[time-nuts] establishing your position w/o gps

Chris Albertson albertson.chris at gmail.com
Tue Jan 24 13:26:12 EST 2012


have you ever tried to measure an angular distance using a hand held
instrument while standing on the deck of a moving boat in the open
ocean?  try it and you will see why they wanted a clock.     You
really can't measure an arc minute reliably we should expect about 15
arc minute accuracy if you are standing on a moving ship.  A few very
skilled people could do better.

The moon moves what? about 10 degrees per day so in practical terms
you can get time to about 30 minutes.   But other sources of error
would add to that.   But still knowing even the hour is very good
that puts you in the correct time zone

On Tue, Jan 24, 2012 at 9:54 AM, jmfranke <jmfranke at cox.net> wrote:
> In addition to the moons of Jupiter, there was a method in direct
> competition with Harrison. It was the Lunar distance method. The Lunar
> distance method used the position of the Earth's moon against the zodiac as
> a clock. The term lunar distance was used because the navigator measured the
> angular distance from the moon to various stars to establish the moon's
> position and then the time was deduced from lunar position tables.
> Developing the lunar distance tables was part of the reason for establishing
> the Royal Observatory.
>
> John  WA4WDL
>
> --------------------------------------------------
> From: "Chris Albertson" <albertson.chris at gmail.com>
> Sent: Tuesday, January 24, 2012 11:36 AM
>
> To: "Discussion of precise time and frequency measurement"
> <time-nuts at febo.com>
> Subject: Re: [time-nuts] establishing your position w/o gps
>
>> On Tue, Jan 24, 2012 at 3:16 AM, Attila Kinali <attila at kinali.ch> wrote:
>>>
>>> On Tue, 24 Jan 2012 11:04:08 +0000
>>> "Poul-Henning Kamp" <phk at phk.freebsd.dk> wrote:
>>>
>>>> In message <20120124115848.312d60bd4fccce4f3e71c136 at kinali.ch>, Attila
>>>> Kinali w
>>>> rites:
>>>>
>>>> >All this talk about telling the time using stars or the sun made me
>>>> > >wonder
>>>> >how did people tell what position their telescopes had back in the days
>>>> >before GPS?
>>
>>
>> Sailingships and trade was what pushed this.   At the time of Columbus
>> he was able to know his latitude within a few 10s of miles but even
>> after returning to Europe he did no know how far around the world he
>> had sailed.  Was it 1/3rd or 2/3rds?  They had no way to know.    The
>> problem was that on one had a clock that should keep time well enough.
>> They used hour glasses on board ship for short duration time keeping
>> but those were of no use on a longer ocean crossing.
>>
>> Later they discovered the idea of common view of the moons of Jupiter
>> and they could measure the time from local noon some even on Jupitor
>> while a person back home did the same thing.  Later when he got back
>> home they compare notes and then know the difference in longitude.
>> Good ocean going clocks were still centuries away.    But in the
>> 1500's they could only know the location after the fact when they
>> returned
>>
>> --
>>
>> Chris Albertson
>> Redondo Beach, California
>>
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-- 

Chris Albertson
Redondo Beach, California




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