Click here to purchase the entire book in PDF format. The second If you think back, way back to a time before clocks and watches, back before we even discovered fire, and then think about how people would have marked time. There were two clues - light and dark. The sun goes up, you wait a while, the sun goes down, you wait a while and the cycle repeats itself. So, we had the concept of a day. In many early civilizations, the time between sunrise and sunset was was divided into 12 equal divisions. Similarly the time between sunset and sunrise was also divided into 12. Why 12? Well, many counting systems were based on 12 once upon a time - which is why we have leftovers still today like 12 eggs in a dozen, 12 inches in a foot, 12 months in a year, and 12 zodiacal symbols. It helped that we have 12 bones in our fingers (3 on each), so you can count to 12 by using your thumb and touching each of your phalanges (look it up if you think that I'm being rude...). The division of one-twelfth of a day was the basis of what we now call an hour. Interestingly, since it was originally based on the amount of daylight in a day, its length would vary depending on when in the year it was measured. The hour was subdivided into 60 equal divisions called prime minutes (60 was used a lot in the counting system of the ancient Babylonians). The prime minute was further subdivided into 60 divisions called second minutes, but these days we're lazy and we just call them seconds. So, for a long time, a second was simply 1/60th of a minute, which was 1/60th of an hour which was 1/24th of a day, which was 1/365th of a year. (It may be of interest to know that this means there are approximately $\pi$ x $10^7$ seconds in a year.)[Wikipedia, ] However, this definition of a second is not really good enough, since it is based on the length of time that takes the earth to go around the sun, or at least for the earth to spin around its own axis. Since this a variable amount of time (the spin of the earth is slowing down, but I wouldn't worry about it affecting your work week...) a new, more stable definition had to be found. That new definition was accepted in 1967 and refined in 1997 by the CGPM as being ``the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state [in a zero magnetic field] of the caesium-133 atom ... at rest at a temperature of 0 K.''[BIPM, ]