= A =
About an 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, or about 3.1 cm (thus, 1
attoparsec/microfortnight equals about 1 inch/sec). This unit
is reported to be in use (though probably not very seriously) among
hackers in the U.K. See micro-.
--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.