<unit, humour> About 3.1 cm (one inch). "atto-" is the standard SI prefix 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, 1 attoparsec/microfortnight is about 1 inch/sec.
This unit is reported to be in use (though probably not very seriously) among hackers in the UK.
(08 Dec 1996)