[time-nuts] establishing your position w/o gps
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.
>>> 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
>> ship without a clock.
>> Classic navigation with a sextant needs a clock and sightings on 3
>> the sky. Each sighting gives you a circle on the globe, or a line if
>> 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
>> 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
>> 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
>> 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.
>> 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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