April 13, 2004
A: FROM Space Shuttle Flight Controller and MENTOR ANDREA GATICA IN TX
Basically, it is that we count down to launch, and then count up during the mission. That way you know how long it is until we are going to launch, and then how long we have been flying, without having to do the math in your head. The countdown clock starts at T-46 hours, so it is not just the final 10 seconds that we do the counting.

There is a "Countdown 101" web page on the KSC PAO website at
http://www-pao.ksc.nasa.gov/shuttle/countdown/sts109/ctdn101.htm that gives more information about the subject and events.
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A: FROM MENTOR BARB KONTOGIANNIS IN CO
A launch event occurs at what we call T-0. Events leading up to the launch are a "minus count" starting from several hours before the launch when critical operations begin. Then after the launch, the time is counted upward, called the "plus count." Think of it as a number line, with 0 being the critical event. In order not to confuse the two activities, space launches are started on the negative side of the number line, pass through 0 at the critical event (typically a launch), and then up again on the positive side of the number line. Activities to launch a rocket don't end when it lifts off. Critical events, both on the ground and in the air, occur after T-0. Some of the more well known events are things like staging when the solid rocket motors come off, or in the case of the shuttle, when the external tank is jettisoned. These occur at T+times. It would be very confusing to count up before a launch, and then count up again afterward. Also, what would be the final launch number you would count up to? 10? 100? 2468? That would be hard to determine, and not very consistent. Zero is a good choice to signify the main event, then you can count down and up from there.
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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.