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-}.
[glossary]
[Reference(s) to this entry by made by: {Commonwealth Hackish}{micro-}{nanofortnight}{quantifiers}]