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    Re: Precision clock
    From: Paul Hirose
    Date: 2008 Jan 09, 09:59 -0800

    Geoffrey Kolbe wrote:
    >
    > Is there a stopwatch out there that can be set to GMT so will act as a
    > chronometer over a period of weeks or months, but has a feature where a
    > button can be pressed  and the display will show the time at which the
    > button was pressed to a tenth of a second or better? Then, at another press
    > of a button, current time is displayed once again in the normal way.
    
    Years ago when I computed satellite orbital elements from visual
    observations, I obtained UTC from from a digital stopwatch. I started at
    a known time by listening to WWV on a radio. Rather than the top of a
    minute, I started the watch on the 10th second of the minute. This
    allowed me to catch the rhythm of the seconds ticks, and hit the button
    at the instant of the tick, rather than reacting to the tick. (WWV omits
    the ticks at the 29th and 59th seconds, which makes this method harder
    to use at the top or bottom of a minute.)
    
    After starting the watch, I would then test for an accurate setting by
    hitting the "split" button (to temporarily freeze the display while the
    watch continued to run) on several ticks, checking for a near zero
    fractional seconds reading each time. With a bit of practice it's no
    great trick to hit the mark plus or minus .02 second.
    
    To time satellite observations, I used the split button and let the
    stopwatch memory save the times. It could hold up to 10, which was more
    than I ever needed.
    
    Of course with this method it's necessary to add the starting time and
    the stopwatch time to recover the true time of each observation. But a
    similar step would be necessary if the stopwatch indicated time of day.
    You'd still have to check its error and apply a correction to each
    observation.
    
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