Countdown
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2008-06-12T18:03:57Z
Cornellrockey
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wikifcation
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A '''countdown''' is the backward counting to indicate the seconds, days, etc. remaining before an event occurs or a deadline expires. Typical events for which a countdown is used include the launch of a [[rocket]] or [[spacecraft]], the explosion of a [[bomb]], the start of a [[racing|race]], or the [[New Year]].
== Rocketry ==
[[Image:STS-121 Launch.jpg|thumb|330px|Seven seconds after launch of [[STS-121]], the [[Press Site-Clock and Flag Pole|countdown clock]] on the grounds of the [[Kennedy Space Center]] News Center is counting up, rather than down.]]
In the context of a rocket launch, the "T minus Time" is the time before launch, e.g. "T minus 3 minutes and 40 seconds". The last ten seconds are usually counted down aloud "10, 9, 8, 7, 6, 5, 4, 3, 2, 1, Lift off!" After a launch, most countdown clocks begin to show Mission Elapsed Time.
The first known use with respect to launching a rocket occurred in [[Fritz Lang]]'s [[1929]] [[film]] ''[[Frau im Mond]]''{{fact|date=November 2007}}. In the film, after each number the words "seconds to go" was repeated.
When counting down to the launch of an [[explosive]], e.g. when testing a new model of gun, it is customary to omit "5" from the countdown sequence because "five" sounds too similar to "Fire!"{{fact|date=November 2007}}.
== Film ==
At the start of films, a countdown sequence is printed on the [[Film leader|head leader]], to aid in synchronizing the film.
The countdown ends just as "two" is displayed so that the countdown isn't shown if the film is put on screen a little earlier than intended. Although this sequence was usually not intended for broadcast, being a cue device for a projectionist or TV producer, it is sometimes used in modern-day [[parody|spoofs]] to signify old film footage.
==New Year's Eve==
{{main|New Year's Eve}}
[[Image:2008TaipeiCityNewYearCountdownParty Firework Taipei101.jpg|thumb|200px|[[Taipei 101]] [[New Year]]'s [[fireworks]] and countdown, 2008]]
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.
Additionally, countdowns are often used to count down the days before special events, such as major holidays and one's birthday.
== External links ==
* [http://www.nasa.gov/returntoflight/launch/countdown101.html NASA Countdown 101]
* [http://www.thecountdownclock.com Countdown Clocks Software]
[[Category:Time]]
[[Category:Spaceflight]]
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