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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: Fred Hebard
Date: 2008 Apr 15, 12:45 -0400
From: Fred Hebard
Date: 2008 Apr 15, 12:45 -0400
Lu, Why billionths of an arcsecond? One arcsecond gets one to 1/60th of 100 feet in traditional surveying, or about 50 cm. One-thousandth of an arcsecond would drop one to 5 mm. I wonder if refraction is a problem here. Fred On Apr 15, 2008, at 12:33 PM, Lu Abel wrote: > > 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 -~----------~----~----~----~------~----~------~--~---