Re: Longitude, was Time Ball, was Signal Stations

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Posted by Jon on October 29, 2007 at 14:57:42 from user Jon.

In Reply to: Re: Longitude, was Time Ball, was Signal Stations posted by Laurence Monkhouse on October 27, 2007 at 19:56:12:

Having learned the arcane science of shooting the sun and deriving one's position, I can appreciate how he did so. Any single sighting, be it on a natural body of a GPS satellite, gives a line of position. If one has a reasonably good idea of one's previous position and course/speed made good, your EP is where the LoP and course cross.

As for the two old sea captains who kept coming up with wildly-different positions for their (presumably stationary) homes, I find it hard to believe they'd overlook the thermal refraction correction, but it's possible that, if they shot the horizon instead of taking half the angle between the sun and it's reflection in a perfectly horizontal surface, an uneven horizon-line could throw them off. I would hope they didn't forget to include their altitude. When I was learning, in central Maine in January, we used a pool of oil shielded from breezes, and sitting on a window-ledge, for our artificial horizon. Aircraft sextants use a bubble level arrangement to the same end - one aligns the object being "shot" with the center of the bubble and gets the angle that way.

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