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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Navigation without Leap Seconds
From: Lu Abel
Date: 2008 Apr 15, 09:33 -0700
From: Lu Abel
Date: 2008 Apr 15, 09:33 -0700
Fred: In theory, yes; in practice, no. To position oneself using star-star distances would require require measuring angles to billionths of an arc-second. Maybe something an astronomer could do, but not something you or I are going to do with our sextants! BTW, I remember a conversation with a radio-astronomer about 20 years ago where he said that his team had measured the distance between two radiotelescopes on opposite sides of the US to within a cm or so using a technique called long-baseline interferometry. But the whole experiment took them a year or so... Lu Abel Fred Hebard wrote: > Completely unrelated, but stemming from the same article. > > The author states that height can only be known to some few cm or > whatever because of variations in gravity, if I remember correctly. > It would seem that this is due to our tradition of assuming we are on > the surface of a spheroid or ellipsoid when doing navigation. > Confining ourselves to a surface makes the trig easier, but couldn't > one position oneself with greater accuracy (with feet firmly planted > on earth, not on a boat) using only stars or stars plus the sun, > ignoring the earth's horizon, by measuring star-star distances? Make > it a true 3-D problem. Or would uncertainties in the positions of > stars still hamper ones efforts, especially uncertainty in their > distance from us? > > Fred Hebard > > On Apr 14, 2008, at 9:50 PM, frankreed@HistoricalAtlas.net wrote: > >> The fascinating article which Fred Hebard linked: >> http://www.physicstoday.org/vol-59/iss-3/p10.html >> includes a detailed discussion about the problems of gravitational >> time >> dilation and extremely accurate clocks. That's the main topic, and >> it's >> great stuff. >> >> The article also mentions leap seconds and navigation: >> "Celestial navigators --that vanishing breed-- also like leap >> seconds. The >> Global Positioning System, however, cannot tolerate time jumps and >> employs a >> time scale that avoids leap seconds." >> >> So here's my question: what's the best way of doing celestial >> navigation if >> leap seconds are dropped from official time-keeping? I don't think >> it should >> be all that difficult to work around, but I'm not sure what the best >> approach would be. Assume we get to a point where the cumulative time >> difference is, let's say, 60 seconds (that shouldn't happen for >> decades, so >> this is just for the sake of argument). Should we treat the >> difference as a >> 60 second clock correction before working the sights? Or should it >> be a 15 >> minute of arc longitude correction after working the sights? Or >> something >> else entirely?? >> >> -FER >> Celestial Navigation Weekend, June 6-8, 2008 at Mystic Seaport Museum: >> www.fer3.com/Mystic2008 >> >> >> >> > > > > > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---