phase of the moon

phase of the moon
    n 1: a time when the Moon presents a particular recurring
         appearance
    

phase of the moon
 n.

   Used humorously as a random parameter on which something is said to
   depend. Sometimes implies unreliability of whatever is dependent, or
   that reliability seems to be dependent on conditions nobody has been
   able to determine. "This feature depends on having the channel open in
   mumble mode, having the foo switch set, and on the phase of the moon."
   See also {heisenbug}.

   True story: Once upon a time there was a program bug that really did
   depend on the phase of the moon. There was a little subroutine that
   had traditionally been used in various programs at MIT to calculate an
   approximation to the moon's true phase. GLS incorporated this routine
   into a LISP program that, when it wrote out a file, would print a
   timestamp line almost 80 characters long. Very occasionally the first
   line of the message would be too long and would overflow onto the next
   line, and when the file was later read back in the program would
   {barf}. The length of the first line depended on both the precise date
   and time and the length of the phase specification when the timestamp
   was printed, and so the bug literally depended on the phase of the
   moon!

   The first paper edition of the Jargon File (Steele-1983) included an
   example of one of the timestamp lines that exhibited this bug, but the
   typesetter `corrected' it. This has since been described as the
   phase-of-the-moon-bug bug.

   However, beware of assumptions. A few years ago, engineers of CERN
   (European Center for Nuclear Research) were baffled by some errors in
   experiments conducted with the LEP particle accelerator. As the
   formidable amount of data generated by such devices is heavily
   processed by computers before being seen by humans, many people
   suggested the software was somehow sensitive to the phase of the moon.
   A few desperate engineers discovered the truth; the error turned out
   to be the result of a tiny change in the geometry of the 27km
   circumference ring, physically caused by the deformation of the Earth
   by the passage of the Moon! This story has entered physics folklore as
   a Newtonian vengeance on particle physics and as an example of the
   relevance of the simplest and oldest physical laws to the most modern
   science.
    

phase of the moon

   Used humorously as a random parameter on which something is
   said to depend.  Sometimes implies unreliability of whatever
   is dependent, or that reliability seems to be dependent on
   conditions nobody has been able to determine.  "This feature
   depends on having the channel open in mumble mode, having the
   foo switch set, and on the phase of the moon."

   See also {heisenbug}.

   True story: Once upon a time there was a {bug} that really did
   depend on the phase of the moon.  There was a little
   subroutine that had traditionally been used in various
   programs at {MIT} to calculate an approximation to the moon's
   true phase.  {GLS} incorporated this routine into a {Lisp}
   program that, when it wrote out a file, would print a
   timestamp line almost 80 characters long.  Very occasionally
   the first line of the message would be too long and would
   overflow onto the next line, and when the file was later read
   back in the program would {barf}.  The length of the first
   line depended on both the precise date and time and the length
   of the phase specification when the timestamp was printed, and
   so the bug literally depended on the phase of the moon!

   The first paper edition of the {Jargon File} (Steele-1983)
   included an example of one of the timestamp lines that
   exhibited this bug, but the typesetter "corrected" it.  This
   has since been described as the phase-of-the-moon-bug bug.

   [{Jargon File}]

   (1995-02-22)