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atomfullerene t1_j20wn41 wrote

Reply to comment by TheLuminary in ELI5: If astronomers use "light-years" for interstellar distances, why do we use AU for interplanetary distances instead of "light-minutes"? by concorde77

Huh, I'd never really noticed before how heavily non-metric distance measurements in space are.

I think the reason is that the benefits of metric come from convertibility. Metric's really handy if you need to estimate the mass of a cubic volume of something, or move between mm and km, or get the amount of energy needed to move some mass some distance.

But in space, your distance measurements are all just distance. You aren't using them with other units, typically, so there's no real drive to make them metric. Instead they stick to the older method of using measurements that are convenient for the particular specific context they are being used in.

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TheLuminary t1_j20xe4d wrote

Yep, that makes sense to be. I am still on team make it all metric though haha.

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_OBAFGKM_ t1_j20zyl4 wrote

metric is actually fairly arbitrary. AU and pc are so much more useful in astronomy because they're derived from actual physical quantities that affect the measurements we make. it's so so easy to write down fundamental astronomical equations in terms of parsecs, whereas if you used metric you would need to include some sort of conversion factor

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TheLuminary t1_j21hatm wrote

I guess so.. but AU doesn't even make sense. Considering the Earth does not have a constant distance from the Sun. So don't you still need a conversion factor somewhere?

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_OBAFGKM_ t1_j21mqnz wrote

it's defined as the average distance

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TheLuminary t1_j2242yx wrote

This feels like a Pi vs Tau argument. Constants can be moved around in equations and units can be changed if enough people wanted.

I get why they don't change now, but I wish they would have. /shrug

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_OBAFGKM_ t1_j226apu wrote

It's not really like that, since tau and pi only differ by a factor of 2.

A useful equation is, for example d = 1/p, where distance is measured in parsecs and p is measured in arcseconds. If you used meters, it's not just a factor of 2, it's something like 3.086×10^(16) d = 1/p. With distances as big as parsecs, there's no intuition you can use to understand the size, so it really doesn't matter what unit you use. It just makes the most sense to use the natural unit instead of the arbitrary one

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TheLuminary t1_j226tz5 wrote

Ah.. yes, I suppose having custom units for those specific equations where the constant is 1 would be handy.

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Future_Club1171 t1_j214zy7 wrote

I mean just like imperial, the units are based on SI which is mostly metric. Anything SI with distance can and is defined in meters. It’s just that using meters to convey such quickly gets messy since the universe isn’t going to make everything near round numbers. To give you a fermi approx of an AU, light takes 498 seconds to reach earth, light travels at 2.998x10^8 meters/second, therefore 1 AU is approximately 1.5x10^11 meters, that’s 150 gigameters, note the earth is only 40 mega meters even by the original meter definition.

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