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Countdown facts for kids

Kids Encyclopedia Facts
Women world cup 2015 countdown
2015 FIFA Women's World Cup countdown at Champlain Place, Dieppe, New Brunswick

A countdown is a sequence of backward counting to indicate the time remaining before an event is scheduled to occur. NASA commonly employs the term "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.

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.

Rocketry

Countdown clock at NASAs Kennedy Space Center on 28 April 2011 (5665401600)
Countdown clock at NASA's Kennedy Space Center at L-11 hours (28 April 2011) of STS-134, Space Shuttle Endeavour

A countdown is a carefully devised set of procedures ending with the ignition of a rocket's engine. Depending on the type of vehicle used, countdowns can start from 72 to 96 hours before launch time.

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;
  • Security personnel prevent unauthorized persons from entering the "keep-out" area.

Film

At the start of films, a countdown sequence is printed on the head leader, to aid in synchronizing the film. In film (but not television) the countdown is in units of feet rather than time units.

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.

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