I’ve been using Mm too, but only when I have a reasonable expectation that it will be understood. IOW, rarely.

But also, I still have the Earth–Moon distance internalized as 385000 km, not 385 Mm, since that is what I learned as a kid.

But stating the Sun–Earth distance as 1 AU is just a tautology, restating the definition, and hence void of information. I suspect you’re jesting. ;-)

> I’ve been using Mm too, but only when I have a reasonable expectation that it will be understood. IOW, rarely.

Yeah, I mainly only use it in chats (where I can immediately explain it if needed) or in person (where the actual pronunciation of "megametres" makes it pretty obvious). "Mm" and "Gm" being somewhat difficult to google due to "mm" and "gm" being different SI units makes it untenable to just expect people to work it out on their own.

> But stating the Sun–Earth distance as 1 AU is just a tautology, restating the definition, and hence void of information. I suspect you’re jesting. ;-)

To be clear, I don't mean that you'd answer "1 AU" if someone asked you how far away the sun was - obviously that's completely worthless information! The discussion was just on what units and prefixes are used for various magnitudes of measurement, and the AU is a fantastic unit to use for things that would otherwise be measured in hundreds of gigametres. For example, if you had a table containing the distances between various celestial bodies, it would be very convenient to have "Sun <-> Earth: 1 AU" in there along with things like "Sun <-> Mars: 1.5 AU".

If someone was to ask me how far is was from sun to earth, and I answered 1AU, and they asked what an AU was?

The point would be to put it in a familiar unit, even though '150 million' isn't easy to visualize.

> If someone was to ask me how far is was from sun to earth, and I answered 1AU, and they asked what an AU was?

You'd probably do the same thing that you would do if they asked what a km was: keep changing the units until you get one that they recognise, and then now they can understand the original unit you used.

(Although in this specific case given that they're asking about the definition of the unit you'd probably provide that context without needing further prompting in the first place, similar to if they asked about the circumference of the earth and you gave an answer in metres)

pretty sure it was a joke

Not a joke, the AU is a pretty common unit of measurement when talking about distances too large for metres and too small for light-years, especially since it's based on one of the most widely understood astronomical distances. Of course using it to describe the distance it's derived from is more than a little tautological, but that wasn't too important in the context of discussing magnitude prefixes.

It's roughly the same thing as describing a distant potentially-habitable planet as being "1.5 times the size of Earth" instead of giving its radius in metres. It's just that AUs wrap that "n times the distance between the sun and the earth" into a nice standardised unit.

...thus making it tautological and extremely pertinent given the context

> Surely "1 AU" is the preferred form for this one.

Everybody knows it’s 4.848×10^12 attoprsecs.