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.

