Why do we use “1 light-year” for an astronomical distance, rather than “9.4607 petameters” or “9.4607x10**15 meters”?
But I guess it’s easier to say “1 billion light-years” rather than “almost 10 billion petameters” or “almost 10 yottameters”. Still - 10x10**24 meters would look OK in print (can anyone visualize quickly “10 septillion meters”?).
However - the limit to light-year distances (as far as we know) is in the low billions, so I guess it’s easier to handle than the metric equivalent.
Do we? I would expect something like parsecs (or kiloparsecs, or megaparsecs…), simply because of the history of measuring such distances.
I’m only a hobby astronomer myself, but my impression is that “serious” astronomy uses parsecs and lightyears as units of measurement for interstellar distances with about equal frequency, whereas popular science astronomy uses almost exclusively lightyears. For distances within the solar system, serious astronomy uses astronomical units and popular science miles/kilometres.
[quote=“Quercus, post:54, topic:855604”]
I was going to nominate Liter for the OP, since I can’t recall ever seeing liter with any prefix other than milli- (except perhaps in a general way “in the microliter range”).[/quote]
I now have to update my post. I just saw a blood test report, with nearly every item reported in a different unit, but milligrams per decilter was the most common (with a few g/dL).
Also picograms and nanograms per milliliter!?!. I can understand trying to change units to get everything to be in the range of 1-100 (You wouldn’t want to trust an MD to deal with decimal points…), but ng/ml is the exact same thing as microgram per liter.
It’s not as relevant any more, but there’s good reason to use AUs rather than meters (or some prefix thereof) in the Solar System: The measurements of other orbits, in AUs, are known to more precision than the size of the AU, in meters. Nowadays, while this is still true, the precision of both are significantly greater than one ever needs… but even less than a century ago, the measurement of the AU left much to be desired.
Similar situations crop up in many other places in the sciences. Another one from astronomy is that the product of Newton’s constant G and the mass of any astronomical body (say, the Sun) is known to greater precision than either one alone. And in chemistry, the mole was useful even before the value of Avagadro’s number was known.