[_Microft]

Kilometer is the largest SI-prefixed variation of meter that is commonly used (among physicists that I know). I would rather suggest that you use scientific notation if you do not want to deal with large numbers of kilometers.

[arghwhat]

We don't stop at millimeters or kilowatt, so there is really no reason to stop at kilometer when megameter is more appropriate.

Scientific notation is also fine, but "210 megameter" is in my opinion easier to read than "2.1 * 10^8 meters", especially for the casual reader.

So while I respectfully make note of your feedback, I strongly disagree and stand by my choice of units. :)

[ksaj]

Given how hard it is to convince most Americans to use prefixed metre measurements instead of inches, feet, yards and miles, I think it would be even harder to convince the world that a megametre, picometre, etc is the way to go.

Not that I don't agree (I fully do!), the ease of using multiples of 10 is way easier. But it is very difficult to introduce change to the masses, no matter how sensible it is.

The US is one of the only countries on the planet that still fully stick to the old system. And they're also one of the only countries that spell 'meter' instead of 'metre.' For everyone else in the world, 'meter' is a measuring gauge or tool, and so is everything that rhymes with 'thermometer' except for the American pronunciation of 'kilometre' that too many have adopted up here.

Likewise, adult Canadians still use pounds at the gym and their body weight, and feet/inches for their height. Young people are far more reasonable about measurements these days.

Change is hard, and seems to get really messy when everyone goes in different directions from the start.

[arghwhat]

As a European born and raised in metric units, all I can do is shake my head and sigh...

... But as we speak about metric measurements I doubt that the US insistence on weird units apply. The average US citizen would presumably be confronted with "mega" and "giga" in their day-to-day lives, and millimeter, micrometer and nanometer are commonly used. Even femtosecond is a common, practical unit that anyone receiving refractive surgery will hear.

Picometer is unpopular because it only makes sense for sub-atomic lengths giving it few practical uses at the current time, not because "pico" is hard to grasp.

Also note that German, Dutch, Danish (my native tongue), Swedish, Norwegian, even Hindi and presumably many others also refer to the unit as "meter", so that particular disagreement is not "the US against the rest of the world".

(All languages have quirks - in Danish, we call a folding ruler in meters an "inch stick", and say the number "53" as "3 and half three's twenties" (skipping the "twenties" part in modern speech). Learn to enjoy the differences rather than hate on them.)

[raattgift]

I think you're right, but with a tiny handful of notable exceptions in astrophysics, where gigametres can be a characteristic or upper length scale.

LISA is practically always described as a gigametre-scale observatory.

One example: "LISA, a gigameter-scale space-based gravitational wave observatory, will explore the gravitational wave universe in the band from below 0.1 mHz to above 0.1 Hz." <https://about.cern/news/announcement/physics/cern-colloquium...>, and a trawl through arxiv will show a common association of Gm and LISA.

Assuming LISA is successfully deployed, gigametre may be seen more commonly.

The only other place I've seen Gm scales in common use is in galactic physics, particularly with respect to turbulence and star and planetary nebula formation (stars have ~ Gm diameters; very large stars like VY Canis Majoris have ~ Tm diameters; star systems like ours have gravitationally bound rocky and icy objects at ~ Tm diameters).

1 petameter ~ 0.1 lightyear; 30 petameter ~ 1 parsec, so those are obvious cutoffs for the SI unit of length in astronomy and astrophysics.

Gigaparsecs and (less frequently) gigalightyears are commonly used in physical cosmology (e.g. <https://duckduckgo.com/?q=gigaparsec+site%3Aarxiv.org&ia=web>).

Penultimately, truly long lengths are typically measured in cosmological redshift z, which is unitless (being a ratio \frac{\delta\lambda}{\lambda}, or 1+z = \frac{a_{now}}{a_{then}} where a is the scale factor), leading to such things as a comoving volume (1 + z)^3. For z > 0.2 one would be using Gpc lengths, or in SI units Ym; or when working in these sorts of comological volumes in 2023's Britain, trevigintillions of acre-feet.

Finally, cf. the excellent printable table at <https://arxiv.org/abs/1303.5961>, the leftmost column (redshift) and the r_comov column (megaparsecs) being the most directly relevant.