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Re: What time is it, really?
From: Greg R_
Date: 2008 Jul 18, 00:18 -0700
From: Greg R_
Date: 2008 Jul 18, 00:18 -0700
--- "Gary J. LaPook"wrote: > It's actually 15.041� per hour (15� 2.5') approximately 361� per > solar day. Hmmm... not questioning your math, but if the Earth rotates 1� beyond a complete rotation every day, wouldn't we need to add leap days every year (i.e. 365� "extra" rotation in a year = 1 extra day + 5� "left over"), instead of approx. every 4? Seems like it should be closer to something like 360.25�/day (?). Then again, it's late and I'm not thinking clearly on this one.... -- GregR --- "Gary J. LaPook" wrote: > Gary writes: > > It's actually 15.041� per hour (15� 2.5') approximately 361� per > solar day. > > gl > > > Bill wrote: > > As understand it, with an earth rotation of 15d per hour, 1 second > time > equals 0.25 arc minute. It follows that 4 seconds time would equate > to 1 > arc minute. > > > >Bill asked > > > > > > > >>>What time is it, really? > >>> > >>> > >>I believe the musical group Chicago answered that question back in > the > >>late '60s... ;-) > >> > >> > > > >And does anyone really care? I do. > > > > > >>>A while ago there was a thread on time and the affect of dropping > >>>leap seconds on cel nav. > >>> > >>> > >>Don't think I was on the list for that thread, but as I understand > it > >>leap seconds are added to UTC as needed to keep it within 0.9 > seconds > >>of astronomical time. > >> > >>The rule that I remember from back when I was first learning celnav > was > >>that your observation time had to be accurate within 4 seconds, > >>otherwise your LOP could be off by up to 1 NM just from that error > >>alone (I interpret that to mean +/- 2 seconds). So I would say that > >>unless you need exceptional accuracy with your celnav sights you're > >>probably OK just ignoring the leap seconds. > >> > >> > > > >As understand it, with an earth rotation of 15d per hour, 1 second > time > >equals 0.25 arc minute. It follows that 4 seconds time would equate > to 1 > >arc minute. An arc minute of longitude would be nominally 1 nm at > the > >equator, but less if the vessel's AP is north or south of the > equator. > >Roughly 1' longitude * cos latitude = fraction of a nautical mile > (ignoring > >oblateness). For example, near an elevated pole 360d longitude > could be > >under 1 nautical mile. > > > >And why--despite the "former" CTA's cavalier attitude towards > >chronometers--would I care? With an artificial horizon, my Astra, > and a 3.5 > >scope, I consider an intercept of 0!0 from an average of 5 or more > >observations from a known GPS position lucky. 0!1 very good. 0!2 > average. > >0!3 fair, and > 0!3 has me checking IC and sextant calibration. > > > >I figure an artificial horizon cuts IE and observation errors in > half, so it > >gives me 0!0 to 0!6 (averaged-observations intercept) as goal to > shoot for > >under ideal conditions. > > > >I have never experienced my ideal conditions. They would include a > crisp > >horizon, clear sky, and a relatively stable (or predictable) > platform. And > >of course accurate UT1 time. But if I ever do... > > > >Bill B > > > > > >> > > > > > > > > > > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---