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.
