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 PERmicrofortnight 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)