[time-nuts] establishing your position w/o gps
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
had moon timing, etc.
>> 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
> 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
> your question.
> Years ago, when a friend was learning navigation, he was reading one of
> 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
> 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
> 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
> correct for the time shift due to speed of light delays as the
> distance changed. (I don't know if they knew if was due to speed of
> 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
> 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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