍓𐍰𐍚𐍠 they use the triple point of what the humans call 'phosphoric acid' (it's a toasty place) and in 𓃾𓂃𓂌𓀩 they use the triple point of the ammonia-methane eutectic
Which is a shame that the temperature scale is still anchored in that arbitrary mix, picked by a lazy apprentice. It's also historically base-10, not like the modern systematic base-8 units.
(Can ammonia actually alloy with methane? Their crystalline lattices don't seem to be compatible, and they don't react together.)
i don't actually know. however, the same considerations would suggest that methane clathrates in water are impossible, which turns out not to be true. methane freezes at 90 kelvin, so if there's a eutectic, it would be lower than that.
Clathrates are different, they are not eutectics. If you try to freeze them, they separate into methane gas and water ice.
Metals can alloy with each other because the shared electron layer "glues" small imperfections in crystals where different metals meet. Can frozen ammonia behave similarly? Liquid ammonia can solvate free electrons, so perhaps frozen ammonia also can?
It'd be interesting to actually try experiments with frozen ammonia.
that's a fascinating idea! frozen electrides! it's a lot freezier than methane (195 kelvin, a difference large enough to make a methane eutectic much less likely) to the point where you might even be able to freeze it with convenient and safe dry ice rather than liquid nitrogen. and, i don't know, maybe if dry ice by itself won't do the trick, maybe dry ice mixed with liquid ammonia will. trying to buy anhydrous ammonia around here gets you put on a list, though, so i'm gonna opt out of this one
there are a lot of solvated ionic eutectics with atmospheric-pressure water ice, despite the absence of metallic bonding
most of the non-dark-matter non-dark-energy mass of the universe finds itself in conditions that prevent the existence of water: black holes, degenerate-matter stars, conventional plasma stars, things like that. almost all of the rest is hydrogen and helium, so water exists there in the same sense that uranium exists here. but, although almost all of the universe is cold enough to freeze hydrogen, its vapor pressure is high enough at the cbr that most of it is vapor, so basically none of that is solid. solid objects do have a substantial amount of water in them, although if our solar system is typical, many of them don't
but sure, if it turns out that there are electron-degenerate-gas vortex intelligences on the surface of white-dwarf stars, they could very likely work out some way to launch robotic probes out of the star into places where water could exist, so that they could measure its triple point. and the folks on 𐍨𐍓𐍰𐍚𐍠 use trace amounts of water in their biology (it's a reasonably powerful base) and have isolated it in liquid form in their cryogenic laboratories. it's accessible in a way that the artifact kilogram and artifact meter and the circumference of the earth aren't
It's funny that I can't even accomplish an emoticon in these comments while some people manage to draw complete scenes or algebraic formulas interleaved with the text.
I must admit that, unlike a lot of HN'ers, I'm using a virtual keyboard on a heldheld device to type comments; I never took the effort of looking up how it is done though it must have to do with extended char sets.
most emoji are banned, as are nonstandard space symbols like thin space and some other things. emoticons (things like :-) and XD) are allowed. generally alphabetic characters and digits are allowed, and some other things, but i think unknown unicode is forbidden by default. https://news.ycombinator.com/item?id=23440551 has some investigation and results which are probably still accurate
i do most of my algebraic formulas with the compose key and a custom compose map mostly written by mark shoulson https://github.com/kragen/xcompose
for the above, after a false start picking random unicode characters, i realized that maybe i shouldn't use a modern language because someone who actually uses the language might feel like i was calling them an extraterrestrial, so i switched to googling archaic scripts. i pasted part of the old permic table from wikipedia
>>> s = '''𐍐 𐍑 𐍒 𐍓 𐍔 𐍕 𐍖 𐍗 𐍘 𐍙 𐍚 𐍛 𐍜 𐍝 𐍞 𐍟
... U+1036x 𐍠 𐍡 𐍢 𐍣 𐍤 𐍥 𐍦 𐍧 𐍨 𐍩 𐍪 𐍫 𐍬 𐍭 𐍮 𐍯
... U+1037x 𐍰 𐍱 𐍲 𐍳 𐍴 𐍵'''
>>> print(''.join(c for c in s if ord(c) >= 0x10350))
𐍐𐍑𐍒𐍓𐍔𐍕𐍖𐍗𐍘𐍙𐍚𐍛𐍜𐍝𐍞𐍟𐍠𐍡𐍢𐍣𐍤𐍥𐍦𐍧𐍨𐍩𐍪𐍫𐍬𐍭𐍮𐍯𐍰𐍱𐍲𐍳𐍴𐍵
>>> len(''.join(c for c in s if ord(c) >= 0x10350))
38
>>> permic = (''.join(c for c in s if ord(c) >= 0x10350))
>>> ''.join(random.choice(permic) for i in range(5))
'𐍨𐍓𐍰𐍚𐍠'
and then i tried out the hieroglyphs range
>>> print(''.join(chr(i) for i in range(0x13000, 0x14000)))
𓀀𓀁𓀂𓀃𓀄𓀅𓀆𓀇𓀈𓀉𓀊𓀋𓀌𓀍𓀎𓀏𓀐𓀑𓀒𓀓𓀔𓀕𓀖𓀗𓀘𓀙𓀚𓀛𓀜𓀝𓀞𓀟𓀠𓀡𓀢𓀣𓀤𓀥𓀦𓀧𓀨𓀩𓀪𓀫𓀬𓀭...
but realized that most of them were unassigned, at least in my font and probably in the current unicode standard (in case someone discovers a new hieroglyph), so i just did this
>>> hiero = (''.join(chr(i) for i in range(0x13000, 0x13100)))
>>> ''.join(random.choice(hiero) for i in range(4))
'𓃾𓂃𓂌𓀩'
you can do all this in python in termux on your phone too (you'll probably have to install it from f-droid) but it's a bit clumsier
it's funny how this conversation has swung from the extreme of universal constants of the universe to the opposite extreme of completely arbitrary and historically contingent things like which ideograms (themselves completely arbitrary) are prevented from being posted by implementation bugs in hacker news
guilty as charged. it gets worse, though: meters are originally based on the size of the earth, but humboldt's expedition (?) fucked up the measurement and now we're stuck with a meter that's significantly too short, nominally defined as the distance light travels in 9192631770/299792458 cycles of the hyperfine transition radiation frequency of cesium-133. you probably think this is a goddamned joke but it's not
i ask you, what the fuck kind of number is 9192631770? is that a hexadecimal encoding of linus torvalds's first child's birthdate? no, it's just a random fucking number derived from the sumerian base 60 numbering system and the average rotational speed of the earth in the 20th century. the kelvin has a similarly filthy history; it's when the thermal energy changes by 1.380649e-23 joules, because that makes the triple point of water come out to 273.16 kelvins. so now we have to preserve those numbers for all eternity like they're the fucking holy writ of the priestesses at delphi in order to interpret scientific papers from the 19th and 20th century. (and don't get me started on the motherfucking calendar. jesus jumping blue christ.)
> guilty as charged. it gets worse, though: meters are originally based on the size of the earth, but humboldt's expedition (?) fucked up the measurement and now we're stuck with a meter that's significantly too short
Uhm, a kilometer is supposed to be 1/40000-th of the Earth's meridional (i.e. from South to North poles) circumference. The modern value is 40008km, so the official meter is juuuust about 0.02% shorter than it should be.
i think it was a ten thousandth of the distance from the north pole to the equator through paris, disregarding the southern hemisphere, which was more difficult to survey. i've been looking for a precise number for how far that distance actually turned out to be, but i haven't been able to find it; i would be delighted if you could!
whether 0.02% sounds ridiculously good or ridiculously bad depends on your frame of reference
https://en.wikipedia.org/wiki/History_of_the_metre#History_o... says that the original mètre des archives in 01799 was machined to within 50μm, which is 0.005%, four times smaller than the error in delambre and méchain's computation. so even at the time that was a pretty large error. since then the measurement uncertainty of the meter has improved by five more orders of magnitude, to about 0.1 part per billion. 0.02% is 200 parts per million, or 200000 parts per billion, which is a lot more than 0.1
You probably know all this, but it is fun to write. Units are parts of models. Used Models were the best in class, at the time they were derived or adopted. Models are wrong and get replaced by less wrong models over time.
That said I think meters and Kelvins are doing their job just fine even for use with rocket science. I'm glad I learned the SI units at school, they reduced the number of constants a lot in physical formulas.
yeah, but there are still a lot of constants even with the si units, which is because they predate the discoveries of many of the constants. more constants generally means more arithmetic operations you can accidentally leave out of your program and constants you can get wrong. this was a bigger concern in the slide-rule era, but the humans are still infallible, so it hasn't gone away entirely
i don't think it makes sense to describe miles or meters or stoney lengths as 'wrong' or 'right'; you can express the fitzgerald contraction or orbital speed or whatever equally well with any of the three, but one of them simplifies the fitzgerald contraction somewhat
I think a key tho in a measurement unit that’s desirable is its relationship to something consistent everywhere in the universe, such that the reference unit is reproducible by anyone. This doesn’t matter much day to day but as a system we should converge to one that anyone anywhere can agree how long a distance may be without a mutually agreed upon calibration but can calibrate without access any specific reference unit. The triple point of water is always what it is, and a colony on mars without easy access to something locked in a room in Paris or whatever can create their own reference measurements from whole cloth and they’ll be the same as the colony at Alpha Centauri now and in 20,000 years.
yes, even before alpha centauri, a big problem with the kilogram artifact locked in a room in paris was that it keeps changing its mass and nobody knows why or exactly how much or in which direction, although the best estimates are that it's a fair fraction of a part per million by now
agreeing on distances with a faraway colony is actually a considerably easier problem than the kilogram; if we transmit them a radio or laser message, they can measure its length to within parts per billion, and we can do that with 01950s technology, while kibble didn't invent the kibble balance (previously known as the watt balance) until the 01970s
(you do have to worry about redshift: one part per billion of redshift is 300 millimeters per second, so you have to know the relative velocity of alpha centauri to within meters per second to correct for it—and, while that's also the 01950s technology of measuring spectral line frequencies to that precision, that same technology is what allows you to do without the radio message entirely)
transmitting or storing the number "9192631770/299792458" can be done with significantly higher exactitude, of course, and can be done over twenty millennia more easily than transmitting a radio message can
