> Thanks, all ...this thread has helped me a lot. > > It reinforces my beliefs of what a 'second' is ...what we were taught from > day 1 in school: > > One solar day/86,400 (A 'solar' day being an earth solar day)
Hi Don, This was true until the 1960's. Hopefully the school curriculum has been updated since then. The definition of a pound, or a foot, or a second has progressed over centuries. We now have a very accurate and convenient definition of the second. And it is no longer 1/86400 of a day; it is no longer 1/31556925.9747 of a year either. The problem with "day" is you have to ask which one; each day has a different length when you measure down in the milli- or microsecond level; the earth doesn't spin very consistently. See: http://www.leapsecond.com/museum/earth/ > So as I try to rationalize all this in my mind, ...a good cesium unit is > just a sophisticated 'counter'. The counter gate is just the solar day. > > It's interesting to note (to ask?): When did someone get smart enough to > start measuring 1/86thousthanth of a day, ....That starts needing a standard > in itself. Exactly. A ~1-meter pendulum can conveniently be made to tick seconds. You can adjust the length until each day you count 86400 ticks. The earth is the standard and the pendulum clock is being measured. What was found in the early 1900's is that some precise pendulum clocks kept better time compared to themselves than to the earth. The conclusion was the earth is less stable than the best pendulum clocks. At that point the pendulum clocks became the standard and the earth became the thing to be measured. This was the beginning of the end for a second being 1/86400 of a day. The same thing is happening today with the cesium definition of the second. Optical clocks are so good that they are being used to measure the cesium clocks. So in years to come it is likely that the definition of the second will again be redefined; this time in terms of an optical frequency. /tvb _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
