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post #1 of 25 Old 12-09-2010 Thread Starter
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Question For Celestial Navigators

For those of you who use your sextant, do you use a chronometer or radio for GMT? Or are digital watches/clocks accurate enough these days that you can just set the time to GMT? Any chronometer, watch, clock recommendations? Looking for a good dual second hand stopwatch too.

Thanks in advance.

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post #2 of 25 Old 12-09-2010
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post #3 of 25 Old 12-09-2010
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I don't use a sextant on my present craft, however, back some years ago when I navigated a KC-135 we used plain ol' wrist watches (w/sweep-second hands) and checked them against the WWV radio signal once a week.

Seemed to work trans-Pacific at 430 knots.
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post #4 of 25 Old 12-09-2010
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how much money do you want to spend is, again, the issue. if you are relying on your sextant for a fix in the middle of the ocean you might want a rolex/tag/etc. if it is more for hobby type use, i use an analog/digi combo with the analog set to local, digi set to gmt and have found that i have a 4secish differential every 3ish days. that is a watch that lists for 150 and got for 50.

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post #5 of 25 Old 12-09-2010
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When I play at CN (and it is just playing since eyeball navigation suffices around here), I use my (non-chronometer) wristwatch, which every once in a while I synchronize via the USNO clocks on the web. I don't set my wristwatch to GMT because it's a constant offset from GMT (though I sometimes forget to subtract the offset!).

I know you're supposed to know the drift rate of your chronometer so that you can estimate chronometer error and adjust the displayed time. So over a period of several months, at noon on Sunday I'd check my watch against the clock you can phone into, and record the error without resetting the watch. Unfortunately I've lost my notes, but I remember being surprised at how little it was, something less than 20 seconds gained over the whole period of several months.

My guess is that the errors you'd expect in CN are probably bigger than the errors that would be produced by using an imperfect watch. It would be an interesting exercise to compute the error in a noon sight if your watch is off by one minute. The sun moves a quarter of a degree per minute, iirc, so at the you'd be out by 15 minutes of longitude for every minute your watch is off, or a quarter minute for every second, or 5 minutes on my own wristwatch after several months of not resetting the watch.

The error might be greater if you're solving arbitrary navigational triangles, but I'll leave that as an exercise to the reader.

In the end what's important is that you have not only an estimate of where you are, but bounds on the error of your estimate. If I know that, at the equator, my watch could be producing up to five miles in error, then I have to keep five extra miles away from any hazards.

As for the radio, I'm not really sure how to get the signal. I have one of those cheap "world radios" that's supposed to receive from anywhere, I have tuned to the frequencies of the various time signals, but to no avail. I suspect that I'm using the wrong sort of radio for the purpose (maybe I need SSB or something), but in any case I'm stuck with my wristwatch.

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post #6 of 25 Old 12-09-2010
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Casio Pathfinder PAW-1500

I have a Rolex that is a mechanical wonder and looks very nice, but to have a very accurate time source aboard for celestial navigation, I use a Casio Pathfinder PAW-1500.

It uses the Colorado time sync signal every night to get itself well within a second of U.S. atomic time (on the occassions where I do check it against shortwave tickers, it is always dead on). Has phases of the moon and tidal state on the main display, plus barometric pressure. And is solar powered.

Any quartz/digital clock that time syncs to the beacon should be similarily accurate.

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post #7 of 25 Old 12-09-2010
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Quote:
Originally Posted by AdamLein View Post
As for the radio, I'm not really sure how to get the signal. I have one of those cheap "world radios" that's supposed to receive from anywhere, I have tuned to the frequencies of the various time signals, but to no avail. I suspect that I'm using the wrong sort of radio for the purpose (maybe I need SSB or something), but in any case I'm stuck with my wristwatch.

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post #8 of 25 Old 12-09-2010
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Quote:
Originally Posted by RhodesSwiftsure View Post
I have a Rolex that is a mechanical wonder and looks very nice, but to have a very accurate time source aboard for celestial navigation, I use a Casio Pathfinder PAW-1500.

It uses the Colorado time sync signal every night to get itself well within a second of U.S. atomic time (on the occassions where I do check it against shortwave tickers, it is always dead on). Has phases of the moon and tidal state on the main display, plus barometric pressure. And is solar powered.

Any quartz/digital clock that time syncs to the beacon should be similarily accurate.
is your rolex less accurate? just curious... i wonder histroically which has a greater chance of failure too... and if your rollie is strapped on you cant drop it overboard!!!

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post #9 of 25 Old 12-09-2010 Thread Starter
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Thanks for the quick replies. That Casio looks pretty cool. Though I wonder, and not at an REI at the moment, is it bulky to wear?

Mick - Budget is as much as you want to drop in the water...

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post #10 of 25 Old 12-09-2010
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Mick - Budget is as much as you want to drop in the water...

hmmmm..... i thought you were awnsering the question 'which boat should i buy...'

unless its your home, of course...

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