Discussion on answer by HDE 226868: Why don't astronomers use meters to measure astronomical distances?

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18:58

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A: Why don't astronomers use meters to measure astronomical distances?

Question: Why don't you measure your height in nanometers? When dealing with a galaxy, or a planetary system, it's convenient to work in units that are comparable to the distances between objects. For instance, an astronomical unit, the mean distance between Earth and the Sun, is $1.49597870700\...

zephyr

To be fair, you'd use the prefix for $10^9$ instead of $10^{11}$, so that $1\:\mathrm{AU}$ is just $150\:\mathrm{Gm}$. Although $1\:\mathrm{AU}$ is certainly more intuitive than $150\:\mathrm{Gm}$.

flawr

The real question would be: "Why don't you measure your height in lightyears?"

Delioth

@flawr What, you don't give your height to people as 1.77196e-16 lightyears?

James K

@Delioth, I will from now on. You're the same height as me. (to 3 sig fig)

Arne

The reason you wouldn't use nanometers for measuring body height is because it's an inconvenient prefix, this is an unrelated problem. Given that meters are perfectly suitable for measuring the sizes of atoms, I don't see why this would be different for large scale measurements, in the end only the prefix changes.

HDE 226868

18:58

@Arne I wouldn't say that any other prefix is less inconvenient, and so the answer is applicable to your premise. Besides, isn't it a lot simpler to avoid prefixes altogether, when possible?

Paŭlo Ebermann

@flawr There is a country where people measure their height in nanolightseconds. (They call it feet, though.)

KAI

I suspect that this answer is incomplete. Physicists often use some kind of MKS units to measure atomic and molecular distances instead of defining some kind of atomic specific unit near unity. So why astronomers invented a bunch of non MKS/CGS units is a valid question. I think you need the historical reasons from SeanLake's answer to help answer this. Astronomers couldn't measure distances with any precision historically, but maybe could measure distances in something like multiples of the Earth-Sun distance. That helped motivate them to memorize new units instead of new SI prefixes.

Sean Lake

@KAI Keep in mind, that I'm pretty sure that measuring the size of the solar system in terms of the Earth-Sun distance predates the MKS system. It's not as archaic as the magnitude system, but still.

Tim

This is why the detection of gravitational waves is one of my favourite discoveries. They're looking at events happening billions of light years away involving objects which weigh 20 times that of the sun compressed into an area less than the size of a small city, with tunnels that are 1000 of metres long and the amount of movement is measured in nanometers (or less?). It's on such a massive, and tiny, scale that it's hard to appreciate, and incredible that it can be detected!

@DavidRicherby I really want to start giving measurements in hectometres, and hextogrammes. Also one could argue that there is a prefix for $$10^7$$ - Hebdo, and $$10^5$$ - Lacta. Unfortunately both have been discontinued in the SI system...

Todd Wilcox

In some cases it helps to know how long it will take light to travel from an object in space to the Earth. We might measure the distance to Mars in light minutes if we are concerned with getting telemetry from a Mars lander. Likewise measuring the distance to stars or galaxies in light years can be more intuitive in other areas than huge numbers of km. It seems analogous to measuring the speed of supersonic capable fighter jets in Mach number when they are transonic or faster, but measuring the speeds of commercial craft in knots or mph.

Martin Bonner

18:58

Professional astronomers don't use "light years" (unless dumbing down for civilians). They always talk about "Parsecs" (or kiloparsecs/megaparsecs as needed.)

Andrew Grimm

@PaŭloEbermann Aussies also measure their height that way.

Jens

$\pi$ seconds is a nanocentury.

AnoE

I feel this question, unfortunately misses the mark completely. The AU is just as arbitrary as the meter, if not more so. 1 AU = 0.15 Tm or 150 Gm; 1 lightyear = 9.5 Pm. Yes, those other numbers have 1 or 2 digits more, but the AU or LJ have no magic meaning in the grand picture; they are just earth-centric arbitrary, historic measures. So arguing that the AU/LJ is either "relevant" or "easier" both do not to work out at all. The correct answer to the question would IMO simply be "it's customary, and some customs are hard to get rid off". The rest seems rhetoric and "I cannot imagine".

HDE 226868

@AnoE You're ignoring the fact that using different distance units puts measurements in context, making it much easier to understand the scales being worked in. Again, this is why nanometers are useless when measuring height; it's substantially easier to use a meter.

AnoE

@HDE226868, I am not ignoring that "fact", I am disputing it. The "nanometer" vs "meter" example is the same kind of rhetoric as in this answer, it makes no sense at all to bring that up.

HDE 226868

18:58

@AnoE It's exactly like the logic in the question. I have no idea why you're saying it doesn't make sense. It's "Why don't we measure [quantity] in [inconvenient unit]?"

Matt

@HDE226868 AnoE isn't talking about using an inconvenient unit. He is talking about using the meter with an appropriate prefix to put it into the right scale. Just because you don't know off the top of your head what the prefix for $10^{15}$ is, doesn't mean you wouldn't know what it was if that was the system you worked in. I can tell you that atto is the prefix for $10^{-18}$ because I have been working with that one a lot lately, but I bet you wouldn't have been able to. Its because you don't use the unit, not because the unit is too confusing or hard to remember.

HDE 226868

@Matt So, 1) I agree with some of what you're saying, although I don't agree that it's not hard to remember all of the relevant prefixes (I doubt most people who deal with such large numbers could remember them all with ease, although some likely can). 2) As has been demonstrated, though, the system of prefixes breaks down; there simply aren't prefixes for most powers of ten. Sure, you can then use 10 [prefix]meters or 100 [prefix]meters, but I still fail to see how that's any more convenient.

I'm added in some of the notes people have been making, because they're quite useful - even those I disagree with. If I've misinterpreted or misrepresented anything people have said, please let me know, and I'll be happy to edit that.

AnoE

Don't worry, @HDE226868, I'm not on a crusade. Thanks for adding those points, it rounds off the answer a bit. Such "why did we pick this-or-that convention" questions are really hard to answer anyway because they're awfully close to opinion-based.

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