April 12, 2004
A: FROM MENTOR JOAN LUSK IN RI
"Why" is always a hard question - if NASA didn't leave a written
record we'd have to read minds - but when there is a specified time =
0, it's clear how far away that time is if you're counting down to
it. If NASA picked a positive number for the time a rocket should be
launched, and counted up 1, 2, 3, ... you'd have to remember that
number and subtract the count from it to know how long to wait for
the launch. So it's simpler to count down. The practice might also
come from the military - D-day minus 3, 2, 3, H-hour minus 3, 2, 1.

Those little bits of video or film before a movie begins also count
down to the start of the movie, for the same reason, I think. But I
guess kids playing hide 'n seek still count _up_ to 100.