Well, as a matter of fact, astronomers typically prefer the parsec over the light-year. A parsec is derived from the AU, is equal to 3.26 light-years, and is defined as the distance an object would have to be from the Sun to experience a parallax angle of 1 arcsecond (that's obviously not ELI5 but it's a digression).

The AU is better than the light-minute because the AU is a distance that we can easily understand. 3 AU is "three times the distance from the Earth to the Sun." I can wrap my head around that. What is 24 light-minutes? How far is a light-minute? You can tell me the equivalent in miles or kilometers, but the number is so big I can't conceive it as easily.

We can't easily conceive a light-year either, but when we get to distances larger than our solar system there isn't any easier measurement that we can grasp because it's way too far out of our range of human comprehension. The parsec doesn't help, and saying "thousands of AUs" doesn't help. We had to pick one, so we went with the measurement that sounds like it's a measurement of time and confuses everyone.

Astronomers do not always use light-years as a unit distance. The chosen unit heavily depends on the context. It could be light years (ly), astronomical unit (AU), parsecs (pc), kilometres (km), or others.

AU is convenient as it represents the distance between the earth and the sun. Using it for the distance between planets gives you a better representation than if it was given in light minutes.

In the same way that it’s more convenient to represent contenance in litres than in meters cubed

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> Light minutes, on the other hand, are a unit of time, not distance. They are used to measure the time it takes for light to travel a certain distance. For example, it takes about 8 minutes for light to travel from the Sun to the Earth, so the distance from the Sun to the Earth is about 8 light minutes

You've contradicted yourself in this paragraph. Light minutes are not a measurement of time, they're a measurement of distance. You know this intuitively because you explained it correctly the final sentence here

Astronomers don't use light minutes within the solar system because they don't really care about light within the solar system, AU is just a more convenient unit.

>Light minutes, on the other hand, are a unit of time, not distance.

Light minutes are a unit of distance just like a light year is. In SI base units are speed is in m/s and time is in seconds so speed * time =m /s * s =m

A light second is exactly 17,987,547,480 m

That is because the speed of light is by definition 299,792,458 m/s so just multiply that by 60 seconds and you ger the distance above. If we improve measurement it is the meter that changes not the speed of light. A meter is the distance light travels in 1/299,792,458 of a second in a vacuum.

Have you guys tried "kajillions of miles"

The AU and parsec, the two most commonly used astronomical distance measures, come from the fact that they depend on the most easily-measured things in the Universe: the parameters of Earth's orbit itself.

The AU is the distance from the Earth to the Sun. Geometry can easily tell us that, say, Venus is about two-thirds as far from the Sun as we are, or that Mars is about half again as far, but it can't actually tell us the Earth-Sun distance directly without some other pieces of information. Thus, a lot of the details of the solar system got worked out using the Earth-Sun distance (that is, 1 AU) as a baseline; as our estimates of the AU got better, so too did our estimates of other things.

Similarly, the parsec also depends on the Earth's orbit. In that sense, the AU and the parsec are closely related. Specifically, the parsec is the distance at which an object's position in the sky changes by 1 second of arc (1 / 3600 of a degree) over a distance of 1 AU. Mathematically, that means it's 1 AU times cotangent(1/3600 degree) = 1 AU times ~206,264, which works out to a little over 3 light-years. We use the parsec because measuring these angles is how we first established how far away the stars are, which let us develop systems for figuring out the distance to more distant stars.

Today, the parameters of the Earth's orbit are known to very high precision in terms of things like the kilometer, but that wasn't always true. And having probes far enough out to have meaningful light-travel delays is even newer.

Today, the AU is defined in terms of the meter, and the meter is defined in terms of light travel, so in a sense we actually do measure with light travel times. We just do it in a weird way for historical reasons.

> Light minutes, on the other hand, are a unit of time, not distance.

...what? no.

Well what do you think "km" stands for?

The units someone uses are ultimately going to depend on what they need them for. Someone working sending interplanetary radio signals might find light-minutes most useful, while someone studying orbits may likely use AU’s. If you’re trying to describe the size of orbits, using the orbit of Earth as a reference is really useful because it’s a distance that’s easy to compare to. It’s the same reason you don’t describe your weight in tones, differences of a few pounds are hard to conceptualize using units that are many times larger/smaller than what you can conceive.

People don’t agree even how to measure how tall we are or how much we weight or anything. Probably not the answer you want but that’s probably why 😝

Kajillion football fields

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I'm pretty sure this answer was written by ChatGPT.

I wish we would have just embraced the metric system the way that computers have.

For example 1AU is approximately 150 Gm (Giga-metre)

[Insert NASA explanation of how many elephant trunks it takes to equal a kajillion football fields]

You say this, but many people don't know the difference between MiB and MB, GiB and GB, etc.

As opposed to "parallax second"?

(I'd say more, but I have a deliver to go do on the Kessel run.)

I think that the practical use for the knowledge of the difference between a MiB and a MB, is much more limited than the knowledge of the difference between a M and a G.

Thus, even if they don't understand that MiB is 2^20 and that MB is 10^6, they will understand that Gx is near enough 3 orders of magnitude larger than Mx. Which in the case of stellar distances, is good enough, IMHO anyways.

Pretty sure you are spot on. Look at all those other posts all at the same time.

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.

Easy.

MiB is Men in Black, MB is Manitoba.

GiB is when you want someone to give you something, and GB is Grand Britain.

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

In short, you always take a unit so that your values are around 1, because they are the number that make sense to us and are easier to compare. In subatomic, you even transform everything so your references are just 1. Weird at first, but powerfull to do calculation easily.

That's why physicist always come up with new units, even though they are just derivative of common units (planck length, time, angstrom, nm, micrometer), To each unit it's reference:

• angstrom: atom

• meter: human

• kilometer: geographical distances (you don't say that Paris is 800 000 meters from Bordeaux, right?)

• AU: solar systems

• parsec: galaxies and beyond (with kpc and Mpc)

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

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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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?

it's defined as the average distance

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

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

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