Same reason for we say 1000 km instead of 1 Mm, or Sun to Earth 150 M km instead of 150 Gm. Some units are usual in some contexts, some are not. In one word, habits.
And don't forget cubic centimeters (or cc) for engines. Anything from 2000 cc down to 50 cc is very common east of the Atlantic Ocean.
Speak for yourself, I've been trying to popularise "1Mm" for years. Admittedly not with a huge amount of success.
> or Sun to Earth 150 M km instead of 150 Gm
Surely "1 AU" is the preferred form for this one.
The point would be to put it in a familiar unit, even though '150 million' isn't easy to visualize.
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.
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)
Everybody knows it’s 4.848×10^12 attoprsecs.
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. ;-)
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".