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)