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For other meanings of the term countdown, see Countdown (disambiguation).

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 race, or the New Year.

Seven seconds after launch of STS-121, the countdown clock on the grounds of the NASA 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^{[citation needed]}. 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!"^{[citation needed]}.

At the start of films, a countdown sequence is printed on the head leader, to aid in synchronizing the film.

The countdown ends before "one" 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 spoofs to signify old film footage.

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.

• NASA Countdown 101

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