A countdown is a sequence of backward counting to indicate the time remaining before an event is scheduled to occur. NASA commonly employs the terms "L-minus" and "T-minus" during the preparation for and anticipation of a rocket launch, and even "E-minus" for events that involve spacecraft that are already in space, where the "T" could stand for "Test" or "Time", and the "E" stands for "Encounter", as with a comet or some other space object, like a spacecraft.
Other events for which countdowns are commonly used include the detonation of an explosive, the start of a race, the start of the New Year, or any anxiously anticipated event. An early use of a countdown once signaled the start of a Cambridge University rowing race.
One of the first known associations with rockets was in the 1929 German science fiction movie Frau im Mond (English: Woman in the Moon) written by Thea von Harbou and directed by Fritz Lang in an attempt to increase the drama of the launch sequence of the story's lunar-bound rocket:
When I count one, two, three, four, ten, fifty, one hundred, the audience doesn't know when it's going to start. But when I count backwards, ten, nine, eight, seven, six, five, four, three, two, one, ZERO! – then they understand.
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Rocketry
A countdown is a carefully devised set of procedures ending with launch of a rocket. Depending on the type of vehicle used, countdowns can start from 72 to 96 hours before launch time.
There are two countdowns proceeding simultaneously:
The T-minus clock, a schedule of planned activities prior to launch (T−0), and
The L-minus clock, a mechanical countdown to the time of launch (L−0).
Except for the last few minutes, which are highly automated and rigid, scheduled activities rarely take exactly the scheduled time, and the T-minus clock only corresponds approximately to the time until launch.
A hold is the suspension of the normal countdown process, during which the T-minus clock is stopped and no planned activities take place. This can be done to investigate a technical process that has gone wrong, or to intentionally delay the launch, e.g. because of bad weather at the launch pad. Most countdown schedules also include some pre-planned built-in holds. These provide an opportunity to perform non-launch activities, handle unexpected issues, or to catch up on the schedule if it is running long.
Under some circumstances, a countdown may be recycled to an earlier time. When that happens, launch personnel begin following the countdown checklist from the earlier point.
During countdown:
Aerospace personnel bring the rocket vehicle to the launch site and load it with payload and propellants;
Launch-center computers communicate with sensors in the rocket, which monitor important systems on the launch vehicle and payload;
Launch personnel monitor the weather and wait for the launch window;
Film
At the start of films, a countdown sequence is printed on the head leader, and is used to synchronize film reel changeovers (switching between reels of film). In film (but not television) the 'Academy Leader' countdown first used in 1930 is in units of feet rather than time units; it starts at 11 and ends at 3 where it cuts to black for the last few feet. In 1959, SMPTE leader was created and measures seconds, not feet. It starts at 8 and cuts to black on the first frame of 2, which is accompanied by a 'pop.' This leader eventually displaced the older Academy and was the only leader used by the end of the optical (film) projection era.
Countdown timer
A countdown timer has been used by an organization/individual etc. in order to complete a task or mission before a deadline. Such devices have been used in various forms of media and popular culture such as game shows, videogames, movies etc.
New Year's Eve
In many New Year's Eve celebrations, there is a countdown during the last seconds of the old year until the beginning of the new year. These countdowns usually end in fireworks. Some celebrations also have countdowns to midnight in preceding timezones.
Independence Day
In Malaysia, the countdown is also used to celebrate independence day, with an event called Ambang Merdeka. The countdown was carried out at 11:59:50 pm and ended with chanting Merdeka 8 times and singing the national anthem.
Palestine Square Countdown Clock
After Iran signed a nuclear pact in 2015 that had a timeline of 25 years to complete, Ayatollah Ali Khamenei responded by saying that it wouldn't take that long for Israel to cease existing. In 2017, the Palestine Square Countdown Clock was unveiled in Tehran, counting down the supposed time to Israel's demise.