It's common to speak of 'thousands of kilometres', but no one ever says 'megametres' or 'gigametres'. Why is this? Is it official, or just a convention? Would it be linguistically correct to say the Earth is about 150 gigametres from the Sun?
I guess the same could apply to kilograms, but here there's a separate unit, the tonne. Would it be technically correct to refer to the universe being '13.7 gigayears old? Is there any consistent official nomenclature across ISO measures for very large numbers?
Added 9/5/12: Since asking this question I noticed that Arthur C. Clarke used the term 'gigayears' and other ISO prefixes in some of his books.
The first answer below uses the argument that saying things like 'yottameters' is not used simply because it's odd. I don't buy that. Even metres and litres sounded odd to me, and 'gigabytes' still does. (I still pronounce it with a soft G or J, as in gigantic, and will always say 'concert', because a music 'gig' sounds silly to me. But of course, sounding silly is no scientific argument against adopting a word or pronunciation.
