NavList:
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, 18:00 -0400
From: Fred Hebard
Date: 2008 Apr 15, 18:00 -0400
Oops, divided twice by 60, not once. You're correct, about 100 feet. So .01 arcseconds to 1 foot, or 30 cm, and .0001 to 3 mm. Only off by a factor of 60! On Apr 15, 2008, at 4:04 PM, Gary J. LaPook wrote: > Gary LaPook wrote: > > How does one arc second get you to 50 cm? One arc minute is 1852 > meters divided by 60 makes 30.9 meters: or 6076 feet divided by 60 > equals 101.3 feet (approximately 100 feet in practice.) in an arc > second. > > gl > Fred Hebard wrote: >> 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 -~----------~----~----~----~------~----~------~--~---