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_OBAFGKM_ t1_j20zyl4 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

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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