NavList:
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 14, 20:47 -0700
From: Lu Abel
Date: 2008 Apr 14, 20:47 -0700
Frank: The passage of time is very real -- it's the thing that physicists worry about -- for example the slowing of time with velocity or gravitational dilation, or the fact that it's the one thing in physics that seems irreversible (although Richard Feynman claimed that a positron (positively charged electron) was in fact an electron moving backwards in time). But answering the question "what time is it?" is a man-made thing. Over time (hah!) mankind has had lots of ways of answering the question. Days started at different points -- sundown, midnight (which in itself is really half a day from noon), sunrise, and so forth. And how to describe the passage of time over that day? We use hours, minutes and seconds. But the French tried to introduce a time measurement of 10 hours of 100 minutes of 100 seconds as part of the metric system (what fascinates me is why meters and grams and such stuck, but metric time didn't stick) At some point we got so good at measuring time (or, accurately, the passage of time) that we could see that the rotational period of the earth varied and that it was slowing. We could average the earth's rotation and thereby figure out very precisely how many vibrations of some atom would constitute 1/86400 of a day. But the earth's slowing required the injection of "leap seconds" so midnight would still occur at midnight and not a second or two or seven earlier. But again, this is a man-made artifact. A second is still a second, a minute is still 60 seconds, an hour is still 60 minutes -- but a day (the rotational period of the earth) is sometimes a bit more than 24 hours. All that GPS requires is that all satellites have the same notion of "what time is it" Yes, GPS doesn't use leap-seconds, but as long as we all agree on what time it is, it doesn't matter if our "time" is off by a second or two from leap-second time, just as GPS doesn't care what time zone we're in or whether we're practicing Daylight time. And that last sentence is the key to your question of "how would I practice celestial navigation if we didn't have leap seconds" As long as I could convert my local time into Almanac time, it wouldn't matter, just as I adjust from local time to GMT. If the celestial data in the almanac were calibrated in leap-second-less time, no correction would be required. But if my watch somehow showed leap-second-less time while the Almanac used them, then I'd have to know how many leap seconds had occurred since 1972 (the year leap-seconds were adopted). Meanwhile, GPS uses a "what time is it" based on UTC in 1980 but not adjusted for leap-seconds thereafter (which means GPS time is 14 seconds fast as compared to UTC). GPS receivers correct for this to give a "true" (ie, leap-second-corrected time display), but buried somewhere in the setup menu for most GPS receivers is an option to display GPS time instead of UTC or local time. Hope this helps. Lu Abel 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 -~----------~----~----~----~------~----~------~--~---