About 31 mm (one inch)."atto-" is the standard SIprefix for multiplication by 10^-18.A parsec (parallax-second) is 3.26 light-yearsan 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)