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What is a second?

  1. Oct 24, 2008 #1
    As the title says what exactly is the defination of a second?

    I am very curios about this.
  2. jcsd
  3. Oct 24, 2008 #2
    the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom

    taken from wikipedia. but I remember learning it in physics.

    Oh, and its the unit of time...
  4. Oct 24, 2008 #3
    lol why is it that?
  5. Oct 24, 2008 #4
    Well, I guess cause its something consistent that we can measure. They want something that is universally constant in nature.

    Its the same with a meter, being defined by distance traveled by light in absolute vacuum in 1⁄299,792,458 of a second.
  6. Oct 24, 2008 #5
    But before could they measure the time it took for the radiation or the speed of light?
  7. Oct 24, 2008 #6
    Well, the first semi-accurate measurement of time came from pendulums. they had second pendumlums and what not. They also had mechanical clocks which showed seconds and minutes and hours like modern clocks. I doubt they were, but they could have beared a resemblance to modern day grandfather clocks. they based there days off of the ascension of stars.

    you can read in depth about it here:
  8. Oct 24, 2008 #7
    Thanks :) Interesting article.
  9. Oct 25, 2008 #8


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    Staff: Mentor

    And the one that you could easily check by yourself before asking...
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