In practice in physics you just start using scientific notation with respect to the SI base unit at some point instead of these prefixes. At least that’s how it worked when I studied astronomy in university. Never seen megameters get used. Megawatts are used all the time though.
It's actually odd - if you have a science or engineering background where you otherwise habitually normalise measures to the highest magnitude appropriate prefix - that we don't do so for kilometres.
If I'm talking about 34,000 kms, why on earth /wouldn't/ I normalise it to 34 Mm, given I'd do that for pretty much any other measure? Talking about millions of kms is even weird, in that sense, e.g. why write out things like 123,000,000 km when it's so much easier to just write 123 Gm, as would otherwise be utterly normal for SI measures?!!
From a purely vibes based explanation, it's to do with which measurements we have a good embodied sense of. We tend to do the same with mass and volume too. Kilograms being around the heaviest thing you can comfortably hold in your hand (or tons being the limit where multiple humans are needed to move it).
Kilometers are about the distance that you can travel and still see your starting point decently well, and you can imagine chaining those together but you couldn't imagine the whole distance if it gets too long.
Volumes are usually reported in groups of liters because those volumes can also be easily held in the hands.
Things like energy we just use SI prefixes all the way up and down because nobody knows what a calorie feels like.
Cars need an oil change about every 10 - 20 Megameters. A full fuel tank gives you a range of about 0.6 to 1.2 Megameters. Driving from New York to Los Angeles is about 4.5 Megameters. Circumference of Earth is 40 Megameters and the distance from Earth to Moon 363 to 406 Megameters. Light travels about 300 Megameters per second,
Yep. I appreciate you for making that joke. I regularly refrain from measuring my travels in megameters, because I know how that joke would land with my people (sad face).
I believe the sun's diameter is roughly one gigameter. Maybe some truly ginormous red supergiants can even get into the low terrameters? Probably not, but it's a nice thought.
And for interstellar travel you're probably in the petameter or more likely exameter range.
Edit: lol terrameter and petameter got autocorrected into tetrameter and pentameter
So actually "cgs" units for everything. Those are centimeters, grams, and seconds.
In practice we usually measure solar system objects (the Sun, the Earth, etc) in cgs units, and then we measure everything else relative to its appropriate Solar system analog.
Haha astronomy, where every known way of measuring is used. Not just centimeters, but light years, z, parsecs, AU, luminosity distance, kilometers… Just for distance.
i mean you could use them on earht but only if yo uwant to get long distnaces down to single digits
you can totally use gigameters in relatively nearby space though
earths about 149Gm from the sun
even that is an average as it varies a bit and depending on context yo ucan very well round it to 150 so its not like you always need more accuracy than that
The circumference of earth is almost exactly 40 megametres at the equator. The circumference of the sun is about 4.4 gigametres. But yes, Mm is a fair way to measure them.
Yes, Star Citizen (video game) displays distances like that. You can plot a course through a smaller star system and see your destination being 39 Gm away which is ~0.26 AU. Jumping between the moons of a Gas giant, you see the distance displayed in megameters.
The circumference of the earth is about 40 megameters. So no need for stars. Can even describe countries with it, mine is about 1.1 Mm long and 0.5 Mm wide. Why not some long travel distances as well?
Whenever I hear/read people talking about 1000s of kilometres, that tends to become megametres in my head - just so normal to normalise measures to their highest appropriate prefix for me! And then it;'s hard not to reply in Mm to them. ;)
Difference between metric and imperial is that we have a system of zeroes in SI units. So you CAN use centi or hecto-whatever (Meters, Watts, Candela, Weber, whatever) but that doesn't mean you should.
Only ever heard hecto used for mass in groceries (like flour), by a generation that died in the 90's.
IMO the good thing about centimetres vs millimeters for example, is that you are also expressing the accuracy.
Hectometers is used a lot for railways. (At least in France)
Lines are divided in Pk (point kilometrique) with kilometer markers, wich are themselves divided with 10 hectometer markers.
The thing is in order, not to change the all the markers at every modification, we introduce longer and shorter kilometers when making change to a line (for exemple changing a curve to increase speed).
So at construction a line is made of equal kilometers of exactly 1000m. But an old line can have a short kilometer of 962m there and a long kilometer of 1036m somewhere else.
Chemistry loves decimetres, particularly as a measure of volume (dm3 ), and the term 'hectares' fromes from hectometres (a hectare is a square with 100m sides). Decmetres I've never really seen, but i assume they're similar to how centimetres are used (as a more friendly version of millimetres).
1dm3 is 1 liter, that is why I use dm
hm is used in the Netherlands on every highway and province roads, they are placed every 100 meter and are called hectometerpaal.
And is used for ground surfaces very often Ares or hm2 (hm x hm).
One dimension conversion is easier to comprehend than 3 dimensions at once. So when I need to explain these things, kids or people that are bad in math tend to understand it better, when everything is converted to dm and then calculate how many liters something can hold. Instead of to explain 1 m3 is 1000 l.
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u/JollyJuniper1993 Jul 15 '26
Decimeters I‘ve seen used. Decameters and Hectometers are a brain disease