About 31 mm (one inch). "atto-" is the standard SIprefix for multiplication by 10^-18. A parsec (parallax-second) is 3.26 light-years an attoparsec is thus 3.26 * 10^-18 light years. Thus, one attoparsec per microfortnight is about one inch per second. This unit is reported to be in use (though probably not very seriously) among hackers in the UK. [Jargon File] (1996-12-08)
