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

jmfranke jmfranke at cox.net
Tue Jan 24 18:47:10 EST 2012


For a rough determination; you are facing due south, or due north when the 
elevation of a celestial body stops increasing with time. The elevation is 
highest when the body is on the observer's local meridian. There are 
exceptions, for instance when observing a body below Polaris, then the body 
reaches the lowest elevation when crossing the observer's local meridian, 
but reaches its highest elevation twelve sidereal hours later.

John  WA4WDL

--------------------------------------------------
>
>>
>>> But how do you untangle longitude and time? How do you know that you are
>>> looking exactly south (or north)?
>>
>> If I understand what you are asking, it's the same problem as navigating 
>> a
>> ship without a clock.
>>
>> Classic navigation with a sextant needs a clock and sightings on 3 
>> objects
>> in
>> the sky.  Each sighting  gives you a circle on the globe, or a line if 
>> you
>> know roughly where you are.  The lines form a triangle.  The size of the
>> triangle is an indication of the accuracy.  You pick the objects so the
>> triangle is roughly equilateral.
>>   http://en.wikipedia.org/wiki/Celestial_navigation
>> You can get time from the moon, so in theory at least, that's an answer 
>> to
>> your question.
>>   http://en.wikipedia.org/wiki/Lunar_distance_%28navigation%29
>>
>> Years ago, when a friend was learning navigation, he was reading one of
>> the
>> old classic texts.  There was a good story about the guy off the coast of
>> England/Ireland who didn't trust his clock, so he did the calculations
>> again
>> assuming his clock was a bit fast and again with it slow.  That gave him 
>> 3
>> parallel lines for each sighting.  Anybody recognize that story?
>>
>>
>> Longitude by Dava Sobel is a good read, especially for time-nuts.  There
>> is
>> also a version with lots of good photographs.
>>
>> One of the techniques they actually considered before Harrison built good
>> enough clocks was to derive time from Jupiter's moons.  They knew enough
>> to
>> correct for the time shift due to speed of light delays as the
>> Earth-Jupiter
>> distance changed.  (I don't know if they knew if was due to speed of
>> light.)
>>
>> You can synchronize two clocks if both sites can see the same event in 
>> the
>> sky,  Occultations are often used for this.
>>
>> With modern technology, radio telescopes are very very good at this.   In
>> order to do VLBI, you need to know where the telescopes are located. 
>> With
>> a
>> big collection of data you can do least-squared fit type calculations to
>> refine the location and clock calibration.
>>
>> --
>> These are my opinions, not necessarily my employer's.  I hate spam.
>>
>>
>>
>>
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