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Old 07-04-2011, 15:04   #16
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Re: What Clock to Choose for Celestial Navigation ?

A Casio.

I keep eye and take notes on how much it advances or slows. Bring it back to normal via our GPS. I know there are time signals on HF too but never used them.

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Old 07-04-2011, 20:15   #17
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Re: What Clock to Choose for Celestial Navigation ?

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Originally Posted by SaltyMonkey View Post
...
accuracy with a sextant within a mile? On a small craft? With a plastic sextant? ROFL!!!
Keep in mind this is an additive error.

If you are in conditions where you normally can shoot within +- 4 miles, but your watch is off 16 seconds, your accuracy is now +- 8 miles.

Getting a position with a sextant is all about adjusting for, and reducing, errors. They all add up.

-dan
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Old 07-04-2011, 22:10   #18
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Re: What Clock to Choose for Celestial Navigation ?

GPS time is the most accurate.
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Old 08-04-2011, 04:56   #19
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Re: What Clock to Choose for Celestial Navigation ?

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Originally Posted by GordMay View Post
You can check the accuracy of your timepiece, without an HF radio,
here ➥ The official US time (NIST & USNO)
Site says it is accurate to 12.7 seconds: Surely we can do better than that? Tony
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Old 08-04-2011, 06:11   #20
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Re: What Clock to Choose for Celestial Navigation ?

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Originally Posted by bvimatelot View Post
Site says it is accurate to 12.7 seconds: Surely we can do better than that? Tony
Where do you read that, Tony?

I read:
... This public service is cooperatively provided by the two time agencies of the United States: a Department of Commerce agency, the National Institute of Standards and Technology (NIST), and the U. S. Naval Observatory (USNO). Readings from the clocks of these agencies contribute to world time, called Coordinated Universal Time (UTC). The time maintained by both agencies should never differ by more than 0.000 0001 seconds from UTC ...
Here About www.time.gov
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Old 08-04-2011, 06:20   #21
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Re: What Clock to Choose for Celestial Navigation ?

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Originally Posted by GordMay View Post
Where do you read that, Tony?

I read:
... This public service is cooperatively provided by the two time agencies of the United States: a Department of Commerce agency, the National Institute of Standards and Technology (NIST), and the U. S. Naval Observatory (USNO). Readings from the clocks of these agencies contribute to world time, called Coordinated Universal Time (UTC). The time maintained by both agencies should never differ by more than 0.000 0001 seconds from UTC ...
Here About www.time.gov
Well - I pulled up the site, clicked on my region (-4) and just under the clock, that's what it said. I've just opened it up again and now it says accurate to 0.4 secs!! Oh well!! T
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Old 08-04-2011, 06:49   #22
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Re: What Clock to Choose for Celestial Navigation ?

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Well - I pulled up the site, clicked on my region (-4) and just under the clock, that's what it said. I've just opened it up again and now it says accurate to 0.4 secs!! Oh well!! T
Re-opened and its 3.9 secs.....
T
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Old 08-04-2011, 06:57   #23
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Re: What Clock to Choose for Celestial Navigation ?

I never noticed that before, Tony.
I wonder why the accuracy varies, like that?
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Old 08-04-2011, 07:06   #24
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Re: What Clock to Choose for Celestial Navigation ?

When trying to choose a watch to use, adjusted all old the timepieces I had to same US timeclock and then recorded their error over time. Found that the newest Casio wristwatch, a SGW- 1008, which also has a handy electronic compass, was accurate to within 1 second over 3 months! It has been 6 months now and it is still within a second or two. Two old Casios with worn out cases, even with new batteries are off a whole lot and not worth trying to use for celestial nav. Didn't know they were adjustable as someone mentioned above. Will have to try that. Seems like the technology must have become better in the newer watches.
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Old 08-04-2011, 07:08   #25
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Re: What Clock to Choose for Celestial Navigation ?

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I never noticed that before, Tony.
I wonder why the accuracy varies, like that?
It has to do with your internet connection, not the clock.
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Old 08-04-2011, 07:25   #26
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Re: What Clock to Choose for Celestial Navigation ?

My cheap $20 Timex gained exactly 20 secs this year from last year. (oops I didn't recheck when I reset it for daylight savings time). I may have clicked it 20 seconds early.
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Old 08-04-2011, 07:44   #27
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Re: What Clock to Choose for Celestial Navigation ?

I check against my country's "national time signal" about once a week. It's a Suunto Vector, a middle-aged "wrist top computer" still sold for about $200 for use by hikers that has clock, stopwatch, alarm, baro, altimeter and compass functions. It also lights up with a dim, greenish light for 5 seconds (preserving night vision) and is waterproof to 10 metres...which I've proved.

Needless to say, that's 90% of what a sailor wants at a fraction of the "yachtsman's chronometer". On the Vector, the graphical way in which the seconds are tallied is particularly suited, I find, to working with a sextant (appearing, advancing and disappearing little squares).

I "rate" it by periodic "hack" type checks. I can never move fast enough to nail it, but I know it's currently four seconds slow, and when three months pass, it will be five seconds slow. I need to change the single coin type battery ($7) about once every 10 months. A method to nail the time within a second would be to pop the battery in at the top of the hour as per the time signal to get the "12:00" on the watch. Then, just adjust the hour to local time.

On board, I would keep a dual local/GMT 24 hour clock for general reference (how far in time zones from ZULU are we?), but the wristwatch is near perfect otherwise.

My only criticism of these class of watches is that they have Lexan watchface crystals that are too easily scratched. I buff it occasionally with baking soda toothpaste (an old furniture refinisher's trick) and this minimizes the scratches.
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Old 08-04-2011, 08:15   #28
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Re: What Clock to Choose for Celestial Navigation ?

From the thread just recently on CF - Cruisers & Sailing Forums > The Life Aquatic Aboard a Boat > Navigation Acceptable Accuracy in Celestial Navigation
it seems to be the consensus that C.N. is good for about 5 nm of accuracy on a small boat - when you can get a sky condition to take sites. Likewise, it is discussed that time accuracy equates to about a 1/4 nm per second of time error.
- - As to source of time information WWV on HF radio is the best and most reliable - if you can receive the broadcasts. There are about 5 different kinds of "time" in use around this world. To get the most accurate "astronomical" time you use WWV and correct the time by the number of double ticks you hear after the "on the minute" beep. That corrects the broadcast time to astronomical time. See: The surveying handbook - Google Books
- - Then you correct your local watch/timepiece to that time. It is hardly likely in the discussed hour or so it takes to complete the sight taking and reduction that your local timepiece will get "off-time" any significant amount.
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Old 08-04-2011, 08:23   #29
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Re: What Clock to Choose for Celestial Navigation ?

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Originally Posted by bill good View Post
GPS time is the most accurate.
Not actually true. The atomic clocks in the observatories are more accurate, and GPS system occassionally has to be updated. However, if you need to use astro, the GPS has probably malfunctioned, so you need an alternative with a known error.
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Old 08-04-2011, 10:27   #30
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Re: What Clock to Choose for Celestial Navigation ?

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Originally Posted by estarzinger View Post
It has to do with your internet connection, not the clock.
Yep - I can believe that! Its back to 0.4 as I write. That's 3rd world internet connections for you.

Otherwise, its a nice website and has a neat little widget if you're in to those sorts of things.

Not really useful for astro nav, which supposes that you are well off the grid and have been for a coupla weeks...In any event, if I get a fix in the right hemisphere I'm quite proud of myself!! Tony
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