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

J. Forster jfor at quikus.com
Tue Jan 24 23:19:10 UTC 2012

```Is the USNO almana/ephemeris still published in hard copy every year? That

-John

===============

-John

================

>
>> 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.
> You can get time from the moon, so in theory at least, that's an answer to
>
> 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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>

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