[time-nuts] Newbie to Time Nuts; Seeking wisdom, re Hydrogen MASER applications
michael.cook at sfr.fr
Mon Nov 13 08:10:07 EST 2017
> Le 13 nov. 2017 à 12:12, Hal Murray <hmurray at megapathdsl.net> a écrit :
> michael.cook at sfr.fr said:
>>>> prior to my senior project most geodetic surveyors used a Wooden boxed,
>>>> marine chronometer, to get sub second UT1 time, or back then, GMT
>>> How did you get the data out of the wooden box?
>> I have a couple of marine chronometers that have electrical contacts
>> closing once a second. This signal is relayed by wires to terminals on the
>> outside of the box.
> That gets you seconds if you count them. How do you get sub seconds? Just
> count time since the PPS using a normal crystal and it will be good enough?
In a sense.. When I was in the merchant navy in the 60s there were no crystal watches, so when taking sights we « transferred time » to a good 1/5sec stepping deck watch previously synchronized to the chronometer which of course was kept in the shelter of the Bridge. As this was done just prior to sights the offset would be known to less than or equal to that increment. Marine chronometers may not be particularly accurate, but they can be extremely stable at about +/- 0.2sec or better per day variation. The daily drift being known from the clocks last rating, getting accurate offset timing from GMT was possible. The clocks themselves were re-rated every year. I’m in France and I don’t think that any borders in Europe were defined by astronomical observation, but in the US I believe that at least some of the state borders were thus fixed. As a second’s error in time will be about a nautical mile in US latitudes, I wonder if anyone has measured with GPS, how good the original surveys were.?
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